Preparation and properties of geopolymer-lightweight aggregate ...

J. Cent. South Univ. Technol. (2009) 16: 0914−0918 DOI: 10.1007/s11771−009−0152−x

Preparation and properties of geopolymer-lightweight aggregate refractory concrete HU Shu-guang (胡曙光)1, WU Jing (吴静)1, YANG Wen (杨文)2, HE Yong-jia (何永佳)1, WANG Fa-zhou (王发洲)1, DING Qing-jun (丁庆军)1 (1. Key Laboratory for Silicate Materials Science and Engineering, Ministry of Education, Wuhan University of Technology, Wuhan 430070, China; 2. China Construction Ready Mixed Concrete Co. Ltd., Wuhan 430074, China) Abstract: Geopolymer-lightweight aggregate refractory concrete (GLARC) was prepared with geopolymer and lightweight aggregate. The mechanical property and heat-resistance (950 ℃) of GLARC were investigated. The effects of size of aggregate and mass ratio of geopolymer to aggregate on mechanical and thermal properties were also studied. The results show that the highest compressive strength of the heated refractory concrete is 43.3 MPa, and the strength loss is only 42%. The mechanical property and heat-resistance are influenced by the thickness of geopolymer covered with aggregate, which can be expressed as the quantity of geopolymer on per surface area of aggregate. In order to show the relationship between the thickness of geopolymer covered with aggregate and the thermal property of concrete, equal thickness model is presented, which provides a reference for the mix design of GLARC. For the haydite sand with size of 1.18−4.75 mm, the best amount of geopolymer per surface area of aggregate should be in the range of 0.300−0.500 mg/mm2. Key words: refractory concrete; geopolymer; lightweight aggregate; thermal property; equal thickness model

1 Introduction Refractory concrete is suitable for using at high temperature (≥200 ℃) and is composed of refractory cementing material, heat-resistant aggregate and/or fillers, which can maintain the necessary physical and mechanical properties at high temperature for long term[1−2]. Refractory concretes, according to different cementing materials, can be divided into Portland refractory concrete, aluminate refractory concrete, phosphate refractory concrete, sulphate refractory concrete, bauxite refractory concrete, chloride refractory concrete, sols refractory concrete and organic refractory concrete[3]. Heat-resistant aggregate can be divided into broken fire-resistant clay brick, clay, clinker, broken high-alumina brick, natural light aggregate (pumice and tuff), industrial wastes (slag, lytag and gangue), and artificial light aggregate (shale haydite, clay haydite and expand perlite)[4−5]. Refractory concrete can be used for the building engineering with fire incipient fault or in high-temperature work environment. With the development of new building structure and new technology, much better properties of concrete are demanded. Some concretes of the building structure

should be of high-strength and heat-resistant characteristics. Many experimental researches on the mechanical and thermal behavior of concrete at constant high temperatures have been reported[6−9]. NEVILLE[10] pointed out that at temperatures approximately above 430 ℃, concretes with siliceous aggregate show a significant strength loss compared to those with lightweight aggregate. At 600 ℃, concrete can lose half of its strength. Above 800 ℃, loss of the bound water in the hydrates may cause a strength loss of 80%, which may lead to the failure of a structure. In Ref.[11], fire resistance of lightweight concretes having a unit weight of 500−1 600 kg/m3 was investigated. It was found that an increase in unit weight resulted in a reduction in the fire resistance of the concretes. TURKER et al[4] investigated the micro-structure and strength of concretes exposed to fire. In their studies, mortars containing ordinary Portland cement and three types of aggregates were respectively subjected to 100, 250, 500, 700 and 850 ℃ for 4 h. Unlike the mortars with quartz and limestone, at high temperatures, crack was observed in the aggregate for the mortars with pumice instead of crack propagation at the interface. Therefore, it was concluded that the interface was strong when pumice was used[12].

Foundation item: Project(2009CB623201) supported by the National Basic Research Program of China; Project(G0510) supported by the Key Laboratory for Refractories and High-temperature Ceramics of Hubei Province, China Received date: 2009−01−06; Accepted date: 2009−06−09 Corresponding author: WU Jing, PhD; Tel: +86−13476233919; E-mail: [email protected]

J. Cent. South Univ. Technol. (2009) 16: 0914−0918

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According to KONG et al[12], ABELES and BARDHAN-ROY[13], concretes containing lightweight aggregate preserve their strength up to nearly 500 ℃. It was stated that the residual strength of LWC after fire decreases linearly from 100% to 40% as a result of increasing the temperature from 500 to 800 ℃. Inorganic alkali aluminosilicate polymers, commonly referred to as geopolymer, are aluminosilicate materials that exhibit good physical and chemical properties and a wide range of potential applications, including precast structures and non-structural elements, concrete pavements and products, containment and immobilisation of toxic, hazardous and radioactive wastes, advanced structural tooling and refractory ceramics, and fire-resistant composites used in buildings, aeroplanes, shipbuilding, racing cars, and the nuclear power industry[14−18]. Because of its characteristics of lightweight, low thermal conductivity and heat-resistance, lightweight aggregate was widely used in the preparation of refractory concrete[19−20]. Geopolymer and haydite have good heat-resistance, so they could be compounded to form a new kind of refractory concrete that should have excellent mechanical and fire-resistance properties. However, so far there have been few reports. In this work, the geopolymer-lightweight aggregate refractory concrete (GLARC) was prepared using geopolymer as binding matrix and haydite as aggregate, and its mechanical performance and resistance to elevated temperature were investigated. The effects of aggregate size and the ratio of geopolymer to aggregate on the performance of the concrete were also discussed. In order to show the relationship between the thickness of geopolymer covered with aggregate and the thermal property of concrete, an equal thickness model was proposed, which could provide a reference for the mix design of GLARC.

2 Experimental 2.1 Raw materials The aluminosilicate mineral used was metakaolin (MK) clay, which was produced from the decomposition of kaolinite clay burned at 700 ℃ for 6 h. The surface area of the MK was 22−24 m2/g. Table 1 lists the chemical composition of MK. The alkali-activator used in this work was a mixture of water glass (soluble sodium silicate), sodium hydroxide (NaOH) and water. The modulus (m(SiO2)/m(Na2O)) of water glass was 3.19 and Baume degree was 39.2. Water glass was adjusted to the required m(SiO2)/m(Na2O) by adding 98% pure NaOH pellets and Baume degree by adding distill water. Table 1 Chemical composition of MK(mass fraction, %) SiO2 63.06

Al2O3 28.93

Fe2O3 2.29

CaO 0.37

MgO 0.68

TiO2 0.56

Loss 0.135

Lightweight aggregate: the shale haydite was obtained from the Guangda Haydite Development Ltd, Hubei province, China. Table 2 lists its main performance. The shale haydite was broken by the jaw crusher, and the crushed aggregate was sieved into two grain size groups of 1.18−2.36 mm and 2.36−4.75 mm, denoted them as S and L respectively. S and L with equal quantity were mixed to obtain sand M with size of 1.18−4.75 mm. Table 2 Parameters of shale haydite Grain Stacking Apparent Water-absorbing Cylinder-mould capacity/% size/ density/ density/ strength/MPa mm (kg·m−3) (kg·m−3） 0.5 h 1.0 h 24 h 5−15

750

1 487

2.6

4.5

5.4

8.8

2.2 Experimental methods NaOH, sodium silicate solution, and water were mixed in a beaker to form the alkali-activated solution (water-glass) with mole ratio of SiO2 to Na2O being 1.2 and the mass fraction of 40%, and cooled down to room temperature. The alkali activator and MK were combined to produce a mixture of molasses-like consistency at a mass ratio of liquid to powder being 0.7 and then beaten up with the haydite sand and dried at 105 ℃ for 24 h according to the ratios given in Table 3. The admixture was transferred into 51 mm×51 mm×51 mm steel moulds at 5 MPa. After 1 d, the specimens were demolded and sealed by a plastic bag to prevent water evaporation, and then cured at a temperature of (20±1) ℃ for 28 d. Compressive strength was tested by a hydraulic material testing system (MTS) according to ASTM C109 — 80. The cross-head speed was 1.27 mm/min. 2.3 Measurement of heat-resistance property The specimens were dried at 65 ℃ for 24 h in a vacuum drying oven, and then heated to 950 ℃ at a speed less than 150 ℃/h, holding for 30 min. The power supply was switched off and the specimens were naturally cooled to room temperature in the oven. By comparing the compressive strength of unheated and heated specimens, the strength loss rate was calculated ( P − P2 ) according to: λ= 1 ×100%[21], where P1 and P2 P1 were initial and residual compressive strengths respectively. From the data obtained, thermal property and its influencing factors of GLARC were investigated.

3 Results and discussion As indicated in Table 3, the initial compressive strength of pure geopolymer is the highest. However, that of GLARC decreases by the addition of haydite sand. This can be contributed to the lower strength of haydite

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Table 3 Mix proportion and test results of specimens Speciment No. MK L1 L2 L3 M1 M2 M3 S1 S2 S3

Grain size of haydite sand/mm − 2.36−4.75 2.36−4.75 2.36−4.75 1.18−4.75 1.18−4.75 1.18−4.75 1.18−2.36 1.18−2.36 1.18−2.36

Mass ratio of gel to haydite sand − 1.0‫׃‬1.5 1.0‫׃‬2.0 1.0‫׃‬2.5 1.0‫׃‬1.5 1.0‫׃‬2.0 1.0‫׃‬2.5 1.0‫׃‬1.5 1.0‫׃‬2.0 1.0‫׃‬2.5

Initial compressive strength/MPa 90.1 79.4 73.6 67.9 82.6 75.3 27.4* 85.4 8.7* △

Residual compressive strength/MPa 20.5 29.4 34.7 38.9 33.8 43.3 − 40.1 − −

Strength loss rate/% 77 63 53 43 59 42 − 53 − −

Note: “*” denotes that moulding is difficulty; “△” denotes that moulding is impossible.

sand and the weaker interface transition zone between gel and aggregate. After being exposed to 950 ℃, the residual strength of pure geopolymer is the lowest, only 20.5 MPa, and the strength loss rate is as high as 77%. On the one hand, the mechanical syneresis of geopolymer occurs when the geopolymer is heated, resulting in the decrease of its strength; on the other hand, the moisture and bound water in geopolymer transform into vapor when they are heated, and the vapor pressure destroys the matrix. The residual strength of specimen M2 is the highest. Specimens L3, M2 and S1 have better high-temperature mechanical property. Specimens M3, S2 and S3 cannot sustain any load at all due to the severe deterioration when they are heated, because less quantity of gel cannot strongly bond the aggregate together. Thus, the grain size of haydite sand and the ratio of gel to haydite sand affect the mechanical and thermal properties of GLARC (see Figs.1 and 2). 3.1 Effect of mass ratio of gel to sand on compressive strength and heat-resistance property It can be seen from Fig. 1 that, when the size of sand is kept constant, the original and residual strengths of GLARC show different development trends: the

Fig.1 Compressive strength and strength loss rate of concrete with different mass ratios of gel to sand(size of sand is 2.36−4.75 mm)

Fig.2 Compressive strength and strength loss rate of concrete with different sizes of sand at mass ratio of gel to sand being 1.0‫׃‬1.5

strength of GLARC decreases with the increase of the aggregate content due to the intrinsic low strength of haydite sand. But after being heated at 950 ℃, the residual strength of GLARC increases by reducing the mass ratio of gel to sand because the moisture and bound water in geopolymer transform into vapor when the specimen is heated, and the vapor tries to release[21]. However, there are no channels in the denser geopolymer paste, water vapor cannot be released and finally the system is destroyed when the vapor pressure reaches the limit. This phenomenon is called as “vapor effect”. The haydite sand is a porous material that can form a lot of holes in the matrix, thus providing a room for the release of vapor pressure. The number of the holes and channels increase with the content of haydite sand. 3.2 Effect of grain size on compression strength and thermal property It can be seen from Fig.2 that, when the mass ratio of gel to sand is 1.0‫׃‬1.5, the initial strength and residual strengths of GLARC increase with the decrease of sand size, and the strength loss rate decreases gradually. When the mass ratio of gel to sand is kept constant, the specific surface area increases with the decrease of the sand size, so the thickness of geopolymer per unit surface of


aggregate is thinner, and the vapor can be released more easily. But if the amount of geopolymer is small enough, it cannot bond the aggregate together firmly. As a matter of fact, besides the damage from the “vapor effect”, the difference in coefficient of thermal expansion between the geopolymer and shale haydite results in the crack in the interface zone, so that the strength declines significantly. This concept is known as the “thermal inconsistency of the ingredients”[22−23]. 3.3 Equal thickness model In fact, if the mass ratio of gel to sand is too low, or the grain size of haydite sand is too small, there will not be enough gel to fill the gap among aggregates. Consequently, the macroporous three-dimensional structure of GLARC forms, as shown in Figs.3 and 4. Just because of the unique porous structure, the GLARC is of good fire resistant property. When it is heated, the bound water in geopolymer is converted into vapor, which is released into the pores of aggregate and structure, thereby the damage to GLARC is mitigated. This is the reason why the refractory property of porous structure GLARC is better than that of the denser system. The mass of geopolymer covering the surface of the

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aggregate can be expressed as the thickness of paste.Changing the aggregate size or the mass ratio of gel to sand is to change the thickness of geopolymer that covers the surface of aggregate. In order to study the effect of the thickness of geopolymer on the heat-resistance property of GLARC, the concept of equal thickness model is presented, that is, the mortar that wrapps each sand is of the same thickness, or the mortar per surface area of aggregate is of the same quantity. And the samples with the same mortar thickness on the surface of aggregate are assumed to possess similar property. The quantity of geopolymer can be expressed as the quantity of metakaolin. In order to facilitate the calculation, it is feasible to express the geopolymer thickness wrapped aggregate by the metakaolin quantity on per surface area of aggregate. Equal thickness model is a kind of imaginary condition, so the following two assumptions are put forward. (1) The aggregate is in the form of balls with the same size, and the average diameter is D. (2) Sand is located in the 40 mm×40 mm×40 mm cube space in accordance with the simple cubic accumulation, that is, a D-diameter ball is put in a D-D cube. So the mass of geopolymer wrapped per surface of haydite sand can be calculated as follows: h=

Fig.3 Porous structure of GLARC

Fig.4 Illustration of test sample with porous structure: (a) Actual picture; (b) Detail view

x x x = = = S Ns 40 × 40 × 40 ⋅ πD 2 V x xD = 6 4000 6 4000π ⋅ πD 2 D3

(1)

where D is the average grain size of aggregate, mm; s is the average surface area of aggregate, mm2; V is the average volume of aggregate, mm3; N is the number of aggregate in every cube (40 mm×40 mm×40 mm); S is the total surface area of all aggregate in every cube; x is the metakaolin consumption in every cube, g; and h is the quantity of geopolymer that covers per unit surface of shale haydite sand, mg/mm2. The quantity of every material is noted accurately during testing, so the value of x is known, and h can be calculated by Eq.(1). The results are given in Table 4. It can be seen from Table 4 that, h values of specimens L3, M2 and S1 are similar, which are equal to 0.356, 0.334 and 0.351 mg/mm2 respectively. Obviously, the thicknesses of geopolymer that wraps aggregate of these three samples are approximately equal, and the compressive strength and refractory properties are better than those of other samples. But when h＜0.270 mg/mm2, for specimens M3, S2 and S3, compressive strengths are very low, even losing the sustaining capacity after being exposed to 950 ℃. When h>0.450 mg/mm2, for specimens L2 and L1, their initial compressive strengths

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are higher, but the residual strengths are lower and strength loss rates are higher. So, h has an optimal bound. When it is in the range of 0.300−0.500 mg/mm2, the relevant samples possess better refractory property. Table 4 Quantity of geopolymer shale haydite sand Minimum Maximum Specimen grain grain No. size/mm size/mm L1 2.36 4.75 L2 2.36 4.75 L3 2.36 4.75 M1 1.18 4.75 M2 1.18 4.75 M3 1.18 4.75 S1 1.18 2.36 S2 1.18 2.36 S3 1.18 2.36 Note: “*” denotes moulding moulding is impossible.

wrapped per unit surface of

the optimal quantity of geopolymer on per surface area of aggregate should be in the range of 0.300−0.500 mg/mm2.

References [1]

D/mm

x/g

h/ (mg·mm−2)

3.555 39.5 0.699 3.555 31.8 0.563 3.555 20.1 0.356 2.663* 32.7 0.433 2.663* 25.2 0.334 2.663* 19.3 0.256 1.770 40.0 0.351 1.770 30.1 0.265 1.770 21.0 0.185 is difficulty; “△ ” denotes

[2]

[3]

[4]

[5]

[6]

[7]

4 Mix design of GLARC Compared with ordinary concrete, there is no recognized mix design method of refractory concrete due to the uncertainty of raw materials and service environment. For the GLARC, the mix design can be based on the equal thickness model, according to the value of h in Table 4 and the experience accumulated in the course of experiment. For haydite sand with size of 1.18−4.75 mm, the optimal quantity of geopolymer on per surface area of aggregate should be in the range of 0.300−0.500 mg/mm2, which can be used as a referenced index of mix design of GLARC. Of course, the optimized mix design method for GLARC needs to be more carefully studied in the future experiments.

[8]

[9]

[10] [11] [12] [13] [14] [15]

5 Conclusions (1) Geopolymer-lightweight aggregate refractory concrete (GLARC) prepared with geopolymer and lightweight aggregate possesses excellent heat-resistance property, and the grain size of haydite sand and the mass ratio of gel to sand both have an effect on the mechanical and thermal properties of this composite material. (2) The thickness of geopolymer that covers the surface of aggregate is the main factor influencing the heat-resistance property of GLARC. If it is too thick, the vapor will destroy the structure when it is released; and if it is too thin, the gel cannot bond the aggregate together firmly. In fact, changing the aggregate size and the mass ratio of gel to sand is to change the thickness of paste. (3) Equal thickness model presented can be used to investigate the relationship between the thickness of geopolymer and the heat-resistance property of GLARC effectively, and provide a reference for the mix design of GLARC. For haydite sand with size of 1.18−4.75 mm,

[16]

[17]

[18]

[19]

[20] [21] [22] [23]

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