origin and also many current gypsum products have the character of composites reinforced with fibre. Besides the often used cellulose and glass fibres, more ...
Czech Technical University in Prague Faculty of Civil Engineering
Ing. Alena Vimmrová, PhD.
FORMULATION OF GYPSUM FOAM MATERIALS
Prague 2009
© Alena Vimmrová, 2009
Alena Vimmrová
Formulation of Gypsum Foam Materials
Czech Technical University in Prague 2009
Content 1
INTRODUCTION ...................................................................7
2
THEORETICAL SECTION...................................................9
2.1 Gypsum....................................................................................9 2.1.1 Characteristics of gypsum.............................................9 2.1.2 Sources of gypsum......................................................10 2.1.3 Production of gypsum.................................................11 2.1.4 Types of gypsum binders............................................14 2.1.5 Setting of gypsum .......................................................15 2.1.6 Hardened gypsum .......................................................18 2.1.7 Modification of gypsum..............................................28 2.2 Gypsum materials and products .........................................31 2.2.1 Gypsum mortars and concretes...................................31 2.2.2 Gypsum blocks ...........................................................32 2.2.3 Gypsum plasterboards and gypsum fibre boards........33 2.2.4 Foamed gypsum..........................................................33 2.3 Formulation of a gypsum composite ...................................39 2.3.1 Components of a gypsum composite ..........................40 2.3.2 Optimization of a multi-component mixture ..............41 3
AIM OF WORK.....................................................................46
4
EXPERIMENTAL SECTION..............................................47
4.1
Used raw materials and chemicals ......................................47
4.2
Instruments and equipment used ........................................53
4.3
Experimental methods and testing procedures..................53
4.4
Survey of prepared compositions ........................................56
5
5
RESULTS AND DISCUSSION ............................................70
5.1 Composition based on gypsum Kobeřice............................73 5.1.1 Modification of gypsum paste properties ...................73 5.1.2 Choice of a foaming system........................................77 5.1.3 Sequential optimization of bulk density and strength.83 5.2 Compositions based on gypsum Rigips...............................88 5.2.1 Reinforcement with fibres ..........................................89 5.2.2 Foamed composites with a granular mineral admixture 90 5.2.3 Formulation of an acid foaming agent ........................95 5.2.4 Monitoring of other properties....................................99 5.2.5 Sequence optimization of a point criterion ...............102 5.2.6 Perlite-foam composite.............................................107 5.2.7 Optimization of material costs..................................108 6
CONCLUSIONS ..................................................................115
7
REFERENCES.....................................................................117
ACKNOWLEDGEMENT: This research has been supported by Czech Ministry of Education, Youth and Sports, under project No. MSM 6840770031
6
1
INTRODUCTION
Currently gypsum is the subject of increasing research attention as a low energy-use and ecologically friendly building binder. There are also being studied possibilities for a greater use of gypsum prefabricates and additionally there is being addressed the question of increasing the resistance of gypsum materials towards water. There are also being researched other ways of how to further increase the possibilities of using gypsum composites especially by improving their mechanical or thermal insulation properties. A common method leading to the improvement of the thermal insulation properties of any solid material is an increasing of the proportion of voids filled with gas in the structure of this material. In order to lower the coefficient of thermal conductivity to a value less than 0,15 W.m-1.K-1 , it is necessary that the proportion of the gaseous phase (pores or gaps) would be more than 60 volume percent. The porosity of quality insulation materials often exceeds the value 90 % of the volume, while natural (technologically normal) gypsum porosity usually is 45 – 55 %. It is then obvious that by increasing the proportion of pores filled in with gas in a structure of hardened gypsum the insulation properties of a gypsum mass can significantly improve and if other user properties can be kept at an acceptable level, there can be discovered some very interesting material. In the professional literature there is however reference to the preparation of materials based on foamed gypsum, although no product of this type is ordinarily produced in our country. The reasons are problems with the stability of foamed gypsum and a considerable decrease in the mechanical properties of foamed gypsum composites. In the context of an increased interest in the wider use of gypsum mass, such a situation is unsatisfactory. Gypsum inorganic foams with optimized properties could serve in the preparation of fire-resistant, thermal insulation materials with very favourable hygienic and ecological properties. In the submitted thesis there are studied the qualities of gypsum foamed by carbon dioxide produced by a double decomposition of carbonates directly inside a setting gypsum mass. The main aim of the experiments carried out was to find an appropriate foaming 7
system and to optimize the composition of a foamed gypsum composite. Regarding its thermal insulation properties and the low energy consumption in its production, foamed gypsum can, in such a way, be an ingredient in products contributing to a concept of sustainable development.
8
2 THEORETICAL SECTION This theoretical section accumulates elements of knowledge gained in the process of solving the matters involved in the thesis, these being focused on problems of the formulation of a material based on foamed gypsum. It consists of three relatively independent parts. In the first part there is summarized general information dealing with gypsum as a binder, while the second part is focused on the problems of gypsum products used in the building industry and the third part is devoted to methodological aspects of the formulation of a gypsum composite material.
2.1
Gypsum Gypsum is one of the oldest binders which humankind has used. Gypsum was known and used by ancient Egyptians and Assyrians. In Europe it has been used since the early Middle Ages.
2.1.1 Characteristics of gypsum From a chemical point of view gypsum is a crystalline hemihydrate of calcium sulphate - CaSO4 .0,5 H2O. This hemihydrate can be most easily gained from natural dihydrate of calcium sulphate (natural gypsum) through partial dehydration carried out by heating. Under normal pressure there is a process of partial dehydration of natural gypsum for gypsum already at a relatively low temperature (circa 150 °C) and in energy terms this is not too demanding. At temperatures of about 180 °C there occurs a complete dehydration to anhydride (Figure1)
9
Fig.1: DTA curve of the hydration of natural gypsum to gypsum and anhydride [1]
2.1.2 Sources of gypsum As a source for gypsum production there is traditionally used natural gypsum (dihydrate of calcium sulphate), created by the evaporation of salty water in closed seas and lakes. Chemically pure calcium sulphate dihydrate (CaSO4. 2H2O) contains 20, 9% chemically bound water. Its bulk density is about 2300 kg.m-3 and it is white or colourless. Commonly found natural gypsum usually contains a certain proportion of clay particles, slate, anhydride, calcium carbonate etc. According to the type and number of impurities the colour of a mineral changes, e.g. to brown, pink, grey. In the Czech Republic the only industrially-used deposit of natural gypsum is near Kobeřice. Gypsum which is produced from it is used for the production of gypsum mortars and mixtures and also for the production of gypsum blocks.
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Another raw material for gypsum production can be synthetic dihydrate, created as a by- product e.g. during the production of phosphate fertilizers and phosphoric acid. A more pronounced usage of phosphogypsum can be prepared from this dehydrate and prevents contamination of the initial raw material by phosphoric acid. So called flue gas desulphurization (FGD) gypsum, which is created during the desulphurization process, especially in power plants, has a substantially higher significance. With regard to the considerable development of desulphurization technologies and regarding also the fact that gypsum prepared from flue gas desulphurization (FGD) gypsum is of a very good quality, this can be counted as a prospective source.
2.1.3 Production of gypsum Gypsum is produced from natural or synthetic dihydrate of calcium sulphate by thermal dehydration. This process is called calcination and is undertaken according to the equation: CaSO4.2 H2O + heat → CaSO4.0,5 H2O + 1,5 H2O According to the method of thermal processing there is derived from the initial raw material binders, which significantly differ in their properties (Table1). Basic types of binders which emerge during calcination are αgypsum and β-gypsum. While β-gypsum is created by the mere heating of natural gypsum under normal atmospheric pressure, αgypsum is prepared by heating in an autoclave with a simultaneous increasing of pressure by saturated water vapour. Both forms of gypsum (α andβ) are hemihydrates of calcium sulphate. They have the same chemical composition but they differ, however, by their degree of aggregation (by the size of crystals and by the frequency of crystal-lattice defects and this result in their considerably different technological qualities (Table 2). α- hemihydrate is commonly considered as a binder of greater quality. It has more compact, regular needle-like particles and has a more configured crystalline lattice, which records a greater strength after the hardening of the paste.
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Table 1: Scheme of the dehydration of the calcium sulphate dihydrate and forms of the CaSO4 CaSO4.2H2O dihydrate (DH) ↓ 150 °C β-CaSO4.0,5 H2O
α−CaSO α− 4.0,5 H2O
α−hemihydrate (α-PH) β- hemihydrate (β-PH) under high pressure in saturated under normal pressure, without water vapour atmosphere saturated water vapour (autoclave) atmosphere ↓
↓
200 - 210 °C
170 - 180 °C
α - CaSO4 III
β - CaSO4 III
α - anhydrite III (α-AIII)
β- anhydrite III (β-AIII)
↓
↓ over 200 °C CaSO4 II
anhydrite II (AII-T, AII-N, AII-E) ↓ Over 800 °C CaSO4 I anhydrite I ( AI)
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Table 2: Differences between α- a β-hemihydrate Type of gypsum Particle size Porosity of particles
α- hemihydrate
β- hemihydrate
Small (10 - 20 mm) Very small (1-5 mm) low
porous
Specific surface
small
high
crystal-lattice defects
small
large
Strength increase
slower
faster
Final strength
higher
lower
β-hemihydrate particles are porous and are irregular in shape. Since β-hemihydrate has a higher frequency of defects in a crystalline lattice, it has even with the same granularity a bigger specific surface than α- hemihydrate and thus also a greater need for batch water. From this then also results the lower strength of hardened β-gypsum. A raw material for gypsum production can be natural gypsum, synthetic dihydrate, sometimes also gypsum shards (e.g. from ceramic forms). Production takes place in such a way that an initial raw material is firstly treated, rid of impurities, crushed and classified into various sizes. Digesters, autoclaves and rotary furnaces can serve for thermal processing of the treated raw material. The calcinated gypsum is then ground to the specified fineness. As high as possible a content of dihydrate in the initial raw material is desirable for production. The presence of anhydrite (anhydrous calcium sulphate) could endanger a production process, and possibly the quality of a product. Undesirable as well is the creation of a higher amount of anhydrite during the gypsum production itself. This can happen due to a higher temperature by a careless calcination.
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2.1.4 Types of gypsum binders Gypsum binders can be differentiated according to many aspects, e.g. according to use, according to the method of production or according to technological properties. According to use, gypsum binders are divided into building gypsum (from which building units, plasters, stucco products, gypsum drywall are produced), into technical gypsum, which is used not only in the building industry (e.g. modelling gypsum in the ceramic industry serves in the production of gypsum forms), into modified gypsum (i.e. gypsum into which admixtures for improving workability, adhesiveness etc. were put). A special group is created by composite gypsum binders (gypsum with an addition of other binders). The recommended types of gypsum for various usage is stated in the standard ČSN 72 2301. According to this standard, individual types of building gypsum are marked according to the fineness of grinding and according to the time of setting (Table 3). According to compression strength they are then ranked by appropriate class of strength (Table 4). Table 3: Types of gypsum according to the fineness of grinding and to the setting time Type
Mark Initial setting time Final setting time max.
quick - setting
A
2 min
15 min
normal - setting
B
6 min
30 min
slow - setting
C
20 min
not given
Type
Mark
Retained on sieve 0,2 mm max. in %
coarse grinded
I
30
middle grinded
II
15
finely grinded
III
2
14
Table 4: Strength classes according ČSN 72 2301 Class
G-2 G-3 G-4 G-5 G-6 G-7 G-10 G-13 G-16 G-19 G-22 G-25
Compressive strength in 2 MPa
3
4
5
6
7
10
13
16
19
22
25
A more precise way for evaluating the fineness of grinding is presented by the setting of a specific surface, which can be carried out by the Blain permeability method described in ČSN EN 196-6. Common commercially accessible gypsums have a specific surface in a range 200 – 400 m2.kg-1 [2].
2.1.5 Setting of gypsum A fast-setting mortar binder can be easily prepared from finely ground gypsum mixed with water. The mechanism of gypsum setting can, in a simplified way, be described by the equation: CaSO4. 0,5 H2O + 1,5 H2O → CaSO4 . 2 H2O + heat A hardened binder is then again created by dihydrate. Gypsum has, after mixing with water, the look of a paste suspension. A part of the gypsum dissolves in water and there is thus created a supersaturated solution of calcium sulphate from which dihydrate CaSO4. 2H2O starts spontaneously to crystallize. Crystals (hexangular monoclinic prisms) of dihydrate gradually grow and in their mutual interacting growth, they create a solid product. Such a mechanism of hydration reaction was already postulated in 1868 by Le Chatelier and further experiments confirmed his idea. Although it is not possible to completely exclude a topochemical reaction (a direct addition of water on hemihydrate without the former dissolving) promulgated by Michaelis, it is not however a reaction with a distinctive influence.
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Fig. 2: Solubility of some types of calcium sulphate in the water
For a description of the kinetics of hydration of the gypsum setting the most fitting proves to be Ridge’s equation [4,5]: dα / dt = k α(1- α)2/3, where α is the degree of hydration, k is the velocity constant and t is time. The rising content of dihydrate induces the strengthening of the structure as is shown in the following figure. The scale on the left side determines the descending depth of the stab with a Vicat’s needle (curve 1), the scale on the right side determines the percentage content of dihydrate (curve 2).
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Fig.3: Kinetics of the gypsum hydration According to [6] for detecting hydration course samples are taken from the setting gypsum at certain time intervals. The samples are immediately mixed with ethanol, which will stop further hydration. The taken samples are rinsed with ethanol on a filter, are sucked out and are subsequently dried at 60°C. The percentage of bound water is then detected by heating at 400 °C. The detected apparent activation energy of the reaction is for βgypsum 17,7 kJ.mol-1 and by its value it confirms that diffusion is the driving process of a hydration process. The strength of hardened gypsum can be increased by drying because thus there will be created from the aqueous sulphate solution embedded among crystals, further crystals of dihydrate, which strengthen the already existing connections among the crystals. The speed of gypsum setting depends on how much anhydride it contains. The higher the temperature of firing the more anhydride it contains, and the more slowly it sets. The setting time is influenced by a number of factors. It can be shortened by prolongation of the mixing time, by lowering the water-cement ratio, by increasing the batch water temperature, or by adding accelerators of the setting (e.g.
17
NaCl). By adding some colloidal materials (milk, agar, casein, keratin) the time of the setting is prolonged. Gypsum setting is accompanied by a growth in volume. Immediately after diluting with water shrinkage occurs, although this happens only during the time when the gypsum is in a plastic state. With the formation of crystals of dihydrate there then occurs a moderate volume increase. The size of the volume growth depends on the fineness of grinding, on the water-cement ratio, and on the addition of the retarders of the setting in ranges from 0,1 to 1% of the total volume. Gypsums with an addition of retarders or accelerators of the setting have a mostly lower volume growth. Reduction of volume growth can be also done by adding sand or limestone. The volume growth is in the main considered desirable, because during it the forms are ideally filled in. If the volume growth is prevented (e.g. by a rigid form), there will take place internal compacting, which leads to higher strength, which may be used for example in the production of building blocks.
2.1.6 Hardened gypsum The basic mechanical property monitored in gypsum is its compression strength. Depending on the quality (class of strength) of gypsum it is 2 to 25 MPa. The class of strength of gypsum sold in our country is stated in the standard ČSN 72 2301 (Table 4). The elasticity module of hardened gypsum ranges from 2000 to 6000 MPa, the ratio of compressive and bending strength being around 3:1. The mechanical properties of gypsum depend on a number of factors, the biggest influence being its moisture. The strength of dried-off gypsum is 2 to 3 times higher than the strength of a moist material. Even small changes in moisture significantly influence mechanical properties. Variation in moisture of 0,1% of the mass can cause changes in strength up to 8%. Since gypsum is significantly hydroscopic, it is necessary to take account of the fact that its strength is influenced by the moisture of the environment in which it is found (Table 5).
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Table 5: Influence of the gypsum moisture on the strength Curing of gypsum
Gypsum moisture
Compressive strength MPa
%
0
13.8
100
air humidity 65 %
0.04
13.6
98.5
air humidity 90 %
0.15
12.9
93.5
full saturation by water
17.5
6.4
46.5
drying at 35 - 40 ° C
Defects due to the influence of moisture are created by incorrect insertion of gypsum products. This happens when exceeding specified moisture of a gypsum product and inserting it without drying off or by the facing of a still insufficiently dry gypsum surface. Building blocks from gypsum may be built into an environment where the relative moisture would not exceed 75 %, but at higher moistures it is necessary to carry out appropriate measures. It has been, however, discovered that in an environment typical for living spaces, any changes of strength are not a fundamental problem, especially due to the fact that after the drying off, gypsum is again able to reach original strengths. Further factors which influence the mechanical properties of hardened gypsum are the quality of input raw materials, the watercement ratio, the age of a material, conditions around the placing of the product (both during setting and hardening, and also during using). The used cement-water ratio especially has a considerable influence on the strength. (Table 6). The highest strengths are reached by mixtures which contain only a minimum amount of water which is necessary for setting and crystallization (with a water-cement ratio about 0,18). These mixtures, however, do not have a sufficiently plastic consistency, and so they cannot be processed. In practice it is necessary to use a substantially higher water-cement ratio, commonly 0,6 up to 0,8 (for 100 ml of water there is allotted 19
120 – 160 g of gypsum). Since α-gypsum needs for the same workability a lower amount of batch water than β-gypsum, the strengths of hardened α-gypsum are higher. Table 6: Influence of water-cement ratio on the strength slow-setting gypsum
quick-setting gypsum
Bulk density
Compressive strength *
Compressive strength *
kg.m-3
MPa
MPa
0.50
1410
14.6
15.8
0.55
1300
13.0
14.0
0.60
1230
11.4
12.0
0.65
1170
10.8
0.75
1040
9.5
w/c
* After 13 days in room temperature + 1 day in 50 °C The dependence of compression strength on the water-cement ratio, in a context of sufficient workability, takes a hyperbolic course. On the hyperbola interlined by experimental points it is possible with the unit water –cement ratio(w = 1) to subtract the strength Qm, which Djabarov [7] suggested to use for the character of gypsum because it allows for every value of a water-cement ratio to be calculated for appropriate compression strength σp according to the formula:
σp = Qm / (w2). The drop of compression strength with a higher water-cement ratio is certainly connected to the growth of volume in a porous system of hardened gypsum. With common water-cement ratios w = 0,7 – 0,8 20
the ratio of pores is 47 – 55 %. Pores create a continuous system, which enables both fast drying ability and fast water acceptance.
In Figure 4 there are specimens of Djabarov hyperbolas. Appropriate values Qm range across 2,2 – 6,9 MPa.
Fig.4: Influence of water-cement ratio on the compressive strength for different types of gypsum After water evaporation there is created a continuous network of air pores, communicating with the surrounding environment of the hardened gypsum. Thus the created pores then create open porosity, the size of which determines the rather considerable gypsum absorptivity. Schiller approximated a real pores network as a system of cylindrical pores [10] and by such a simplified system he derived a theoretical
21
relationship between porosity P and compression strength of gypsum σp: σp = q . log(Pkr/P), where q is a qualitative characteristic and Pk ris critical porosity responding to the mass with zero strength. While the value q can rely on the chosen technology of a preparation of gypsum suspension (impurities, additives), the value Pkr for the given gypsum is constant. The relationship can be understood only as a recommendation towards an appropriate image of a regressive function for the levelling out of the experimental data. The real shape of pores is complicated to the extent that it goes beyond the mathematical description and Schiller in his derivation worked moreover with pores of a uniform average size. On the basis of some older works [11-13] Lach suggested a more precise relationship including the influence of the size of pores [7]:
σp = σ0 .D-k.e(-b.v /1000), where σ0 is the theoretical strength at zero porosity, D is a median of the diameter of pores given by mercury porozimetry, v is the volume of these pores (mm3.g-1), e is a base of natural logarithms and k, b are constants. The practical utility of this relation is, however only small given the number of constants unknown beforehand. Pore size distribution is however, in spite of that, an important characteristic of a porous space. With increasing temperature the structure of gypsum suspension significantly changes. With the change of temperature there also changes the difference between the solubility of hemihydrate and dihydrate and even if the character of the curve of the division of pores size remains the same, the medium size of pores is moving towards higher values (Figure 6). This phenomenon is connected with another characteristic of hardened gypsum, which is the anisometry of natural gypsum small crystals, expressed as a ratio of small crystals length to their width. This measure is influenced both by the value of the water-cement ratio and the temperature of the gypsum setting (Figure 7).
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Fig. 7: Influence of the mean length L and width D of the crystals of the calcium sulphate dihydrate on the water-cement ratio and temperature
23
Fig. 5: Pores size distribution curves of a dihydrate, developed from α and β-hemihydrate [14]
Fig. 6: Influence of the mean size of the pores on the setting temperature and porosity [14]
24
The change of the mean length and width of natural gypsum crystals, however, can be caused also by some additives. A widening and shortening of natural gypsum crystals will be caused by an admixture of citric acid, which is often used as a retarder of gypsum setting (Figure 8).
Fig. 8: Crystals of the calcium sulphate dihydrate, developed from the suspension of hemihydrate only ( left) and from suspension with the citric acid (right) [15]
If at least 20 % of the total volume of pores is not filled in with water, gypsum products are considered as frost resistant. The values of the thermal conductivity of hardened gypsum mass is stated in the standard ČSN EN 12859. At bulk densities 600 up to 1200 kg.m-3 the thermal conductivity of dry gypsum ranges from 0,2 up to 0,6 W.m-1.K-1 (Table 7). The specific thermal capacity of dry gypsum is 840 up to 1050 J.kg-1. K-1. The linear thermal expansion of gypsum is 20 . 10-6 K-1, and is then about twice greater as the linear thermal expansion of concrete. From a viewpoint of fire protection, gypsum products are considered as fireproof. Fire resistance of gypsum results from the content of crystalline-bound water in a material, which is around 18% of the mass. At temperatures of above 110°C gypsum dehydrates and loses strength, but expelled water however, creates on the surface a layer of steam, which lowers the temperature of materials and thus protects against fire. 25
Gypsum has considerable ability to absorb moisture (it is hygroscopic), which applies positively especially for moisture regulation in living spaces. Volume changes due to the influence of moisture changes are at the same time relatively small and linear length changes do not exceed 0,01 % (the comparison with concrete is 0,0 up to 0,08 %). Table 7: Coefficient of the thermal conductivity of hardened gypsum (ČSN EN 12859) λ23-30
Bulk density -3
(kg.m )
(W.m-1.K-1)
600
0.18
700
0.22
800
0.26
900
0.30
1000
0.34
1100
0.39
1200
0.43
1300
0.47
1400
0.51
1500
0.56
The dependence of the creeping of hardened gypsum (w/c = 0,8) on moisture is shown in the following Figure 9. Curve 1 shows values gained at a tension action of 1,2 MPa for a period of 28 days and Curve 2 shows creeping if the same tension action is 90 days.
26
Fig. 9: Creeping of gypsum in dependence on permanent moisture In case of drying up (removing porous water) plastic creeping ceases (Figure 10). At the same tension and the same water-cement ratio it is possible to monitor the sample for dropping moisture (Curve 3) for insignificant shrinkage (curve 2) and for creeping with a more gradual character (Curve 1). At the same time with the drying up of the sample, the shrinkage and also the creeping stops (terminal point Ke).
Fig. 10: Creeping and shrinkage of the drying gypsum [7]
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2.1.7 Modification of gypsum Admixtures can be put into gypsum to improve some of its properties. In principle it is possible to divide these admixtures into the following groups: accelerators retarders fungicidal agents substances improving the resistance of hardened gypsum against water pigments plasticizing agents. Setting retarders and accelerators can be divided into five classes [16]: I. Substances changing the solubility of hemihydrate and dihydrate, without creating hardly dissolving layers on the surface of these compounds. (These can be both strong and weak electrolytes and non-electrolytes); II. Substances creating crystallizing nuclei (e.g. CaSO4 . 2 H2O, CaHPO4 . 2 H2O); III. Surface active substances capable of absorption onto a surface of hemihydrate or dihydrate, which at the same time can also retard the creation of new crystallizing nuclei; IV. Substances creating on a surface of hemihydrate or dehydrate hardly dissolving protection layers (e.g. boric acid, alkali borates and phosphates); V. Combined setting retarders/accelerators, acting at the same time through more mechanisms. Setting regulators of the class I can increase the solubility of hemihydrate and cause acceleration of the setting (e.g. NaCl, KCl, Na2SO4 ), and others can restrain solubility, acting then as retarders (e.g. NH4OH, ethanol). Setting regulators of class I and II display a distinctive efficiency threshold beyond which there is no reason to increase the concentration of the setting retarder/accelerator. Beyond this threshold the class I accelerators can display also an opposite effect.
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Fig. 11: Relative setting time (ratio between the original and new setting time) in dependence to amount of the setting regulator [16]
Most frequently there are used admixtures of the class III, retarding the setting (e.g. glue, keratin, citric acid, molasses), but when using them there is necessary a certain caution because every treatment of the setting causes, at the same w/c ratio, a decrease in strength. The effect of the setting regulators of class I and III is mutually independent and it is possible to determine the resulting relative time of the setting RtV, with the simultaneous use of both types of setting agents as a product of the relative time periods responding to the use itself of both setting retarders/accelerators: RtV = RtI . RtIII . A detailed comparison of the regulation effect of a number of common substances on velocity of hydration of gypsum hemihydrate brings us work [2] which claims an acceleration effect of all inorganic acids and a retarding effect of all hydroxides. Magnesium
29
chloride hydrate, calcium chloride, zinc chloride behave neutrally or they retard weakly, while other chlorides are more likely to accelerate. All soluble phosphates, carbonates and hydrogen carbonates are retarders. Sulphates (except magnesium sulphate) have an accelerating effect. The found values can be generalized in such a way that salts of strong acids and strong alkali are accelerators, the same as salts created from strong acids and weak alkali. Salts of weak acids with a strong alkali have, on the contrary, a retarding effect. During the course of hydration pH of the solution moves towards a neutral value. It is possible to assume that hydrogen and hydroxyl ions are absorbed by growing crystals. Formerly, as setting agents there were often utilized mixtures created by the treatment of raw materials of natural origin. The products thus gained were not completely and unambiguously chemically defined and it was difficult to ensure a clearly reproducible effect. Previously there was utilized, for example, a product gained by the decoction of keratin with a lime suspension enriched with sodium hydroxide [2]. Even today commercially accessible setting agents are often mixed but they are, however, usually prepared from chemically distinct components. Recently there appeared a review indicating good results gained by using a mixed retarder created by tartaric acid and calcium hydrate [17, 18]. Belonging to modern retarders are organic derivatives of phosphorous acid: HEDP (1-hydroxy-ethan -1,1-bisphosphorous acid) and DPPDC (2,3 dicarboxyl – butan – 1,1- bisphosphorous acid), which moderate crystallizing processes in concentrations at the level of 0,001 % [19]. The use of fungicidal agents can be necessary with gypsum mixtures containing organic additives sensitive to biologic attacks [20]. Improving the resistance of hardened gypsum against water by some modification of gypsum binder has not progressed so far sufficiently for a complete solution, as the surface treatment of gypsum by coating or possibly by impregnating (e.g. borax, fluorosilicate, water glass, dextrin, paraffin, keratin water, synthetic polymer) proves much easier. A certain increase in resistance against water can be attained with the help of polymer additives or also with the help of cement. [21]. An interesting possibility is presented by the strengthening of a structure 30
of hardened gypsum with the help of CSH phase, created in a gypsum binder after adding micro silica [22]. Plasticization of gypsum paste, enabling the lowering of the w/c ratio, was utilized as far back as the Middle Ages. Prescriptions dating from that time using colloidal extract from Radix althaea officinalis, have so far been put into effect for restoration of gypsum products [7]. At present, when lowering the water-cement ratio of gypsum paste, the main focus of interest are plasticizers used during concrete processing, or their equivalents [23].
2.2
Gypsum materials and products Mortars and concretes based on sulphate binders are mainly used in the interior construction of structures. By using them on exterior walls it is necessary to take special measures towards ensuring the protection of these products.
2.2.1 Gypsum mortars and concretes A mixture of water and gypsum is in practice often branded as gypsum mortar, even when it is actually gypsum paste. The mixture becomes mortar after adding a fine aggregate. If also a coarse aggregate is added to the gypsum mortar, it passes actually as a gypsum concrete. Less hydrated water is bound in a product with an increasing content of aggregate. Also the amount of free water is lower, which leads to a lower moisture in the gypsum products and to their drying up better. As a binder in plasters there is used either pure gypsum mortar or a lime-gypsum composite binder, or possibly a gypsum-cement composite binder. Lime-gypsum mortars are mostly mixed in the ratio of 0,2 up to 2 parts of gypsum, 1 part of lime hydrate and 3 parts of sand. An admixture of gypsum to the lime mortar enables the application and smoothing of coatings in one working cycle. Gypsum carries away water from mortar, and so the mortar more easily pre-sets. Gypsum cement mixtures are suitable for a preparation of hollow perforated blocks, because the addition of cement improves the consistency and rheology of mortar. It is possible, however, to add to 31
gypsum mortar only a small amount of cement (up to 3 % of binder content). In the case of a greater amount of cement there is a danger of the cracks in a finished product as the result of an additional creation of complex calcium sulphoaluminate (ettringite). Industrially-produced ready-mixed plaster mixtures are produced in two types – smoothing and felting. Both types differ by granulometry of aggregates and by the method of application. Smoothing ready- mixed plaster contains aggregate with granulometry circa 0, 06 mm and its final treatment is carried out by steel trowel. Felting mixed plaster contains grains up to a size usually of 0, 8 mm and the final treatment is carried out by spinning the surface with a felt trowel. Other industrially-produced gypsum products are construction blocks, plasterboards, structural boards and wall blocks, sheeting and acoustic parts. A disadvantage of gypsum materials is their considerable brittleness and low tensile flexural strength. That is why people in the past tried to reinforce gypsum with various fibres of vegetable and animal origin and also many current gypsum products have the character of composites reinforced with fibre. Besides the often used cellulose and glass fibres, more recently polypropylene fibres are also being used. [24]. Locally, there are being produced gypsum composites with coir, sisal fibres or with de-fibered wood.
2.2.2 Gypsum blocks Gypsum blocks are defined in the standard ČSN EN 12859 as industrially-produced structural elements from calcium sulphate and water, which can contain fibres, fillers, aggregates and further admixtures and pigments. Gypsum blocks can be either full or with shaped cavities. According to their bulk density gypsum blocks are classified into these groups: with high bulk density (from 1100 up to 1500 kg.m-3), with medium bulk density (800 up to 1100 kg.m-3), with low bulk density (600 up to 800 kg.m-3).
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2.2.3 Gypsum plasterboards and gypsum fibre boards A gypsum plasterboard was patented by the end of the 19th century in the USA. In Europe it started to gain recognition in the first half of the 20th century. Now it is among the most frequently used materials for interior construction. Gypsum drywall boards consist of a core, which is created from a gypsum mixture, paper fibres and glass fibres, and from a special cardboard, which covers the gypsum core. The production of gypsum drywall is undertaken in such a way that a gypsum paste with appropriate admixtures is distributed proportionately on a paper strip. A gypsum layer is covered by an upper layer of paper and is allowed to harden. The boards thus created are cut to the necessary size and possibly they are further surface treated. Gypsum fibre boards are produced from a gypsum mixture, water and cellulose under an increased pressure. The content of cellulose fibres in a mixture moves from 8 to 20 %. Boards are most frequently produced from α-gypsum. Gypsum boards reinforced with fibres need not be fitted with cardboard on the surface and the term gypsum fibre board is mostly used just for boards without cardboard. They are produced as an impregnated in the standard way. In Scandinavian countries boards are produced from wood particles (fibres, cuttings or chips) bonded by gypsum. During the production of these boards very little water is added (water-cement ratio 0, 15 – 0, 19), because wood particles with natural moisture usually contain a sufficient amount of water necessary for the hydration of a binder. These boards have properties comparable with the top quality gypsum plasterboard. In comparison with classical chipboards they have substantially higher fire resistance. To recognise some complexity, it is necessary to state that there are also produced gypsum plasterboards containing fibre reinforcement in the gypsum mass. To some advantage there are used glass fibres which are not in any way attacked by the gypsum. Most frequently used are glass fibres 8-12 µm in diameter and 15-50 mm in length.
2.2.4 Foamed gypsum In the case of gypsum products we are always dealing with a porous material, because a not negligible content of the pores in the setting 33
gypsum is created as a result of the fact that batch water, during the formation of a strong crystalline structure, brings itself to bear only partially. As has been already said, water not consumed, dispersed in a setting system, creates a characteristic porous structure created mainly by micro pores. It is possible to reach a further increase in the proportion of a porous structure (lightening) in several ways. Further pores can be implanted into a gypsum composite by a porous filler but it is also possible to create further macro pores directly in the gypsum binder itself and in this way to create gypsum foam. A lightened porous structure in a gypsum binder is gained either by mechanical aerating with the help of a foam generator or by foaming and foam creation with the help of a gas developing admixture. With regard to the favourable thermal, acoustic, ecological and hygienic properties of a lightweight gypsum binder there is increased interest in materials made on this basis and there have appeared articles which deal with both the preparation of such a binder [25, 26] and the properties of appropriate products [27]. Lightweight gypsum composites with a macro porous structure, created directly in a gypsum binder are in past literature identified as porous gypsum, whereas under this term there are subsumed both products prepared by mechanical aerating, and products with a chemical foaming admixture. In the following text there is used, for indicating gypsum materials with a chemically foamed binder, the word combination foamed gypsum. In the commonly accessible domestic national literature [7] it is possible to find only a brief mention of the fact that abroad there are produced from foamed gypsum, multilayer prefabricated elements, and that for gas developing in a gypsum suspension there can be used hydrogen peroxide, calcium carbonate and aluminium phosphate or sulphate waste liquor and sulphuric acid. After hardening, foamed gypsum prepared in this way has a compression strength 0, 35 - 0, 40 MPa, with a bulk density 400-500 kg.m-3. A significant characteristic of foamed gypsum is its macroscopically distinct heterogeneity of structure in which alternate all phases: a gaseous phase (above all in macro pores created by foaming gas), a liquid phase (present as water in pores and micro pores) and a solid
34
phase (a hardened gypsum binder also possibly containing fillers and fibrous reinforcement). The first experiments with the preparation of foamed gypsum were made apparently at the end of the 19th century. We may come to such a conclusion from later patent documents, which in their introductory parts refer back to a well-known state of technology. Later, a number of papers, published most frequently only in the form of patent documents, dealt with this set of problems. However, a more profound study dedicated to a set of problems relating to production and application has been lacking so far. Original work for foaming of a gypsum paste obviously used carbon dioxide, created by the reaction between sodium hydrogen carbonate and some mineral acid (especially sulphuric acid) or acid alum. Gammarra mentions these compositions in the introduction to his patent from 1930 [28]. According to Gammarra, a source of carbon dioxide can also be calcium carbonate naturally present in natural gypsum. In the Gammarra patent cited, for foaming there is used a reaction between calcium carbonate (naturally present in gypsum raw material) and an added acid component, created by the mixture of tartaric acid with calcium chloride. Easily soluble calcium chloride firstly reacts with tartaric acid on the creation of a less soluble calcium tartrate and at the same time the created hydrochloric acid then reacts with calcium carbonate: CaCl2 + H2C4H4O6 → CaC4H4O6 + 2HCl 2HCl + 3CaCO3 → CaCl2 + H2O + CO2 There is also possible a direct reaction of tartaric acid with calcium carbonate, but this reaction, however, probably did not appear as sufficiently intensive to the author. Gammarra uses the principle of a gradual production of inorganic acid also in the following patent [31], in which the main foaming reaction presented is that of trihydrogen phosphoric acid with calcium carbonate. The gradual creation of phosphoric acid in the system is ensured by the reaction of soluble acid phosphate salts (e.g. sodium hydrogen phosphate) with tartaric acid. The reaction then takes place according to the equations:
35
Na2HPO4 + H2C4H4O6 → Na2C4H4O6 + H3PO4 2H3PO4 + 3CaCO3 → Ca3(PO4)2 + 3H2O + 3CO2 Soluble sodium tartrate however also reacts with the calcium ions present, which can be attributed to the dissolved calcium sulphate: Na2C4H4O6 + CaSO4→ CaC4H4O6 + Na2SO4 For successful foaming however, we also need optimum velocity of the creation of foaming gas, and the optimization of the development of CO2 may require an increase in the content of the carbonate component. We therefore come across in a number of publications the idea of increasing the content of carbonates with an admixture of calcite or ground dolomite. Also hydrogen carbonates can be used. Mainly this relates to patents originally coming especially from the former USSR and Poland. It is not without interest that in the case of an acid component this often deals with an acid-reacting salt or relatively weak acid. An acid reaction of a solution of iron (III) chloride, for example, uses for gypsum foaming a soviet patent, according to which there can in this way be gained product with higher strength than when using aluminium sulphate [32]. Potassium hydrogen carbonate (2 – 9 %) is used in a combination with boric acid (0,5 – 2 %) in the German patent [33]. A basic gypsum binder based on α - hemihydrate is supplemented with calcium hydrate or portland slag cement, a foam stabilizer and a retarder. In the patent paper there is stressed the necessity to grind boric acid to at least gypsum fineness. The stress on the active influence of boric acid is in apparent contradiction with the fact that according to [26] in the environment of a gypsum suspension there takes place a spontaneous disintegration of hydrogen carbonate in an alkali metal according to the equation: CaSO4 + 2 NaHCO3 → Na2SO4 + Ca(HCO3)2 Ca(HCO3)2 → CaCO3 +H2O + CO2
36
Obviously even in this case it is not possible to ignore the dynamics of the whole process and it is therefore appropriate to accelerate the development of carbon dioxide by the presence of an acid. The use of sludge, dropping out during sugar refining, for strengthening the content of a carbonate component, is described in [34]. The used saturated sludge contained 50 – 75 % CaCO3. For decomposing the carbonate there was used aluminium sulphate in an amount 1,2 –3,0 % of gypsum paste mass (saturated sludge in amount of 0,5 – 1,5 % of gypsum paste mass). For increasing the strength, and for the stabilization of a porous structure there also was added cut glass fibre 7 – 10 mm long in the amount 0,2 –0,5 % of gypsum paste mass. The Polish patent [35] describes a gypsum foam production, during which calcium lignitic fly ash, reacting with ammonium chloride, which is weakly acid, serves as a gassy admixture and to the foamed mixture there is added urea formaldehyde resin for increasing water resistance. The European patent [36] proposes the use of hexafluoromagnesium silicate (MgSiF6) as an acid component, reacting with a considerable excess of magnesium limestone. Of interest is the use of a hydrophobic treatment (by aluminium stearate) for a surface treatment of hexafluor silicate. The purpose of the treatment is obviously the retarding of a hydrolytic decomposition, from which there is created its own effective acid (hydrofluoric acid). Neither is there neglected however the use of strong mineral acids themselves. For example sulphuric acid (together with limestone) is used during the production of a special foamed gypsum coating [37]. Using concentrated sulphuric acid added into a gypsum suspension together with limestone, it is possible to get a foamed material of varying bulk density. Sulphuric acid at the same time accelerates the setting of a mixture, which prevents a foam collapse [38]. Since foamed gypsum has relatively low tensile and bending strength, a number of patents are dedicated to ways of increasing the strength of any resulting mixture. For example according to [39] a composite with a good bending strength will be attained from gypsum foamed by magnesium limestone and oxalic acid, tartaric acid, maleic acid or by succinic acid with the simultaneous use of a wetting agent based on a condensate of naphthalene sulphonic acid
37
with formaldehyde. The influence of the w/c ratio on the strength of foamed gypsum was studied by Saduaksonov [40]. It is stated that plasticizers retard the foamed gypsum setting [41] and that, in a contrary fashion, for accelerating the foamed gypsum hardening we can use natural gypsum [42]. An admixture 0,5 – 1 % of natural gypsum will shorten workability by half to a still acceptable strength reduction of around 30 %. The most comprehensive work was published on the topic of foamed gypsum by Colak [25]. For the production of foaming gas he used both aluminium sulphate and potassium aluminium sulphate (the creation of CO2 by the reaction with carbonates contained in gypsum) and ammonium hydrogen carbonate (the creation of CO2 by hydrolytic decomposition). At the same time with this formulation there was used citric acid (as a retarder), carboxy methyl cellulose (as a viscosity regulator) and sodium lauryl sulphonate (as a surfaceactive substance). Foaming by carbon dioxide released from carbonate or hydrocarbonate is obviously the most frequent method of foamed gypsum preparation. For a complex view it is necessary, however, still to mention other methods. The use of a reaction of an isocyanate group with batch water [43] belongs among the less common ways of carbon dioxide creation in a gypsum binder. In the first place, isocyanate provides, along with water, an unstable carbamic acid, which breaks down into carbon dioxide and amine: R-N=C=O + H2O → R-NH-COOH → RNH2 + CO2 The created amine is also a substance with active proton and therefore it participates in the further reaction: R-N=C=O + RNH2 → R-NH-CO NH-R . Substituted urea is then the final product of the reaction. If the used isocyanate is multifunctional, the reaction obviously leads to oligomere or polymere products, which can have a positive influence on the final properties of a composite. Another possibility for the preparation of foamed gypsum presents the use of oxygen as a foaming gas. The necessary oxygen is
38
released in a gypsum paste by a catalytic decomposition of an aqueous solution of hydrogen peroxide (6%), added into alkalinized batch water. 2 H2O 2 → 2 H2O + O 2 For the peroxide decomposition there is recommended cobaltous sulphate [44], but it is also possible, however, to use the catalytic effect of manganese ore (MnO2) [45]. Another approach to foamed gypsum preparation presents the use of low-boiling 1-butene (boiling point –6,3 °C) or butadiene (b.p. –4,5 °C). The appropriate substance is absorbed in pores of calcium zeolite (molecular sieve), from which it is released after mixing with water. In this way it is suggested that gypsum products with a bulk density 950 kg.m-3can be attained [46].
2.3
Formulation of a gypsum composite
As composites, there are identified multi-component heterogenous materials which contain at least two phases (of which at least one must be solid), and which indicate between phases a distinctive, macroscopically recognizable boundary interface. The phases must have properties different from their adjacent environment, though they need not be, however, of a different state of matter. The phase is substantially understood as a relatively homogenous area, in which a certain composition prevails. Typical composites are materials created by a hardened binder (matrix) and a solid aggregate. However, foamed materials also have a composite character containing to a higher degree in the solid matrix, bubbles and pores. It is characteristic of composites that their properties partly depend on the properties of individual components and their bulk proportion in the mass of a composite (additive effect), and partly on a mutual interaction of these components (synergic effect) [47].
39
2.3.1 Components of a gypsum composite A composite nature is quite typical for materials based on gypsum. Gypsum composite is already a material created by hardened gypsum binder itself (hardened gypsum paste). The composite character of a hardened gypsum paste is indicated by the fact that it is created by a solid dihydrate phase, densely interpenetrated by macroscopically distinct pores created by excessive water (the water which does not participate in a hydration reaction). A number of these pores is dependent on the water-cement ratio used and their inundation factor depends on whether and how the set gypsum was dried. Even with the suggested composition of the simplest gypsum material created by a set gypsum binder itself, it is possible to use different substances and select a different approach both in line with the choice and in line with the dosage. A basic parameter of the formulation is the water-cement ratio, the size of which is as a rule chosen on the basis of the required fluidity of the whole mixture. Just this requirement for sufficient workability leads to a considerable excess of the water-cement ratio over the value 0,186 (which is the value theoretically necessary for hydration of gypsum hemihydrate). For treatment of the properties of gypsum binder, there is offered on the market a number of admixtures. This deals, above all, with the setting retarders/accelerators, plasticizing substances and substances influencing rheological properties. Besides special additives determined directly for gypsum masses, there are also principally usable some commercial concrete admixtures, especially if their effect is based on a treatment of interfacial interactions among water, mineral particles and air in the setting composite. In practice used gypsum mixtures contain as a rule a mixed setting retarder/accelerator and a certain proportion of aggregate (which also can be mixed), a common one being an admixture of reinforcing fibres and an admixture of other substances, especially substances regulating aeration, improving the adhesive properties of a composite or increasing the resistance of a gypsum composite against water. In the case of foamed gypsum, in addition to this, there comes into
40
being a system of admixtures creating foaming gas, and this is most often a binary one. For a complete system for any setting gypsum composite there is then the typical presence of a higher number of mutually influencing components.
2.3.2 Optimization of a multi-component mixture Optimization of the composition of a multi-component mixture is an experimentally and computationally demanding task and for it to be carried out, different approaches are possible. A classical method of sequential optimization is created by a gradual application of one- factor optimizations. After getting an optimum answer for one factor, we keep its found value and try to optimize another factor. This method was used for the optimization of a gypsum plaster composite by Arikan [49]. When studying the influence of every individual admixture, it is possible in the process to follow the recommendation of an appropriate producer of an admixture, or standard data in the literature. Several formulations around a recommended dosage are chosen and the one used is the one which proves to be the best. The disadvantage of this approach is the fact that experiments are given preference in the area where a good result is expected. A more risky (but possibly an optimum) area will stay neglected. More correct therefore proves a systematic examination to a larger extent of dosage application. To reduce the number of experiments it is appropriate to proceed in such a way that we choose other experimental points on the basis of a comparison of successful previous experiments [48]. If the resulting curve gained is analytically easily expressible, and does not have to deal with the curve monotonously increasing or decreasing, it is possible to speed up the finding of an optimum on the basis of a zero value for the first derivation. Even if by the sequential optimization it is possible to improve considerably the qualities of an appropriate composite, the resulting solution need not be a real optimum, because during this process
41
there is insufficient attention to the interaction (synergy) of components. By the simultaneous action of more factors, it is possible to examine their influence with the help of factor experiments. Instead of monitoring the response of a system for one variable factor we can observe, when carrying out the factor experiments, a response to current change in a whole combination of factors. So that it is possible to evaluate the results gained, individual combinations must be created according to a certain plan in such a way that they implement certain mathematical mutual relations. The whole process is therefore sometimes noted as the technique of a planned experiment. The method of factor experiments is very effective, but notably sensitive to the quality of input parameters. It is appropriate especially in that type of case, where one is working with a large amount of data of an obviously quantitative character, which can be (at least in principle) assigned to certain points of continuous functions, but where the course of these functions is not undermined by a disproportionately big measurement error. To lower the measurement error, it is possible, however, to use repeat experiments, but when monitoring a larger number of factors the number of necessary experimental formulations then grows very quickly. Higher factor experiments work on the principle of an analysis of scatter with the help of the sum of the squares. The total variance (S0) – represented by the sum of the squares of deviations of individual observations from the total average – we will divide into individual components (Si), belonging to individual factors. We gradually examine interactions among individual factors, at all levels (from two factors gradually up to (n-1) factors). Finally there will remain a so-called residual variance (Sr), which is caused by nonmonitored and non-captured incidental influences. The significance of individual factors will be gained by the ratio of the variance of the given factors with a residual component. On the basis of a total examination of the resultant field it is possible to carry out the consequent selection of possibly found interactions for further more detailed judgement. The result of an analysis carried out in this way can, however, hardly be a direct particular optimized formulation of a material, as the 42
gained results can only be a guide for a factor determination, whose current change (if you like, mutually bound change) presents a rational way for reaching the optimum. A method of this type for drafting gypsum plaster was used by Böse [50]. At the present time there are promoted various methods of multidimensional sequential optimization in optimizing tasks when solving chemical engineering problems and for optimizing analytical methods. The composition of a subsequently examined composite is determined by a properly chosen algorithm on the basis of results gained with previous formulations. To determine the direction and the size of a composition change there are used various gradient or comparative techniques [48]. Despite the fact that these methods have, to a certain extent, the character of a direct search for the extreme, they can be very effective. The most effective at the moment are considered algorithms, which are a modification of the original Nelder-Meadov method of seeking the extreme of a multivariate function with the help of a flexible simplex [51]. A modified method of a flexible simplex is therefore a part of a number of currently widely-used statistical packages. For example in the program Mathematica 4.2 (Wolfram Research Inc.), the NelderMead algorithm [52] is included as the procedure Nminimize [53]. A generally accepted and user-friendly program for carrying out optimization by the method of a flexible simplex is the program MultiSimplex (Grabitech Solution AB) [54]. The essence of a flexible simplex method is the search for the optimum of the function with m variables on the basis of its values in an m + 1 element set, which creates in m the dimensional mathematical space apexes (B0 up to Bm ) of a virtual polyhedron (simplex). The next stage of an optimizing process consists of four operations, based on a linear transformation of an initial simplex. After evaluating function values in individual corners there will be abandoned corner Bh as having the least favourable value of an optimized function and on the axis determined by this abandoned point and the centre of gravity of the remaining points (by centroid) there is established a new point. This new point Br is created on the opposite side towards the centroid, whereas the absolute value of its distance from the centroid varies only slightly from the original distance between the centroid and the abandoned point. This 43
operation is called reflection. According to the success of this operation there follows either expansion rendering the point Be, or contraction of a newly created vector giving the point Bk The terminal point of the best vectors gained in this way, together with the remaining points of the original simplex, creates another simplex. If it is necessary to get to the corner with a more favourable value, the total simplex possibly is reduced (reduction) by creating appropriate points Bj. By repeating this process we arrive where the rotated, turned over and pulsating simplex approaches the sought-out point by some of its corners. In its proximity then this most often leads to the reduction of the simplex and through this to a more detailed examination of the optimum environment. If we designate the centroid as the point C and the point with the most favourable value as Bd, the programmer’s recording of individual optimizing operations is as follows: C = (1/m)* ΣBj , (j =0,1,2,3 …….m , j ≠ h), Br = (1 + α)*C - α*Bh, Be = β*Br + (1- β)*C, Bk = (1-γ)*C + γ*Bh, Bj = Bj + [(Bd - Bj)*δ] , (j = 1,2,3,……m).
For a successful optimization, of considerable importance is the right choice of starting points. A detailed analysis is provided by Öberg [55]. His knowledge, together with optimum values of coefficients α, β, γ and δ is implemented in the program MultiSimplex.
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Fig. 12: Geometric interpretations of the optimization operations for two-dimensional simplex (W - trial with the least favorable response value, R – reflection value, E – expanded reflection value, C+ - value after the contraction of the reflection value, C- - reflected contraction)
The advantage of a gradual evaluation of individual designs for seeking the optimum composition of a composite, is the possibility to eliminate any such point of the simplex which is physically impossible (the negative content of some component). It is also very easy to penalize exceeding the limit of a fixed area of dosage of some component. At the same time it is also possible to derive a certain bonus from fulfilling an appropriate additional criterion. The optimizing criterion used therefore can have a relatively complex form and can respect various user requirements. In the literature there was not found any mention of the use of sequential optimization for the formulation of gypsum materials. This is in general surprising, because along with the advantages of a sequential approach there is also the possibility to combine both qualitative and quantitative factors, and the possibility of making a basic judgement on any interactive bond among more factors with a tolerable number of experimental formulations, which would prove very appropriate for the formulation of gypsum composites.
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3
AIM OF WORK
The properties of hardened gypsum can be considerably influenced not only by change in the amount of the main initial components (gypsum and water), but also by a whole number of admixtures influencing the solubility and crystallization of individual forms of calcium sulphate. Substances active on the surface and saline electrolytes are among the most significant additions regulating gypsum setting and hardening. For improving adhesive properties, polymer dispersions are added to gypsum, and rheological properties are treated by thickening substances based on cellulose according to need. For improving mechanical properties there is used the lowering of the water-cement ratio with the help of plasticizers or the reinforcing of a gypsum matrix with glass and cellulose fibres. To a certain extent, it is possible to combine gypsum with other binders (e.g. lime or cement), which also can lead to an increasing of strength. Up till now only a small amount of attention has been given to the possibility of the lightening of gypsum by foaming with the help of admixtures for that purpose. It can be relied upon in terms of the same technology, by which it is in principle possible also, directly in situ, to prepare in energy terms a not demanding and in hygienic terms a fitting material, with good thermal-insulation and fireprotective properties. The wide range of possible additions gives major options for seeking out the choice of technologically interesting combinations of foamed gypsum materials. Applying modern multidimensional optimization methods which respect possible interactions of individual components can bring significant improvement in their properties. Gaining knowledge usable for the formulation of quality composite materials based on chemically foamed gypsum is the aim of the submitted work. To fulfil this aim there will be carried out tests of gypsum compositions prepared in the laboratory, foamed by carbon dioxide created by a chemical reaction inside the setting binder. After finding the appropriate foaming system subsequent work will be focused especially on the optimization of the total composition proportion used for the preparation of a foamed gypsum material, and on the specification of the technology of preparation, as well as 46
on the defining properties and the possible use of materials gained in such a way.
4
EXPERIMENTAL SECTION
In the experimental section there are collected data about used raw materials and chemicals. Here are specified the apparatus used and the testing methods. There follows a passage given over to the methodology used for the evaluation of foamed gypsum composites and after that there is presented a survey of all progressively tested formulations (formulae).
4.1
Used raw materials and chemicals In this chapter there are given the raw materials used together with the data which their producers give in relation to them.
White gypsum G-2 BII - producer: Gypstrend Kobeřice Retained on 0,2 mm sieve: max. 15 %, on average:0,2 –1,5 % Setting: 6 – 30 min, on average: 6 – 12 min Strength in MPa after 2 hours: min. 2 MPa, on average: 3,0 – 5,5 MPa CaSO4.1/2 H2O + CaSO4: min. 95 %, on average: 97,4 % pH : 5 – 8, on average: 7,4 MgO, water soluble: max. 0,1 %, on average: 0,0013 % 0,01 % Na2O, water soluble: max. 0,06 %, on average: Cl-, water soluble: max. 0,01 %, on average: 0,0029 % CaCO3 1-2,5 % Al2O3 0,1-0,4 % SiO2 0,2-0,6 % TiO2 0,5-1,2 % Fe2O3 0,1-0,3 % Insoluble residue in HCl: 1,0-2,5 %
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White gypsum - producer : RIGIPS Ltd. Retained on 0,2 mm sieve : 0,42% Retained on 0,63 sieve : 19,25 % Passings on 0,63 sieve : 80,33 % Compressive strength after 24 hours: 19 MPa Paste of normal density: v/s = 0,83 95,9 % CaSO4.1/2 H2O + CaSO4: Content of MgO ≤ 1,00 % Initial setting time 2:05 min Final setting time 5:35 min Hydration 96,47 % Specific surface 2620 cm2/g Vinnapas RI 551 Z - producer: Wacker Polymer System In water redispergeable dispersed powder of terpolymer based on ethylene, vinyl laurate and vinyl chloride. In the masses which contain gypsum or anhydrite as a binder, it improves tensile flexural strength, impact resistance, elasticity, retention property and workability and at the same time it lowers the absorption capacity of the masses modified by it. Dry residue 9 % mass. Ash content 13 % mass. Loose bulk density 450 kg.dm-3 Limit size of particles ax. 2 % above 400 µm Prevailing particles 0, – 9,0 µm Minimum film-forming temperature 0 °C Walocel MKX - producer: Wolff Cellulosics Water soluble pulverized methyl hydroxyl ethyl cellulose with a medium to high degree of esterification. It improves consistency, workability, increases the strength and retention properties of mixtures for the undercoat. Moisture content: 7 % Viscosity of the solution with a content 2 % Walocel MKX: 6000 70000 mPa·s at 20°C
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Melment F10G - producer: Stachema Kolín, Ltd. Dried powder of sulphonated polycondensate products based on melamine. Used for plasticizing and water reduction. Loose bulk density: 450 – 650 kg.m-3 Stachement NN – producer: Stachema Kolín,Ltd. Plasticizing and liquefying admixture. Water solution of sulphonated polycondensate based on naphthalene. Dark brown liquid. Density: 1 200 ± 30 kg m-3 Stachesil - producer: Stachema Kolín,Ltd.. Powdered, chlorideless, mineral admixture in concrete mixtures considerably improving workability. Appearance: grey powder Loose bulking density: 200 - 250 kg.m-3 Size of particles: 0,1 - 0,2 µm Moisture: 1,0 mm min 66 % SiO2 Fe2O3 max 3 % max 18 % Al2O3 CaO + MgO max 6 % Na2O+K2O max 8 % Perlite EP 150 PB (Perlite coarser) – producer Perlit Praha Spol. Ltd. Loose bulking density: 110 – 130 kg/m3 Granularity: 0 - 2,0 mm , max 5 % > 2,0 mm SiO2 min 66 % Fe2O3 max 3 % Al2O3 max 18 % CaO + MgO max 6 % Na2O+K2O max 8 % With the following laboratory chemicals the producer is not stated. The reality that these can be counted as substances of analytic clarity, guarantees their quality within a sufficient tolerance. Boric acid p.a. Potasium alum p.a. Citric acid p.a. Salicylic acid p.a. Sodium hydrogencarbonate p. a. Tartaric acid p.a. Ammonium sulphate p.a. Phthalic acid p.a.
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4.2
Instruments and equipment used
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Vicat’s automatic apparatus E 040, producer MATEST, with a needle penetrometer and conic head
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Mixer for dry mortar mixes E093,producer MATEST
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Compressive machine FP 100, producer VEB Industriewerk Ravenstein , used ranges 0-10 kN, 0-20 kN
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Thermal conductivity measuring device ISOMET 104, producer Applied Precision, Ltd., with a needle probe ranging 0,03 – 1,0 W/m.K
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Ultrasonic measuring device C372, producer MATEST
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Pull-off machine COMTEST OP 3, producer COMING PLUS,a.s.
•
Scales KERN EW 6000-1M, producer KERN
•
Scales KERN EW 3000-2M, producer KERN
•
Drier HS 61A, producer Chirana, range 50 -200 °C
•
Device FEUTRON 4110 for measuring thermal conductivity, producer Feutron, Greiz
•
Mixer B026, producer Matest
•
Manual drill tool with a spiral mixing head
4.3
Experimental methods and testing procedures
Preparation of mixtures •
According to the standard ČSN 72 2301 Gypsum binders, paragraph 7 - manual mixing (test No.1-175).
•
Mechanical mixing: 30s at low running speed, 30s wiping off, 30 s mixing at low running speed (test No. 176 - 311).
53
Determination of gypsum paste with a normal consistency •
Method of operation according to the standard ČSN 72 2301 Plaster binders, paragraph 6.1 – flow test after lifting the cylinder (test No. 1-243).
•
According to the standard ČSN EN 13279-2 Gypsum binders and gypsum plasters – Part 2: Test methods, paragraphs.4.3.2 A dispersal method (from the test No. 244).
¨ Determination of the setting time •
According to the standard ČSN 72 2301 Plaster binders, paragraph.6. –Vicat’s apparatus with needle penetrometer (test No.46-160)
•
According to the standard ČSN EN 13279-2 Gypsum binders and gypsum plasters - Part 2: Test methods, paragraphs 4.4.1.Knife method ( test No. 245-270)
•
According to the standard ČSN EN 13279-2 Gypsum binders and gypsum plasters - Part 2: Test methods, paragraphs 4.4.2 Vicat cone method (test No. 281 – 311)
Shapes and sizes of sample test pieces •
For tests of bulk density: prisms 20x20x100 mm (test No.65160)
•
For tests of compressive and flexural strength: standard sample pieces according to the standard ČSN 72 2301 and ČSN EN 13279-2: prisms 40 x 40 x 160 mm (test1-46, 161-311)
•
For tests of heat conductivity: plate 40x160x144,8 mm (test No.271), prisms 40x40x160 mm (test No.281-299), plate 250x250x20 mm (test No. 327, 328)
54
Bulk density determination •
Direct measurement of sizes
•
Weighing after drying at 50 °C to the stabilized mass
Determination of compressive and flexural strength •
Tested on pieces after drying at 50 °C to the stabilized mass
Procedure according to: •
ČSN 72 2301 Plaster binders, paragraphs7, 8 (test 1-46)
•
ČSN EN 13279-2 Gypsum binders and gypsum plasters – Part 2: Test methods , paragraphs 4.5.4.Establishing flexural strength and paragraphs 4.5.5 Determination of compressive strength (test No. 176-311)
Determination of thermal conductivity •
With a device ISOMET 104 by the method of a needle probe
•
With a device FEUTRON 4110 by the method of a guarded hot plate
Determination of the modulus of elasticity •
Determination of a dynamic modulus of elasticity with a device MATEST C 372 based on the velocity of the sonic wave
Determination of the adhesive strength of gypsum plaster on substrates •
According to ČSN EN 1015-12 Methods of test for mortar for masonry. Determination of adhesive strength of hardened rendering and plastering mortars on substrates – with a device COMTEST OP3 , with manual control
55
4.4
Survey of prepared compositions
In the following tables 9 – 20 there are collected compositions of the individual tested mixtures. Even if individual experimental compositions are not presented in strictly chronological order in these tables, they are provided with helpful sequential numbers. These numbers are used in the following text for the unambiguous identification of an appropriate prescription. The number of actually prepared mixtures was in fact much greater, because the majority of compositions were used repeatedly. Individual batch numbers are not differentiated in the working text and are indicated either by a composition or by the number of the appropriate composition. In the cases where the appropriate composition is a component part of an optimizing series of experiments (two factor optimizations or sequence optimizations) the values of the optimizing quantities were as a rule gained as the average from the measurements on the three repeatedly prepared compositions. The gained data xi were, before inserting into an optimized set, tested by the Dean-Dixon test [64], whereas values Q1 and Qn were compared with critical value Qa valid for a = 0, 95 .
R = xmax – xmin
56
Tab.8: Critical values Q for rejection of outliers [65]
Number of values
Q (a = 0,95)
3
0,941
4
0,765
5
0,642
6
0,560
7
0,507
8
0,468
57
Gypsum producer
Kobeřice
Nr.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
water/gypsum ratio
0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,70 0,60 0,65 0,63 0,87 0,87 0,87 0,87 0,87 0,87 0,87
Al2(SO4)3
%
Citric acid
%
Tartaric acid
other type
Acid component
%
%
Melment 0,5 1 1,5 2 2,5 Stachement Stachement Stachement Stachement Stachement Stachesil Stachesil Stachesil Melflux Melflux Melflux Melflux Melflux Melflux Melflux Melflux Melflux glass fibres glass fibres glass fibres glass fibres glass fibres glass fibres PP fibers
0,5 1 1,5 2 2,5 5 10 15 0,1 0,2 0,5 0,75 1 1 1 1 1 0 0,04 0,08 0,2 0,3 0,05 0,05
%
other admixture
Note
Tab.9: Composition of the gypsum mixtures
58
Retardan
Vinnapas
Walocel MKX
Silipon
CaCO3
NaHCO3
59
0,87
0,87
0,80 0,87 0,87
0,87
53
54 55 56
57
0,87 0,87 0,87 0,63 0,63 0,63 0,78 0,78 0,78 0,98 0,98 0,98 0,70 0,70 0,70 0,86 0,86 0,87 0,60 0,80 0,80
water/gypsum ratio
52
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
Gypsum producer
Kobeřice
Nr.
%
Citric acid
0,4 0,3
0,5 0,2 0,5 0,5 0,5
Al2(SO4)3
%
Tartaric acid
other
potasium alum
boric acid
potasium alum
potasium alum
boric acid
type
Acid component NaHCO3 1
0,5
0,5
0,5
0,1
0,3
0,1
0,1
0,5
0,17 0,1 0,1 0,1 0,1 0,5 0,5
%
%
Vinnapas 3 6
3 6
3 6
3 6
3 6
Melflux Melflux Melflux
1 1 1
Melflux 1 Melflux 1 Melflux 1 Melflux 0,5 Melflux 0,5 Melflux 0,5 pure gypsum
% celuloze fibers 0,05
other admixture
Note
Tab.10: Composition of the gypsum mixtures
Retardan
Melment
Walocel MKX
Silipon
CaCO3
0,87
0,87 0,87
0,87
0,87
0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87
59 60
61
62
63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84
water/gypsum ratio
58
Gypsum producer
Kobeřice
Nr.
%
Citric acid
0,3 0,3 0,2 0,2 0,4 0,6 1 1 0,6 0,4 0,6 0,4 0,6 0,6 0,6 0,6 0,6 0,4 0,4 0,4 0,4 0,6
Al2(SO4)3
%
Tartaric acid
other
potasium alum
potasium alum
boric acid boric acid
potasium alum
type
Acid component
1 1
1
0,5
0,5
%
NaHCO3 0,1 0,2
0,2
CaCO3 0,1 0,2 0,2 0,4 1 2 3 5 5 5 10 10 15 20 5 12 15 5 10 12 15 2
0,2
0,1
Silipon 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03
%
dihydrate dihydrate dihydrate dihydrate dihydrate dihydrate dihydrate dihydrate dihydrate dihydrate dihydrate dihydrate dihydrate dihydrate dihydrate dihydrate dihydrate dihydrate dihydrate
0,2 1 5 3 5 5 5 10 5 5 5 5 5 5 5 5 5 5 5
%
other admixture
Note
Tab.11: Composition of the gypsum mixtures
60
Retardan
Vinnapas
Melment
Walocel MKX
Gypsum producer
Kobeřice
Nr.
85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115
water/gypsum ratio
0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,87 0,60 0,70 0,80 0,80 0,80 0,80 0,80 0,80
Citric acid
% 0,6 0,6 0,5 0,5 0,5 0,6 0,6 0,6 0,8 0,8 0,8
Al2(SO4)3
%
0,2 0,4 0,6 0,8 1 1,2 0,6 0,8 1 1,4 2 4 2 2 2 3 4 5 0,4 0,4
Tartaric acid
other type
Acid component
%
CaCO3 4 6 3 5 8 3 5 8 3 5 8 5 5 5 5 5 5 8 8 8 5 5 5 5 5 5 5 5 5 0,6 1
Silipon 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03
% dihydrate dihydrate
% 5 5
other admixture
Note
Tab.12: Composition of the gypsum mixtures
61
Retardan
Vinnapas
Melment
Walocel MKX
NaHCO3
Gypsum producer
Kobeřice
Nr.
116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146
Al2(SO4)3
%
Citric acid
%
0,6
water/gypsum ratio
0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,70 0,74 0,78 0,80 0,80 0,80
0,4 1 1 1 1 1 1 5 7 10 2 5 10 2 5 10 2 5 10 10 15 20 2 5 10 5 5 5
Tartaric acid
other
síran amonný síran amonný síran amonný
type
Acid component
%
5 5 5
2
CaCO3
62 2 2
2 2 3 4 5 7 10 5 5 5 1 1 1 2 2 2 8 8 8 4 5 5 0,6 0,6 0,6 1 1 1
Silipon 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03
%
%
other admixture
Note
Tab.13: Composition of the gypsum mixtures
Retardan
Vinnapas
Melment
Walocel MKX
NaHCO3
0,80
0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,80 0,98 0,98 0,98 0,98 0,98 0,98 0,98 0,98 0,98 0,98 0,98 0,98 0,98 0,98 0,98
148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175
water/gypsum ratio
147
Gypsum producer
Kobeřice
Nr.
%
Citric acid
5 2 3 3 6 4 6 8 4 6 8 1,2 1,2 1,2 2,4 2,4 2,4 3,6 3,6 3,6 4 4 10 10 8 8
1,2
Al2(SO4)3
%
Tartaric acid
other
phtalic acid
type ammonium sulphate
Acid component
5 1,6
%
CaCO3 1,5 1,5 2 1 1 2 2 2 2 2 5 5 5 1 2 3 1 2 3 1 2 3 1 1 1 1 1 1
5
Silipon
63 0,05 0,05 0,05 0,05 0,05 0,05
0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03 0,03
0,03
%
0,07 0,07 0,07
PP fibres PP fibres PP fibres
%
other admixture
Note
Tab.14: Composition of the gypsum mixtures
Retardan
Vinnapas
Melment
Walocel MKX
NaHCO3
Gypsum producer
Kobeřice
Nr.
176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206
water/gypsum ratio
0,75 0,75 0,75 0,75 0,85 0,85 0,85 0,85 0,86 0,90 0,97 0,96 1,07 0,92 0,94 1,10 1,27 1,12 0,94 1,03 0,89 0,90 0,86 0,74 0,68 0,69 0,61 0,64 0,54 0,58 0,63
Citric acid
%
Al2(SO4)3
% 8 10 8 10 10 8 10 8 10,286 8,367 7,551 10,239 11,359 11,199 12,113 10,717 11,075 11,024 10,472 11,708 11,532 12,541 9,135 10,286 11,429 8,694 8,041 8,321 7,481 10,413 11,102
Tartaric acid
typ
other %
CaCO3 3,3 2,7 2,7 3,3 3,3 3,3 2,7 2,7 2,614 2,418 1,977 2,812 2,868 2,86 2,134 2,572 2,508 2,517 2,938 3,057 3,04 2,365 3,066 2,614 2,271 2,32 1,83 2,812 2,868 2,844 2,885
Silipon 0,015 0,025 0,025 0,015 0,025 0,025 0,015 0,015 0,026 0,03 0,037 0,023 0,022 0,035 0,028 0,029 0,031 0,043 0,021 0,019 0,032 0,024 0,025 0,014 0,008 0,009 0,001 0,017 0,018 0,018 0,006
Walocel MKX 0,29 0,29 0,23 0,23 0,23 0,29 0,29 0,23 0,221 0,279 0,303 0,3 0,335 0,33 0,281 0,279 0,273 0,274 0,293 0,324 0,32 0,268 0,285 0,221 0,187 0,269 0,289 0,203 0,16 0,255 0,263
%
Melment 1,2 0,8 1,2 0,8 1,2 0,8 1,2 0,8 0,743 1,127 1,29 0,752 0,528 1,075 1,153 1,254 1,481 0,934 0,706 0,459 1,008 1,068 0,867 1,257 1,486 0,939 0,808 0,864 0,696 1,283 0,906
Vinnapas 5,5 5,5 4,5 4,5 5,5 4,5 4,5 5,5 4,357 5,316 5,724 5,666 6,249 4,88 5,989 5,128 4,942 6,255 5,549 6,074 4,713 5,774 4,819 5,643 6,214 4,847 4,52 5,946 6,669 5,073 6,237
Retardan 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1
%
other admixture
Note
Optimization of the bulk density - Multisimplex
Acid component
Tab.15: Composition of the gypsum mixtures
64
NaHCO3
207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237
Gypsum producer
Kobeřice
Rigips
Nr.
water/gypsum ratio
0,60 0,56 0,70 0,54 0,70 0,59 0,56 0,66 0,58 0,68 0,65 0,63 0,83 1,07 1,07 1,07 1,07 1,07 1,07 1,07 1,07 1,07 1,07 1,07 1,07 1,07 1,07 1,07 1,07 1,07 1,07
Citric acid
%
Al2(SO4)3
11,359 11,359 11,359 11,359 11,359 11,359 11,359 11,359 11,359 11,359 11,359 11,359 11,359 11,359 11,359 11,359 11,359 11,359
% 9,417 9,25 9,813 11,768 8,942 9,706 10,796 9,405 7,73 11,429 10,504 7,887
Tartaric acid
type
other %
CaCO3 20 2,868 2,868 2,868 2,868 2,868 2,868 5 10 50 2,868 2,868 2,868 2,868 2,868 2,868 2,868
2,166 2,614 2,678 1,836 2,934 2,531 2,16 2,74 2,957 2,271 2,442 2,216
Silipon 0,022 0,022 0,022 0,022 0,022 0,022 0,022 0,022 0,022 0,022 0,022 0,022 0,022 0,022 0,022 0,022 0,022 0,022
0,013 0 0,019 0,011 0,014 0,005 0,008 0,012 0,015 0,008 0,01 0,018 0,528 0,528 0,528 0,528 0,528 0,528 0,528 0,528 0,528 0,528 0,528 0,528 0,528 0,528 0,528 0,528 0,528 0,528
1,337 1,49 0,972 1,025 1,156 1,305 1,164 1,158 0,74 1,486 1,3 1,433
Walocel MKX 0,335 0,335 0,335 0,335 0,335 0,335 0,335 0,335 0,335 0,335 0,335 0,335 0,335 0,335 0,335 0,335 0,335 0,335
Melment
% 0,272 0,207 0,269 0,201 0,268 0,222 0,209 0,253 0,309 0,187 0,218 0,22
Vinnapas 6,249 6,249 6,249 6,249 6,249 6,249 6,249 6,249 6,249 6,249 6,249 6,249 6,249 6,249 6,249 6,249 6,249 6,249
6,733 6,086 5,646 6,127 5,657 5,985 6,067 5,76 5,381 6,214 6,006 5,292
Retardan 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1
0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0,1
65
1 5 10 15 20 30
0,5 1 2 5 10 15 1
wood splinters wood splinters wood splinters wood splinters wood splinters wood splinters
mineral fibres mineral fibres mineral fibres microsilica microsilica microsilica microsilica
%
other admixture
Note
Optimalizace objemové hmotnosti - Multisimplex
Acid component
Tab.16: Composition of the gypsum mixtures
NaHCO3
Gypsum producer
Rigips
Nr.
238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268
water/gypsum ratio
1,07 1,07 1,07 1,07 1,07 1,07 0,94 0,94 0,94 0,94 0,94 0,94 0,94 0,94 0,94 0,94 0,94 0,94 0,74 0,74 0,74 0,70 0,60 0,50 0,60 0,70 0,50 0,60 0,70 0,60 0,50
Citric acid
0,1 0,1 0,1 0,5 0,5 0,5 5 10 10 7,7 5,143 3,43 3,43 5,14 5,14 3,43 2,57 5,14 3,43 5,14 3,43 3,43
%
Al2(SO4)3
5 10 5 5 2,5 2,57 5,14 6,86 6,86 5,14 5,14 6,86 7,7 5,14 6,86 5,14 6,86 6,86
5 10
5 10
% 11,359 11,359 11,359 11,359 11,359 11,359
Tartaric acid
other
boric acid
type
Acid component
0,03 0,08 0,15
%
CaCO3 2,614 2,614 2,614 2,614 2,614 2,614 2,614 2,614 2,614 2,614 2,614 2,614 2,614
2,868 2,868 2,868 2,868 2,868 2,868
Silipon 0,022 0,022 0,022 0,022 0,022 0,022
66 1,257
1,257 1,257 1,257 1,257 1,257 1,257 1,257 1,257 1,257 1,257
0,528 0,528 0,528 0,528 0,528 0,528
Walocel MKX 0,221 0,221 0,221 0,221 0,221 0,221 0,221 0,221 0,221 0,221 0,221 0,221 0,221
Melment
% 0,335 0,335 0,335 0,335 0,335 0,335
Vinnapas 5,643 5,643 5,643 5,643 5,643 5,643 5,643 5,643 2,8 2,8 5,643 5,643 2,8
6,249 6,249 6,249 6,249 6,249 6,249
Retardan 0,1 0,1 0,1 0,1 0,1 0,1
cement cement cement
% 1 5 10
other admixture
Note
Tab.17: Composition of the gypsum mixtures
NaHCO3
Gypsum producer
Rigips
Nr.
269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299
water/gypsum ratio
0,50 0,50 0,50 0,55 0,50 0,50 0,55 0,55 0,55 0,55 0,55 0,55 0,70 0,50 0,70 0,50 0,70 0,50 0,74 0,68 0,81 0,58 0,59 0,55 0,53 0,46 0,64 0,44 0,63 0,58 0,61
Al2(SO4)3
% 6,86 6,86 6,86 6,86 6,86 6,86 6,86 6,86 6,86 6,86 6,86 6,86 7,33 7,33 6,00 7,33 6,00 6,00 5,73 6,13 5,25 6,81 6,73 7,02 7,15 5,74 6,93 7,45 6,36 7,59 7,09
Citric acid
% 3,43 3,43 3,43 3,43 3,43 3,43 3,43 3,43 3,43 3,43 3,43 3,43 3,67 3,67 3,00 3,67 3,00 3,00 2,87 3,07 2,63 3,41 3,36 3,51 3,57 2,87 3,47 3,72 3,18 3,80 3,55
Tartaric acid
type
other %
CaCO3 2,614 2,614 2,614 2,614 2,614 2,614 2,614 2,614 2,614 10 20 40 2 4 4 2 2 2 3,6 3,2 1,28 3,32 2,13 1,59 3,38 3,93 2,48 1,32 3,33 3,86 2,75
Walocel MKX % 0,221 0,221 0,221 0,221 0,221 0,221 0,221 0,221 0,221 0,221 0,221 0,221 0,2 0,3 0,2 0,3 0,3 0,2 0,18 0,21 0,14 0,26 0,25 0,28 0,14 0,22 0,21 0,22 0,21 0,23 0,16
Melment 1,257 1,257 1,257 1,257 1,257 1,257
1,257 1,257 1,257
Vinnapas 2,8 5,643 5,643 5,643 5,643 5,643 5,643 6 6 4 4 6 6 7,2 6,4 5,36 5,84 4,74 3,9 4,63 4,08 5,52 6,69 4,67 4,16 3,65
5,643 2,8
2,8
wood splinters wood splinters wood splinters
5 10 15
%
other admixture
Note
Foamed plaster optimization Multisimplex
Acid component
Tab.18: Composition of the gypsum mixtures
67
Retardan
Silipon
NaHCO3
Gypsum producer
Rigips
Nr.
300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328
water/gypsum ratio
0,58 0.5 0.5 0,50 0,50 0,50 0,53 0,53 0,53 0,55 0,55 0,55 2,60 2,00 3,00 2,60 2,70 2,05 2,10 2,10 1,06 0,77 0,80 0,80 0,67 0,70 0,60 0,58 0,68
Al2(SO4)3
% 7,09 6,96 7,35 6,67 7,33 8 6,67 7,33 8 6,67 7,33 8 7,33 7,33 7,33
7,33 7,33 7,33
7,33 7,33 7,33
7,33 7,33 7,33 7,33
Citric acid
% 3,55 3,48 3,67 3,33 3,67 4 3,33 3,67 4 3,33 3,67 4 3,67 3,67 3,67
3,67 3,67 3,67
3,67 3,67 3,67
3,67 3,67 3,67 3,67
Tartaric acid
type
other %
CaCO3
68 2,34 2,34 2,34 2,34
2,34 2,34 2,34
2,34 2,34 2,34
2,75 4,24 2,34 2,34 2,34 2,34 2,34 2,34 2,34 2,34 2,34 2,34 2,34 2,34 2,34
%
Walocel MKX 0,16 0.13 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2
Vinnapas 3,65 5,68 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35 8,35
Perlite 30 30 30 30 30 20 20 20 5 5 5 5 5 3 1 1 3
Perlite finer
%
other admixture
Note
Two-factor optimization
Acid component
Tab.19: Composition of the gypsum mixtures
Melment
Silipon
NaHCO3
329 330 331 332 333 334 335 336
Nr.
water/gypsum ratio
0,70 0,75 0,70 0,72 0,72 0,72 0,72 0,72
Al2(SO4)3
% 7,33 7,33 7,33 7,33 7,33 7,33 7,33 7,33
Citric acid
% 3,67 3,67 3,67 3,67 3,67 3,67 3,67 3,67
Tartaric acid
Gypsum producer
other type
Acid component
%
CaCO3 2,34 2,34 2,34 2,34 2,34 2,34 2,34 2,34
%
Walocel MKX 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2
Vinnapas 8,35 8,35 8,35 8,35 7 6 5 4
Perlite 5 7 7 7 7 7 7 7
Perlite coarser
%
other admixture
Note
Tab.20: Composition of the gypsum mixtures
69
Melment
Silipon
NaHCO3
5
RESULTS AND DISCUSSION
Before the first proposal for the composition of a foamed gypsum binder, the following reflections were made towards the establishing of a reasonable starting dosage of individual components. In the gypsum binder itself (not containing any foaming component) it is necessary to take into account the regular creation of a relatively large number of pores. Porosity is at the same time strictly connected with the used water-gypsumt ratio. Since the density of the solid hemihydrate phase (2310 kg.m-3) during the transformation into dihydrate, does not change significantly, it is possible to find approximately the relation between the water-cement ratio and the future porosity of the system on the basis of the simple mass balance. In the following balance equations, excepting the water-gypsum ratio w, there is also the projected mass of batch water mw, starting mass of gypsum ms, volume of batch water Vw, volume of water necessary for hydration Vh , volume of created dihydrate matrix Vm, total volume of hardened gypsum Vc and finally, porosity P and porosity in per cents P% . From the stoichiometry of the hydration equation come the results that water consumption for dihydrate creation is 18,6 % of the hemihydrate mass. If we consider common gypsum in which there is a content of hemihydrate of calcium sulphate 97, 5 %, then it is valid that: w = mw / ms Vw = mw /1000 = (ms . w ) / 1000 Vp = Vw – Vh = ((ms . w ) / 1000)- (ms . 0,975 .0,186/ 1000) = ms .(w – 0,18) /1000 Vc = Vm + Vp = (1,18 ms /2310) + ms .(w – 0,18) /1000 = ms . (1,18/2,31 – 0,18 + w)/1000 P = Vp/ Vc = (w-0,18) / (w + 0,33)
70
If we assume that hydration will take place only from 72 %, water consumption will go down from the original 18 % of a mass of unhardened gypsum to 13 % and we will get an equation: P = (w-0,13) / (w + 0,36), which is identical with the relation proposed for the porosity calculation by Schiller [9]. In technical practice porosity is most frequently expressed in per cents, which means as a one hundred multiple of a porosity value without dimension: P% = 100. P . With the usual values of the water-gypsum ratio used in the preparation of masses based on β- gypsum, i.e. with w = 0,6 – 0,7 we can then (according to the above given formulae ) rely on a porosity 45 – 55 %. Any particular value certainly depends on the specific content of hemihydrate and the extent of hydration. For the bulk density ρv of hardened gypsum it is then, at the same time, valid that: ρv = ((ms + mw) / ((w+0,36). ms)).1000 ρv = ((1+w) / (w+0,36)).1000 The bulk density of freshly set gypsum is based on results according to this relation ranging 1500 -1600 kg.m-3 (for w = 0,7 – 0,8). In reality we find values 10 – 25 % lower. This difference can be credited to air mixed into the composite during its preparation. In the course of time there certainly also appears a drop in bulk density as a consequence of the evaporation of chemically unbound water from a hardened system. If we contemplate that the bulk density of set gypsum is circa 1500 kg.m-3, in order to reduce its bulk density to 500 kg.m-3 we need to create through the use of the foaming admixture, 2 dm3 of other pores for every1 dm3 of the original gypsum paste. If we assume bubbles about one millimetre in size, then the pressure inside individual bubbles will not much differ from atmospheric pressure. The pressure necessary for overcoming forces of surface tension with bubbles of this size is small and neither does hydrostatic 71
pressure inside a foamed mixture play (with the usually prepared amount of gypsum mass) a more significant role. Since the growth of atmospheric pressure will certainly not exceed 10 %, it is possible to use for the estimate of the substantial amount of gas n necessary, the universal equation of state of ideal gas [29]. If we denote pores volume V, pressure in pores P , temperature in pores T and the number of moles of foaming gas n, then with using basic units, an universal gas constant R has the value 8,314 J mol-1 K1 and it is valid that: n = P.V / (R.T) n = 110000. 0,002 / (8,314 . 293,15) n = 0,1 Under the conditions prevailing inside pores (T = 293,15 K, P = 110000 Pa) then there corresponds to the required volume (0,002 m3 ) roughly 0,1 mole of foaming gas. In order to get this amount of carbon dioxide by the acid decomposition of calcium carbonate there is necessary about 10 g of calcium carbonate. Since for the preparation of 1 dm3 of gypsum paste there is necessary about 800 g of gypsum, from the stoichiometry viewpoint, the foaming reaction gives a reliably sufficient content of calcium carbonate amounting to 1,25 % of gypsum mass. Such an amount is not, in gypsums of natural origin, exceptional in any way. In the case of domestic gypsum a producer states for the content CaCO3 a value 1,0 – 2,5 % [30]. Even if preceding calculations indicate that for a substantial gypsum foaming, the calcium carbonate present can naturally suffice (it is after all in harmony with data in [28]), it is obvious that it will be also necessary to investigate formulations containing an admixture of a carbonate proportion. It is actually probable that the mere quantity of material does not determine the effectiveness of foaming but neither does it determine rate of the foaming gas formation, which may be considerably influenced by an increase in the concentration of carbonate. The dosage of an acid component should in principle correspond with the amount of the carbonate component, but even here it is obvious that the kinetics of the whole process will determine the most effective dosage, because it deals with bimolecular reaction, the 72
rate of which is influenced by the concentration of both reacting components. Concerning the dosage of other substances used in the formulation of foam composite it proved functional to start with their normal dosage, used in the preparation of „ordinary“ (not foamed) composite. With substances not commonly used in the technology of gypsum materials preparation, the starting dosage can be chosen analogous with a similar concrete admixture or analogous with similar use in composites of another kind. Experiments with gypsum paste without a foaming admixture can provide valuable information about the effect and dosage of other substances.
5.1
Composition based on gypsum Kobeřice
For experimental work, firstly there was used gypsum from a firm Gypstrend Kobeřice, which seemed to be the most accessible. This concerned a relatively cheap type of chemo- gypsum with strength class G-2. On control strength setting there was measured a strength 3,3 MPa.
5.1.1 Modification of gypsum paste properties The first part of the experiments was in harmony with the thoughts given above and applied to compositions of a non-foamed type. The aim was to achieve some basic knowledge about the behaviour of a tested gypsum binder and to check the effect of some admixtures. First of all there was checked the possibility of reducing the water content in a gypsum mixture by an appropriate plasticizer. Those preparations tested were: Melment F10G (which is designed especially for gypsum), Stachement, Stachesil and super plasticizer Melflux PP 100F. A flow test was carried out according to the standard ČSN 72 2301. The flow test results are displayed in Table 21. The biggest influence was with super plasticizer (Melflux PP 100F), which decreased the 73
water content in a mixture of normal density almost by 30 %. In contrast, there was not manifested any principal difference between the product especially designed for gypsum and those preparations designed for general purpose.
Tab. 21: Results of flow test – diameter in cm Plasticizer Amount [%] 0*
Melment F10G
Stachement NN
Stachesil
Melflux PP 100F
18,0
0,1
21,5
0,2
27,0
0,5
27,0
25,3
31,0
0,75
32,5
1
26,5
27,6
1,5
31,0
28,9
2
31,0
29,6
2,5
30,7
28,0
34,2
5
14,0
10
15,0
15
12,0
* mixture with normal density
74
A two-factor optimization of a gypsum mixture was another phase of the experiments, during which the influence of the combination of plasticizer and the dispersion on varying dosages was detected. Melflux PP 100F was chosen as a plasticizer and Vinnapas RI 551 Z was used as the dispersion. There were tested mixtures in a combination of the following dosages: Melflux – 0 %, 0,5 % and 1 % gypsum mass, Vinnapas – 0 %, 1 %, 2 % gypsum mass. Test specimens 40 x 40x 160 mm in size were subjected to a test for flexural strength and compressive strength according to the standard ČSN 72 2301. Test results are given in Table 22 and in Figures 13 and 14. Tab. 22: Flexural and compressive strength of the gypsum mixtures Dispersion
Flexural strength [ MPa]
Compressive strength Rt [ MPa]
0%
1%
2%
0%
1%
2%
0%
3,1
3,7
3,73
6,11
6,44
6,83
0,5 %
3,47
3,31
3,42
6,63
5,89
6,12
1%
1,14
1,56
1,44
3,13
3,89
3,3
Plasticizer
In Figure 13 it is seen distinctively that a higher content of plasticizer significantly lowers flexural strength. Dispersion surplus increases material strength somewhat but in combination with the plasticizer, however, dispersion is not able to compensate for strength loss caused by the plasticizer. Also compressive strength is negatively influenced by the plasticizer, as is visually demonstrated in Figure 14. In this case the dispersion influence is essentially irrelevant.
75
Fig. 13 : Influence of plasticizer and dispersion on the flexural strength
In the following stage of experiment the influence of various types of fibres on mechanical properties was investigated. There were tested mixtures with glass, polypropylene and paper fibres but the results of workability tests, however, were not such as to make it possible to recommend tested fibre admixtures for further work with foamed compositions.
76
Fig.14.: Influence of plasticizer and dispersion on the compressive strength
5.1.2 Choice of a foaming system In yet another stage of experiment there was the selection and optimization of a foaming system for the preparation of a chemically foamed gypsum mixture. As a foamed gas there was used carbon dioxide, which is created as a result of the reaction between an acid and a carbonate component. The added carbonate component was on the one hand sodium hydrogen carbonate (cooking soda) or pulverized calcium carbonate (limestone). For the function of an acid component there was tested a whole range of admixtures brought into consideration. With the narrowing choice there is arrived at: boric acid, tartaric acid, o-phytic acid, citric acid, aluminium sulphate, ammonium sulphate and potassium sodium sulphate.
77
During the following foaming experiments there was then investigated bulk density and workability properties and also potentially flexural strength and compression strength. ¨ As is shown in Figure 15, the bulk density of the composition foamed with tartaric acid and calcium carbonate (limestone) reduces with the increasing content of an acid component.
Fig.15: Bulk density of the gypsum composition, foamed by the tartaric acid and calcium carbonate (X-axis: tartaric acid [%], Y-axis: bulk density [kg.m-3], Z-axis: calcium carbonate [%]).
In the case of a carbonate component, the revealed dependence is not unambiguous. It is obviously connected with the substantial quantity of limestone used (surplus). In experiments recorded in Figure 16 the maximum dosage of dihydric tartaric acid (applied to 100g of gypsum) is 9,3 mmol and the minimum dosage of added calcium carbonate (which is also dihydric) is 30 mmol. From the results it is obvious that at least a triple surplus of limestone with the given fineness is sufficient and any increase serves no purpose.
78
Fig.16: Bulk density of the gypsum composition, foamed by the tartaric acid and calcium carbonate (X-axis: tartaric acid [%], Y-axis: bulk density [kg.m-3], Z-axis: calcium carbonate [%]).
For the reaction between aluminium sulphate and calcium carbonate it is possible to write the following equation: Al2(SO4)3 + 3CaCO3 + 6H2O
2Al(OH)3 + 3CaSO4 + 3H2O +3CO2.
In experiments recorded in Table 23 there is applied to 100 g of powdered gypsum 10 – 30 mmol of calcium carbonate and 2 – 6 mmol of aluminium sulphate. The theoretical equivalent amount of calcium carbonate necessary for a reaction with the given amount of aluminium sulphate is 6 – 18 mmol of calcium carbonate and the resulting foaming effect should be therefore influenced by both the sulphate and calcium carbonate. Nevertheless, the influence of calcium carbonate is not apparent. This led to the assumption that as a reacting partner limestone, also naturally present in gypsum, obviously presents itself. Analytically it was verified that the amount of limestone present in the powdered gypsum itself is about 20 mmol 79
to 100 g of gypsum. Limestone is then, in all cases given in Table 23, present in the surplus. Tab.23: Bulk density and strength of the gypsum composites, foamed by calcium carbonate and aluminium sulphate Aluminium sulphate Calcium carbonate Bulk density [kg.m-3]
Flexural strength Ry [MPa] Compressive strength Rt [MPa]
1,2 %
2,4 %
3,6 %
1%
781,6
564,6
469,9
2%
806,6
598,0
499,3
3%
809,8
635,7
515,5
1%
1,19
0,6
0,52
2%
1,33
0,57
0,39
3%
1,27
0,57
0,35
1%
2,23
0,95
0,68
2%
2,31
0,79
0,41
3%
2,23
0,82
0,35
80
Fig.17: Bulk density of the gypsum composition, foamed by the aluminium sulphate and calcium carbonate (X-axis aluminium sulphate [%], Y-axis: bulk density [kg.m-3], Z-axis: calcium carbonate [%]).
Fig.18: Bulk density of composition, foamed by the aluminum sulphate and calcium carbonate (count from Fig. 17) 81
Fig.19: Flexural strength of composition, foamed by the aluminum sulphate and calcium carbonate (count from Tab. 23)
Fig.20: Compressive strength of composition, foamed by the aluminum sulphate and calcium carbonate (count from Tab. 23) 82
5.1.3 Sequential optimization of bulk density and strength In the two factor experiments given in the previous chapter in Table 23 there continued experiments with the sequential optimization of foamed gypsum, containing seven variables (viscosity regulator, plasticizer, adhesive admixture, carbonate foam-forming component, acid foam-forming component, defoamer and water-gypsum ratio). With a sequence optimization directed only to reducing bulk density, the favourable (low) values of bulk density are achieved at the expense of strength. It is desirable, however, that any optimization criterion leads rather to a certain compromise based on arriving at adequate strength values for an adequately low bulk density.
700
600
500
400
300
200
100
0 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Fig. 21: Example of the results of the sequential optimization of the bulk density [kg.m-3] carried out by the simplex method
83
2,5
2
1,5
1
0,5
0 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Fig.22: Decrease of the compressive strength [MPa] after the sequential optimization of the bulk density, carried out by the simplex method,
As another tested optimization criterion there was therefore chosen the coefficient of bearing capacity Kn expressing strength in MPa applied to a bulk density of 100 kg.m-3 This modulus is mentioned in the literature as a useful criterion for the formulation of products made from autoclave cellular concrete [57]. During its maximization the drop in bulk density should not be accompanied by too considerable a loss in tensile strength. If we denote the bulk density of the evaluated foam in kg.m-3 ρv and compression strength in MPa as Rc , then: Kn = 100.Rc /ρv . Since a sufficiently strong composite with an, if possible, low bulk density is desirable, value Kn should be as high as possible. With products from autoclave cellular concrete, value Kn is in the range 0,4 – 1,0. With autoclave cellular concrete a good result is considered to be a modulus of carrying capacity to a value 0,6 and higher. In an optimizing experiment there was therefore sought a maximum value Kn in line with the fact that during optimization it is also 84
necessary to respect certain not to be exceeded values of both the key user properties. For the maximization of the coefficient of bearing capacity of the foamed gypsum composition a program Multisimplex was used, which proposes individual optimizing steps on the basis of gradually attained values creating the points of a flexible simplex [51]. The initial simplex, which is planned on the basis of limit values in applying doses to individual components, was created by eight points (the number of optimized components extended by one). The ninth point then presents the first optimizing step. As is obvious from Figure 23, the value of criterion Kn increased by two thirds during the five steps in comparison with a maximum value Kn found in the initial simplex. The value reached in the fifth optimization step (13th experiment in a row) is at the same time, very close to the greatest discovered value (16th point). The optimized compositions (prescriptions 176 -183, 199 - 203, 205 - 207, 209, 211, 212, 214) at the same time show values of bulk density and compressive strength on a level comparable with common autoclave cellular concrete, which can be considered a satisfactory result During the optimization process 4 prescriptions, proposed by the program Multisimplex, had to be excluded for poor workability. Workability however, was not directly a part of the optimization criterion, but especially in the case of extreme values, it cannot be ignored. The program builds in exclusion beyond the criteria (the exclusion of an unsatisfactory prescription is in a selective algorithm of Multisimplex dealt with by a separate command).
85
0,6
0,5
0,4
0,3
0,2
0,1
0 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Fig.23: Results of the sequential optimization of the coefficient Kn carried out by the simplex method (seven-dimensional simplex – first 8 values create initial simplex – the optimization process starts from 9th value)
900,0
800,0
700,0
600,0
500,0
400,0
300,0
200,0
100,0
0,0 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Fig.24: Development of the bulk density [kg.m-3] during the sequential optimization of the coefficient Kn carried out by the simplex method. 86
6
5
4
3
2
1
0 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Fig.25: Development of the compressive strength [MPa] during the sequential optimization of the coefficient Kn carried out by the simplex method.
3,5
3
2,5
2
1,5
1
0,5
0 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Fig.26: Development of the flexural strength [MPa] during the sequential optimization of the coefficient Kn carried out by the simplex method.
87
In the following figure (Figure 27) it is seen that the structure of optimized foamed gypsum is on the whole uniform, but there are distinctly visible sporadic large bubbles, which can have a negative impact on strength and thermal properties.
Fig.27: Comparison of the fracture area of the testing prisms (40x40x160mm), made from pure gypsum (left) and from foamed composite (right)
5.2
Compositions based on gypsum Rigips
Regarding problems resulting from work with gypsum of the lowest strength class, for the next experiments there was acquired FGD gypsum, used for the production of Rigips boards. In a study dedicated to verifying the basic qualities of domestic FGD gypsum it is stated that from this gypsum it is possible to prepare samples of a strength up to 25 MPa [66]. During a strength check test, samples made from especially supplied gypsum showed a compressive strength 12 MPa with a flexural strength 4,7 MPa. 88
5.2.1 Reinforcement with fibres As is stated in Chapter 5.5.1, foamed gypsum reinforced with glass, polypropylene and paper fibres is problematic regarding change in its workability. While gypsum foams themselves, thanks to the content of air bubbles, are easily spread and coated on, the admixture of fibres already applied in small dosages causes deterioration in workability. This deterioration is at the same time more considerable than with materials that are not foamed. The same deterioration in workability qualities was also monitored during experiments with other types of fibrous admixtures. Mineral fibres were shown to be inappropriate for workability, and from the viewpoint of the resulting qualities, of little effectiveness. Wood fibres, although they show a considerable influence on flexural strength (Figure 28), also show other unsuitable properties which disqualify them as well.
Fig.28: Flexural strength of the foamed gypsum composite with wood fibres
89
Fig.29: Structure of the foamed gypsum composite with wood fibres
5.2.2 Foamed composites with a granular mineral admixture For the series of experiments monitoring changes in the bulk density and compressive strength of foamed gypsum after adding a granular aggregate, there was used microsilica, calcium carbonate (in an amount in multiples rising above a substantial quantity of an acid component) and Portland cement CEM I 42,5. As an initial raw material (later modified by the granular admixture) there was chosen a mixture according to prescription 188, providing a very light foam with a bulk density 250 -300 kg.m-3. While microsilica and Portland cement are substances which are supposedly able positively to contribute to the building of a solid phase in the setting gypsum mass, calcium carbonate can be considered as an inert aggregate, in a substantial quantity rising three times above the substantial quantity of the acid component. As is obvious from Figure 30, an addition of microsilica very considerably increases the bulk density of a foamed composite (almost by 50 % with a 15 % admixture of microsilica to the gypsum mass). A microsilica admixture thus considerably limits the development of a porous system in a foamed composition. It is thus also necessary to restrict the monitored increase of strength (Figure 31), for a decrease in porosity. 90
Despite the fact that in the literature [22] there is mentioned the possibility of the creation of hydrated calcium silicates (CSH phase) it cannot be presumed that the increase in strength is to any significant extent caused by the creation of some stronger structure of this type. The decisive influence of the lowered porosity is obvious also from another experiment based on adding calcium carbonate. For reaching the same increase in bulk density, limestone must be added in a bigger amount than microsilica. As shown in Figure 32, a comparable increase in bulk density can be reached only with a 50 % admixture of calcium carbonate to the gypsum mass. Since the added amount rises considerably above a quantity which can theoretically react with an acid foaming component, we can consider the added limestone as an inert aggregate. At the same time with the bulk density there is nevertheless an increase in strength similar to the case of microsilica. Similar results were also gained when using cement (Figures 34 and 35). Even if in this case the results of strength setting had a larger scatter, the total trend corresponds to the previous cases and to the growth of strength the porosity decrease in the system contributes especially. It is obvious that it is possible to use a fine granular admixture as a certain regulator for the degree of foaming. This brings another possibility during the optimization of foamed gypsum composites. Especially noticeable is the fact that in the composites with a fine granular admixture there occurs a minor creation of big bubbles and the structure of the composite on the fracture proves more uniform in general.
91
Fig. 30: Bulk density of the foamed gypsum (composition Nr. 188) with the microsilica
Fig. 30: Compressive strength of the foamed gypsum (composition Nr. 188) with the microsilica
92
Fig. 30: Bulk density of the foamed gypsum (composition Nr. 188) with the calcium carbonate excess
Fig. 30: Compressive strength of the foamed gypsum (composition Nr. 188) with the calcium carbonate excess
93
Fig. 30: Bulk density of the foamed gypsum (composition Nr. 188) with the cement
Fig. 30: Compressive strength of the foamed gypsum (composition Nr. 188) with the cement
94
An interesting alternative formulation in this connection proves to be the use of light granular aggregate, the help of which can in some cases achieve good and steady values of strength and thermal conductivity in a composite based on foamed gypsum (Figure 36). On the basis of these considerations there were carried out experiments focused on the formulation of a foamed gypsum mass with an admixture of expanded perlite. These experiments are presented specifically in Chapter 5.2.6.
Fig. 36: Structure of foamed gypsum without the granular admixture (left) and with the 5 % of perlite.
5.2.3 Formulation of an acid foaming agent While the choice of a carbonate component of the foaming system did not present a special problem and that calcium carbonate (in the form of floated whiting) initially seemed the most advantageous, the choice and formulation of an acid component was the subject of a number of individual experiments. After their evaluation it was possible to state that good results from the viewpoint of the foam production itself were gained with finely pulverized aluminium sulphate and also with citric acid. The indisputable advantage of both substances is the fact that these are completely non-problematic ecologically. Aluminium sulphate is
95
used for potable water treatment and citric acid is a common food additive. Moreover, both substances are used as a setting retarder/accelerator, whereas aluminium sulphate is according to [16] an accelerator of class I and citric acid acts as a retarder of class III. By the appropriate combination of these substances it is then possible to achieve, besides the foaming effect, also a desirable period of workability. This fact is illustrated by Figure 37.
Initial setting time [min]
300 250 200
150 100 50 0 0
10
20
30
40
50
Amount of citric acid in the acid agent [%]
Fig.37: Relation between the initial setting time and the amount of the citric acid in the acid agen
While the originally used aluminium sulphate (the product of purity p.a.) was finely pulverized and could be used as part of a dry, foamable mixture completely without problems, technical aluminium sulphate (used in the water industry) is coarsely crystalline. Grains 3 – 7 mm in size must be ground before adding into a dry mixture, which of course means a certain complication. The necessity for grinding would cause in practice an increase in production costs. An alternative way of using technical aluminium sulphate presents a process during which the aluminium sulphate will dissolve in water. A concentrated water solution of aluminium sulphate (circa 40 %) is
96
even commercially accessible, and thus the dissolving operation could also be cancelled. Because of a possible hydrolysis of aluminium sulphate it is desirable that the concentration of aluminium sulphate in the solution be in tenths of per cents. Regarding sufficient exactness in dosage, it is, however, desirable to work with a more diluted solution. During the dilution to around 15 % the hydrolysis of aluminium sulphate is already distinct and the solution grows turbid. By hydrolysis there are created positively charged polymers of aluminium hydroxide capable of making flat negatively charged colloid and suspended substances contained in the water, whereas the precipitated particles create separable flakes. This phenomenon is used during the use of aluminium sulphate in the water industry. The answer was found in the preparation of a combined solution of aluminium sulphate and citric acid. In an acid environment created by citric acid the hydrolysis of aluminium sulphate is suppressed and the solution remains clear (Figure 38).
Fig.38: Turbidity of the aluminium sulphate solution (right) in comparison to the solution containing citric acid and aluminium sulphate together (left).
97
At the same time the use of citric acid does not introduce any new factor into the formulation, because already the pulverized citric acid has been used as a setting retarder . When considering the composition of such a solution, this is based on the fact that the amount of citric acid (monohydrate) will make half the amount of crystalline aluminium sulphate. This ratio of both components was deduced empirically purely on the basis of workability results. Given that it is reasonably diluted by adding water to the total volume of 1 dm3 there was finally selected a solution prepared from 150 g of citric acid and 300 g of crystalline aluminium sulphate. Such a solution contains about 0,72 mol of citric acid and 0,48 mol of aluminium sulphate. The reality that during this dosage the molar ratio of citric acid and aluminium sulphate appears as 3:2, is more or less accidental and it is not necessary to attach to it any special importance. In addition to the fact that the solution in practice does not grow turbid, the use of a combined solution of citric acid and aluminium sulphate also brings another advantage. By this combined solution it is automatically ensured that when adding the solution into a dry mixture there will always be ensured the same ratio of aluminium sulphate and citric acid. This significantly reduces the sensitivity of the subsequently prepared foaming composition to a possible inaccuracy when applying a dosage of the acid component. The combined solution of aluminium sulphate and citric acid can be therefore considered as a practically advantageous agent for the preparation of gypsum foam materials, which is in the text which follows identified as an acid foaming agent. With the same dosage, dry compositions with ground aluminium sulphate and pulverized citric acid behave practically the same as two-component compositions into which is added an acid foaming agent mixed with batch water.
98
5.2.4 Monitoring of other properties In this chapter there are put together the results of measurements of some qualities of foamed gypsums and gypsum composites in their totality, which were gained in the course of the dissertation work and have not been given in the text so far. Some of these results made a contribution during considerations about the further image of optimizing experiments, while others had the function of bringing to light information important for considerations about the further use of foamed gypsum materials.
Fig. 39: Influence of the moisture on the coefficient Kn
99
Fig.40: : Influence of dispersion on the adhesive strength
Fig. 41: Relation between bulk density and thermal conductivity of foamed gypsum
100
Fig. 42: Relation between moisture and thermal conductivity of foamed gypsum
Fig. 43: Relation between moisture modulus of elasticity of foamed gypsum 101
5.2.5 Sequence optimization of a point criterion It proved in the preceding experiments that it is not appropriate to push to an extreme only one simple physical property (bulk density), but that the optimization of the coefficient of bearing capacity consisting of a combination of two characteristics (bulk density and compressive strength), and which are to certain extent opposites, gives better results. At the same time it proved that between bulk density and strength there exists a really tight connection. Better properties of compositions with an optimized value of the coefficient of bearing capacity are essentially connected with the fact that this criterion has a tendency to keep bulk density on distinctly higher values than is the minimum value. On the basis of gained experience, as meaningful as such a criterion can prove, this will ensure the keeping of bulk density in a timetested zone and at the same time will take into consideration workability properties enabling the application of a material in normal conditions. There was therefore created a combined point criterion, outlined as the sum total of four point-estimated characteristics (consistency, setting, foam constancy and bulk density). This criterion is further designated as criterion A. For optimization with the help of this point criterion there was selected a formulation containing 5 variable components (water, acid foaming agent, calcium carbonate, dispersion and rheologic additive). The first six points in the following Figure 44 present a starting simplex. Optimization itself starts on point 7
102
Tab. 24: Combined point criterion A Consistency 6 points
-
4–5 points
-
2-3 points
-
1-2 points
-
applied excellently, does not run off, holds well to a vertical base applies well, but more compact, or negligible run-off application more difficult ( too compact or mild run-off) application problematic (thin or too solid)
0 points
-
not possible to apply, does not hold to a base
-1 points
-
not possible at all to work with the mixture
4 points
-
initial setting time > 45 minutes
3 points
-
initial setting time > 40 minutes
2 points
-
initial setting time > 30 minutes
1 point
-
initial setting time > 20 minutes
0 points
-
initial setting time < 20 minutes
0 points
-
foam is stable
-1 point
-
foam moderately settles
-2 points
-
foam collapses over time
5 points
-
ρ = 500 – 550 kg/m3
4 points
-
ρ = 450 – 500 kg/m3 or ρ = 600 – 650 kg/m3
3 points
-
ρ = 400 – 450 kg/m3 or ρ = 650 – 700 kg/m3
2 points
-
ρ = 350 – 400 kg/m3 or ρ = 700 – 750 kg/m3
1 point
-
ρ = 300 – 350 kg/m3 or ρ = 750 – 800 kg/m3
0 points
-
ρ < 300 kg/m3 or ρ >800 kg/m3
Setting
Foam stability
Bulk density
103
14
12
10
8
6
4
2
0 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Fig. 44: Sequential optimization of the point criterion A
After evaluating the behaviour of the gradually optimized mixtures, the point criterion was revised. From the criterion there was removed foam stability due to the fact that non-stable foams will be classified as completely unsatisfactory (with the help of the exclusion function). Slightly adjusted also was the point evaluation of remaining properties. The new criterion is further denoted as criterion B. Mixtures acquired with the help of point criterion B show good workability properties and with on the whole a favourable value of bulk density 700 kg/m3 they also have sufficient mechanical and thermal insulation properties. In principle this deals with a practically applicable composition, even when the created foam structure is not always uniform. The presence of large bubbles may disturb the strength of the foam composite (Figure 46).
104
Tab. 25: Combined point criterion B Consistency 7 points
-
5-6 points
-
3-4 points
-
1-2 points
-
0 points
-
applies well, light, does not run off, holds to a vertical base applies well, but more compact, or runs off only negligibly application more difficult ( too compact) or runs off mildly thin, runs off or too solid not possible to apply, (too fluid or too dense) does not hold to a base
Setting 3 points
-
initial setting time > 45 minutes
2 points
-
initial setting time > 30 minutes
1 point
-
initial setting time > 20 minutes
0 points
-
initial setting time < 20 minutes
5 points
-
ρ = 550 – 650 kg/m3
4 points
-
ρ = 500 – 550 kg/m3 or ρ = 650 – 700 kg/m3
3 points
-
ρ = 450 – 500 kg/m3 or ρ = 700 – 750 kg/m3
2 points
-
ρ = 400 – 450 kg/m3 or ρ = 750 – 800 kg/m3
1 point
-
ρ = 350 – 400 kg/m3 or ρ = 800 – 850 kg/m3
0 points
-
ρ < 350 kg/m3 or ρ >850 kg/m3
Bulk density
.
105
14
12
10
8
6
4
2
0 1
2
3
4
5
6
7
8
Fig. 45 : Sequential optimization of the point criterion B
Fig. 46: Structure of the foamed gypsum composite 106
9
5.2.6 Perlite-foam composite In the effort to gain a material with greater stability of qualities there was a move to the development of a perlite-foam composite. During the experiments with granular admixtures it expressly proved that with the help of these admixtures it is possible to regulate the porosity of a system and gain a more uniform porous structure. But at the same time in conjunction with this, there resulted a growth in bulk density and a deterioration in thermal-insulation properties. The reasoning behind the work on a perlite-foam composition was thus the effort to gain a system with a more uniform pore structure with minimum impact on the bulk density of the composition. It was already known that expanded perlite gets on well with a gypsum binder. Perlite-gypsum plaster (from non-foamed gypsum) has been successfully used for fire prevention purposes for a long time. Recently, however, there have appeared certain problems with the production of perlite-gypsum plaster, but this however does not mean that the perlite and gypsum combination itself would be somehow problematic. To gain unambiguously reliable information about the reasons leading to a lowering in the production of perlite-gypsum plasters is not possible for reasons of business secrecy, but it is possible to assume however, that the main reasons were problems with fibres used to increase plaster resistance against cracks with a fire exposure. This problem does not occur in the case of foamed gypsums. Foamed gypsum composites prove very resistant against the creation of cracks and with the use of fibres during their formulation it is not so significant. During the first experiments to formulate perlite-foam composite, there were tested gypsum foams with a considerable admixture of perlite amounting to 30 % of the gypsum mass (gypsum hemihydrate). In this way there were created composites that had very low values of bulk density (200 – 250 kg.m-3), but unfortunately the decrease in bulk density was accompanied by an unacceptable decrease in strength. By lowering the perlite amount by one third (to 20 % of the gypsum mass) bulk density grew to 280 - 300 kg.m-3 (that is approximately
107
by 20 %). Compressive strength, however, was in the best case around 0,5 MPa. In the next experiments therefore, the amount of perlite was further decreased by up to 1 % of the gypsum mass. Even with this dosage the influence of perlite was noticeable, but the bulk density went up to 700 kg.m-3 . After evaluating the data gained, there was selected for the next task a prescription containing perlite amounting to 7 % of the gypsum mass. Regarding the fact that the loose bulking density of perlite is roughly 7 - 9 times smaller than the loose bulking density of gypsum, this involved the mixing of approximately two volume parts of gypsum with one volume part of perlite. The bulk density of the hardened composite in this case reliably stayed on the value 600 kg.m-3and compressive strength held to the value 2 MPa. The values arrived at are in essence attainable also with foamed gypsum itself but it seems however, that perlite-foam compositions are more stable in the sense that they provide results with a smaller variability.
5.2.7 Optimization of material costs From the table given below it is obvious that for a typical perlitefoam composition, the total price of components is around 15 CZK per one kilogram of hardened, naturally dried-off perlite-foam composite. Since such a composite has a bulk density around 600 kg.m-3, approximate material costs for the production of 1 m2 plaster are 9 CZK (at a considered thickness of 1 cm). This is on the whole a favourable result because material costs for the production of 1 m2 of pre-fabricated perlite-foam gypsum would (with the same thickness) be around 11 CZK. From the calculation in the table given above it is obvious that half of the material costs are costs for the polymere dispersion Vinnapas. In such a case it is definitely worth attempting to decrease the content of this polymere dispersion.
108
Tab. 26: Material costs Price of component
Mass of component
Price of component in the mixture
(Kč/kg)
(kg)
(Kč)
Gypsum
6,00
1,000
6,00
Hydratation water
0,05
0,186
0,01
Residual moisture
0,05
0,040
0,00
Perlite
13,40
0,070
0,94
Aluminium sulphate
7,50
0,073
0,55
Citric acid
55,00
0,037
2,02
Calcium carbonate
8,00
0,023
0,19
Wallocell
289,00
0,002
0,58
Vinnapas
141,00
0,084
11,77
Mixture together
14,95
1,4753
22,05
Components of perlite-foam mixture
Therefore there were carried out three further experiments, during which the admixture of polymere dispersion was gradually decreased. Apart from the usual parameters (bulk density and strength), during these experiments there was also monitored the adhesive strength on substrate, which is the most heavily influenced by the dispersion agent admixture. From further given results (Figure 47 – 50) it is obvious, that decreasing the dispersion allowance to 6 % represents a technically tolerable measure.
109
Fig. 47: Influence of the dispersion on the bulk density
Fig. 48: Influence of the dispersion on the compressive strength
110
Fig. 49: Influence of the dispersion on the coefficient of the bearing capacity
Fig. 50: : Influence of the dispersion on the adhesive strength Decreasing the dispersion content to 6 % has at the same time a quite considerable economic effect. With such a decrease material costs go
111
down 13,5 %. For 1 m2 of plaster we can then calculate approximate material costs that amount to 7,80 CZK. Even if it only concerns an approximate calculation, which does not include the costs of the processing necessary for the preparation of the mixture, the fact that it derives from retail prices of the used components, it gives to this calculation a sufficient credibility for the assertion that the prescription so elaborated operates within the boundaries of realistic costs. 5.2.8 Pilot-plant tests The preparation of perlite-foam composite was also verified on a pilot-plant scale. Into a mixer with a forced planetary circulation with volume of 25 litres, there was introduced water containing an acid foaming agent (3800 ml water + 1960 ml acid foaming agent) and subsequently there was poured an additional amount of 8,7 kg dry mixture. With the composite prepared in such a way a part of wall in a testing laboratory was plastered. The material rendered well and it was possible to smooth it with a steel or PVC trowel until to achieve a satisfactory final appearance.
Fig. 51: Plastering with the PVC trowel 112
From such a prepared material there were at the same time taken testing samples, which showed an average compressive strength of 2,3 MPa and a bending strength of 0,6 MPa. With a bulk density of 620 kg/m3 a pilot-plant prepared composite had a thermal conductivity of 0,13 W/m.K. The alternative way of preparation, corresponding to the probable method used in practice, is that of mixing with a spiral mixing adapter in a round plastic vessel. For verifying the perlite-foam composite behaviour while mixing in this way there was used a 4,3 kg dry perlite-gypsum mixture and an appropriate amount of liquid foaming agent diluted with water to 2,9 l. The prepared material was rendered on a slightly wetted testing wall in the same way as in the previous case.
Fig. 52: The preparing of the mixture by the spiral mixing adapter During the drying out of the material no cracks formed, and on the substrate slightly wetted beforehand the rendered layer had a good adhesivity, even over a large area. During plastering of a dry absorptive substrate there can occur prematurely water being sucked from the composite and this brings about a poor adhesivity to the substrate. This effect will manifest itself especially on the edge of the plaster-faced area. The behaviour of the perlite-foam mass is not exceptional in this respect, as 113
common plaster masses on a gypsum or lime cement base also behave in the same way. On non-absorptive (metal) substrates, the adhesivity of a perlitefoam layer is in general good, which gives good prerequisites for the use of perlite-foamed gypsum for functional measures of fire prevention, thus serving to protect steel constructions. Therefore there was carried out an approximate test of perlite-foam composite behaviour when heating by flame. During this test a board of perlite-foam composite 20 mm in thickness was put on a laboratory tripod and heated by Bunsen burner (type Z1 Kavalier a.s.). On the upper surface of the board there was put in the location of heating, a sheet of tissue paper and on it a match. The match took flame in 28 minutes, at the moment when the paper started yellowing faintly. Heating was then finished. On the lower part of the board there arose in the location of firing, star-shaped little cracks (Figure 53), which only in the middle of the fired circular area, went through to the upper part. There did not occur any dropping off from the board mass. During this improvised test the perlite-foam material then proved to be fire-resistant. It stands to reason, however, that the prerequisite for real practical use would be the necessary results of tests of evidence for a fire prevention effect which should have to be carried out in a specialized accredited test room.
Fig. 54: Lower part of the tested perlite-foam plate after firing
114
6
CONCLUSIONS
The performed experiments showed that during optimization of multi-component composites it is useful to combine approximate single factor experiments, by which the constant effect of individual components is detected, with two factor experiments, which will enable a more precise idea of the mutual interaction of two components with a similar or opposite effect. On the basis of the results gained it is then appropriate to propose a sequential experiment, evaluated by a method of the flexible simplex, in which can be monitored the influences of a bigger number of components simultaneously. For the evaluation of individual points of the sequence experiment, which is necessary for decision about any further procedure in a flexible simplex, it is advantageous to compose a complex criterion, into which are suitably projected key optimized properties. Any optimizing criterion, however, must not be mechanically pushed to the extreme, without simultaneously checking whether some of the key parameters do not have a too inappropriate value. The surrounding environment of any sequentially found optimum can be re-examined with the help of an appropriately chosen two factor experiment, which at the same time will show how the result gained is sensitive to changes in the examined factors. If in the composition, there is present a component which considerably influences the total cost, it proves useful to put again into the conclusion of the whole optimization process a one factor optimization, focused on the possibility of lowering the content of this component, with the aim of reaching the most favourable possible material cost. It proved that, if we start from gypsum binder with sufficient strength, it is possible in this way to formulate a foamed plaster gypsum mass with adequate parameters. What proved appropriate for this purpose was a gypsum binder Rigips, with an admixture of polymere dispersion which acts at the same time as a plasticizer and as a substance which improves adhesion. Of the foaming systems tested, the best value proved to be calcium carbonate, reacting with a combined foaming agent which is created by a mixture of aluminium sulphate and citric acid. Both acid
115
components can be a part of a dry composite mixture if they are finely ground. Alternatively, it is possible to use a liquid foaming agent, created by a combined solution of aluminium sulphate and citric acid. For treatment of rheological properties there was used an admixture of methyl hydroxy cellulose and plasticizer based on sulphonated melamine formaldehyde. The influence of both these components, however, is not too significant and the use of SMF plasticizer was finally abandoned. As regards the results, so far there has been given a review to only a limited extent [67-69], because it seemed obvious that in connection with the work there have been gained elements of knowledge which can inspire solutions leading appropriately towards some form of legal patent specification. This presumption in the end proved well founded. [70-71]. The aim set out in this thesis was fulfilled. It was documented that with the use of combined optimization procedures and with the use of appropriate additives it is possible to prepare composites based on foamed gypsum with interesting properties for users. Beyond the scope of the original assignment there was processed an economical form of liquid acid foaming agent and there was created a new type of hybrid-lightened perlite-foamed composite. Optimized foamed gypsum and perlite-foamed composites have good prerequisites for practical use as fire preventive, maintenance or thermal-insulation renders. Their further development will depend on the results of more extensive practical tests which it will be necessary to implement in collaboration with a possible producer. A suitable platform for such tests could be an appropriately focused project of applied research.
116
7
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[69] Vimmrová, A. - Svoboda, L.: Sekvenční optimalizace při formulaci sádrové pěny. In: Construmat 2006 [CD-ROM]. Nitra: Slovenská poľnohospodárska universita v Nitre, 2006, s.20-24 [70] Vimmrová, A. - Svoboda, L.: Hybridně lehčená sádrová hmota. Přihláška PUV 2007-19028 [71] Vimmrová, A. - Svoboda, L.: Kyselé činidlo pro přípravu pěnové sádrové hmoty. Přihláška PUV 200719040…………………………. ……….
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