Anhydrite, blast-furnace slag and silica fume composites

Cite this article

Research Article

Guerra-Cossío MA, González-López JR, Magallanes-Rivera RX, Zaldívar-Cadena AA and Figueroa-Torres MZ (2018) Anhydrite, blast-furnace slag and silica fume composites: properties and reaction products. Advances in Cement Research, https://doi.org/10.1680/jadcr.17.00216

Paper 1700216 Received 29/11/2017; Revised 21/03/2018; Accepted 21/03/2018

Keywords: blast furnace slag/ blended cements/calcium sulfate

ICE Publishing: All rights reserved

Advances in Cement Research

Anhydrite, blast-furnace slag and silica fume composites: properties and reaction products Miguel A. Guerra-Cossío

Antonio A. Zaldívar-Cadena

Cuerpo Académico de Materiales Alternativos en Ingeniería, Facultad de Ingeniería Civil, Universidad Autónoma de Nuevo León, Nuevo León, Mexico (Orcid:0000-0003-0336-5112)

Cuerpo Académico de Materiales Alternativos en Ingeniería, Facultad de Ingeniería Civil, Universidad Autónoma de Nuevo León, Nuevo León, Mexico

Javier R. González-López

Mayra Z. Figueroa-Torres

Cuerpo Académico de Materiales Alternativos en Ingeniería, Facultad de Ingeniería Civil, Universidad Autónoma de Nuevo León, Nuevo León, Mexico (corresponding author: [email protected]) (Orcid:0000-0003-0887-1292)

Cuerpo Académico de Materiales Alternativos en Ingeniería, Facultad de Ingeniería Civil, Universidad Autónoma de Nuevo León, Nuevo León, Mexico (Orcid:0000-0002-6823-7384)

Ricardo X. Magallanes-Rivera Facultad de Ingeniería, Universidad Autónoma de Coahuila, Coahuila, México

Mixtures of a composite binder based on anhydrite synthesised from waste plaster from the ceramics industry complemented with granulated blast-furnace slag (BFS) and silica fume (SF) were studied. Small additions of potassium sulfate, calcium hydroxide and Portland cement clinker were used as chemical activators. The addition of BFS and SF to the systems was found to improve the properties and hinder the dissolution of gypsum in wet environments. The presence of ettringite was visually verified by scanning electron microscopy. The samples did not show considerable expansion or loss of mechanical properties for up to 56 d. The strength obtained by the compounds after 56 d was 20 MPa, far exceeding the typical compressive strength of gypsum plaster (about 4 MPa). The main hydrated products were gypsum, ettringite and calcium silicate hydrate type products.

Introduction The construction industry, like many others, is now focusing on sustainability objectives by reducing the environmental impacts of the production and use of construction materials. The ecological/economic needs to seek alternative cementitious materials to Portland cement are due to two main reasons (Barcelo et al., 2014): the high levels of carbon dioxide produced in the limestone decarbonisation process and the high amount of energy needed to reach the 1450°C required by clinkerisation, which is mainly achieved by the incineration of fossil fuels. These factors make the construction industry one of the most significant sources of anthropogenic carbon dioxide production due to the manufacture of its main material. In 2016, more than four billion tonnes of Portland cement were used globally (USGS, 2017) and it is estimated that carbon dioxide emissions from the cement industry in recent years represent around 5% of total anthropogenic carbon dioxide (IEA, 2009). One of the most viable options to this problem is the reuse of industrial wastes (Gartner and Hirao, 2015), either as fillers in cement matrices or in the manufacture of composite cements. The term composite cement refers to cements that contain a mineral addition in combination with Portland cement and a variety of by-products from other industries are currently accepted and used as mineral additions (Kosmatka et al., 2004). Composite cements usually contain only one component in addition to Portland cement (Alahrache et al., 2016; Jalal et al., 2015; Sua-iam and Makul, 2015); those containing two or more components are called multi-component cements

(Escalante-Garcia et al., 2017; Jeong et al., 2015; Singh Gill and Siddique, 2017). Multi-component cements can also be produced by combining two or more cementitious materials without the addition of Portland cement to the system (Magallanes-Rivera and Escalante-Garcia, 2014a; Robayo-Salazar et al., 2017). These can be produced by grinding all of the constituents together or by adding the various components during mixing (Odler, 2000). The use of alternative construction materials with a calcium sulfate matrix is a proposal for the reuse of different wastes, such as wastes from the ceramics industry and foundries and residues from the manufacture of raw materials. The use of gypsum as a cementitious material requires a temperature of around 150°C to produce its main cementitious product, plaster of Paris. This results in an energy consumption of 24–33% of that needed for clinker manufacture (Gartner, 2004) and also results in reduced carbon dioxide release as decarbonisation of the raw material is not involved. However, the main disadvantage of gypsum is its high solubility in the presence of water (approximately 2 g/l), which restricts its use to indoor environments with very little moisture. In addition, its mechanical strength is very low compared to that of Portland cement. Investigations have thus focused on the improvement of parameters such as the mechanical strength, dimensional stability and workability of composites cured under different conditions, the use of different sources of calcium sulfate and the production of binary and ternary cements using additions of supplementary cementitious materials such as fly ash, slags, silica fume (SF), metakaolin (Camarini and De Milito, 2011; Escalante-Garcia et al., 1


Anhydrite, blast-furnace slag and silica fume composites: properties and reaction products Guerra-Cossío, González-López, Magallanes-Rivera, Zaldívar-Cadena and Figueroa-Torres

Offprint provided courtesy of www.icevirtuallibrary.com Author copy for personal use, not for distribution 2009a; Singh and Garg, 1995a, 1995b). These studies have shown the feasibility of improving the behaviour of the resulting materials. Although these types of cement composites could not replace Portland cement in many applications, there are many cases where it would be possible to use them as an important alternative, depending on the development of future research. For example, Singh and Garg (1992) produced mixtures of phosphogypsum with Portland cement and a pozzolan (either fly ash or blast furnace slag (BFS)). The reported compressive strength was higher in blends with BFS (35 MPa) than in blends with fly ash (22 MPa) after 28 d of curing. This material was proposed as a raw material for the manufacture of brickwork and as masonry mortar. Improvements in the physical properties of gypsum, such as higher mechanical strength and hindering of its dissolution, are due to the formation of hydrated calcium silicate (C–S–H) phases, which engulf the gypsum crystals, keeping them away from moisture. There are two ways of using calcium sulfate as a cementitious material, depending on the degree of hydration. The first is the use of the hemihydrated form of calcium sulfate (CaSO4.½H2O) (referred to as HH in this paper), which is the commercial form of utilisation of calcium sulfate; its reaction is accompanied by an intense release of heat, fast setting and low mechanical strength development (Odler, 2000). On the other hand, unlike HH, the reaction of calcium sulfate in its anhydrous form (AH) is very slow if not nil, so it requires chemical activators to function as heterogeneous nucleation sites to form crystals of the dihydrated form of calcium sulfate (CaSO4.2H2O) or gypsum (referred to as DH in this paper) (Singh and Garg, 1995a), developing mechanical properties superior than those of HH. The aim of the work reported in this paper was to improve the mechanical properties of waste calcium sulfate by adding BFS and SF for the manufacture of hydraulic mortars. To do this, use of an anhydrous calcium sulfate phase activated by chemical agents was investigated.

Experimental details Materials Waste gypsum from spent jiggering moulds from a local sanitary ceramics factory was used as the source of calcium sulfate. It was conditioned by pulverising and calcining at 500°C for 6 h to obtain anhydrite (AH). Analytical-grade potassium sulfate (K2SO4) and calcium hydroxide (Ca(OH)2) were used as activators for the AH reaction. Granulated BFS was obtained as a by-product from steel manufacturing, while the SF used was a non-densified commercial type. A very small amount of Portland cement clinker was used in order to act as an alkaline activator for the BFS (Daimon, 1980; Ding et al., 2014; Gruskovnjak et al., 2008). Silica sand according to the relevant ASTM standard (ASTM, 2011a) was used as fine 2

aggregate. Table 1 shows the physical properties and chemical composition (main oxides) of the raw materials as obtained by X-ray fluorescence. Preparation of mortars A series of mixtures was obtained by varying the contents of the constituent cementitious materials (AH, BFS and SF), maintaining two SF/BFS ratios (0·25 and 0·1765) and maximising the AH content between 50% and 70% by weight (Table 2). Similar SF/BFS ratios have been tested in other investigations, where it was found that the presence of SF hindered the formation of ettringite. However, SF can also interfere in the formation of gypsum, affecting the development of mechanical properties at early ages. The activating additives were added in proportions of 1–4% of potassium sulfate and calcium hydroxide and up to 3% of clinker. The activators provided suitable pH conditions for the formation of reaction phases at an early age. The water/cement ratio was set at 0·45 for all the mixtures according to the observed workability, and a proportion of sand:cementitious material of 2·75:1·00 was maintained for all the systems, as suggested by the relevant ASTM standard (ASTM, 2010a). A Hobart planetary motion mixer with an approximate capacity of 5 l was used to mix the mortars. In accordance with the mixing method of the ASTM standard (ASTM, 2010b), all of the cementitious materials and their activators were added to the mixing water and then the sand was added slowly and steadily over a period of 30 s. The total mixing time, in accordance with the ASTM standard, was 4 min. As shown in Table 2, the systems were named Table 1. Chemical composition and physical properties of raw materials

Sodium oxide (Na2O): wt% Magnesium oxide (MgO): wt% Aluminium oxide (Al2O3): wt% Silicon dioxide (SiO2): wt% Sulfur trioxide (SO3): wt% Potassium oxide (K2O): wt% Calcium oxide (CaO): wt% Ferric oxide (Fe2O3): wt% Blaine fineness: cm2/g Density: g/cm3

AH

BFS

SF

Clinker

0·1 0·1 0·3 0·7 57·0 — 39·7 0·1 5629 2·84

0·3 9·5 9·6 31·7 3·0 0·5 42·9 0·5 3514 2·75

— 0·2 — 96·7 — 1·0 1·2 0·2 6851 2·18

— 0·8 2·5 10·4 1·8 1·1 77·8 4·1 1399 3·03

Table 2. Mixture formulations Mix ID 50-40-10 50-42·5-7·5 60-32-8 60-34-6 70-24-6 70-25·5-4·5 100

AH: wt%

BFS: wt%

SF: wt%

50 50 60 60 70 70 100

40 42·5 32 34 24 25·5 0

10 7·5 8 6 6 4·5 0



Offprint provided courtesy of www.icevirtuallibrary.com Author copy for personal use, not for distribution according to the percentage content by weight of the main constituents of the mixtures (AH, BFS and SF). Characterisation The compressive strength of 50 mm cubic samples (ASTM, 2010a) and the dimensional stability (according to length change) of prismatic specimens (ASTM, 2011b) were recorded from 1 to 56 d. In terms of curing, all the specimens were fully immersed in water at 23°C until mechanical testing was carried out. Compression tests were performed in a hydraulic press with a capacity of 60 T (Instron 600DX) using a load rate of 500 N/s. Some pieces of the crushed cubes were dried and immersed in methanol for 72 h to stop the hydration and then dried for an additional 24 h at 40°C to remove the methanol. Micrographs of both fractured and polished samples were taken on a Jeol JSM-6510LV scanning electron microscope with a tungsten filament. Both types of samples were gold coated and observed under secondary electron mode. Using the polished samples, X-ray energy dispersive spectroscopy (EDS) analysis was performed by means of an Oxford spectrometer. In addition, some fragments were ground by hand and then subjected to X-ray diffraction (XRD) analysis using a PANalytical diffractometer (model Empyrean, CuKα radiation, 40 kV). These analyses were conducted from 2θ = 6° to 72° with a step size of 0·026°.

Results and discussion Physical properties Figure 1 shows the compressive strengths of the mortars as the average of three samples per test date (the vertical bars represent standard deviations). All of the systems had compressive strengths below 5 MPa at 1 d. However, at 7 d, most of the systems exceeded a strength of 12 MPa, except for the pure

Compressive strength: MPa

20

15 50-40-10 50-42·5-7·5 60-32-8 60-34-6 70-24-6 70-25·5-4·5 100-0-0

10

5

0 1

7

28 Curing time: d

Figure 1. Compressive strength development of AH–BFS–SF mortars cured under water at 23°C

56

AH system (mix 100-0-0) and mix 70-24-6, which was the system with the least amount of BFS. At early ages, the phase formed was mainly gypsum from AH, along with the possible formation of hydrated calcium silicate, activated by the addition of alkaline compounds. At 28 d, most of the systems had strengths above 17 MPa; the exceptions were mixes 100-00 and 50-42·5-7·5, which both showed a loss of compressive strength of approximately 8%. Between 28 and 56 d, the strengths tended to stabilise. Two systems stood out, with strengths close to reaching 20 MPa: mix 50-40-10 (18·97 MPa) and mix 70-25·5-4·5 (19·21 MPa). These strengths far exceed the typical compressive strength of gypsum plaster of about 4 MPa (Odler, 2000) and the results agree with other reports of similar mixes containing any reactive phase of calcium sulfate with BFS (Escalante-Garcia et al., 2009a; Magallanes-Rivera and Escalante-Garcia, 2014b; Martinez-Aguilar et al., 2010). The strength results place this material as a viable alternative for the manufacture of masonry elements or elements with low mechanical requirements since, at 28 d, they exceeded the minimum compressive strength required by ACI 318 (17 MPa) to be considered as a structural material (ACI, 2014). The solubility of gypsum in wet environments is evident from the behaviour of mix 100-0-0 in Figure 1: the strength of this system did not exceed 6 MPa throughout the considered period and, after 7 d, its strength decreased to reach a value of zero at 56 d, unlike the positive strength development of the other systems containing BFS and SF. This provides evidence that the composites formed by the reaction of BFS and SF within the cementitious systems complemented the gypsum effectively in terms of strength gain and dissolution resistance of products in wet environments: the composites engulf the gypsum crystals, giving them impermeability (Bentur et al., 1994; Escalante-Garcia et al., 2009a; Magallanes-Rivera and Escalante-Garcia, 2014a). The formation of ettringite (3CaO.Al2O3.3CaSO4.32H2O) in these kinds of systems is possible, due to the sulfate content and the presence of reactive aluminium phases provided by the BFS (the reactive aluminium phases provided by the clinker can be considered negligible because the quantity of clinker used was very low (less than 3%)). When ettringite is formed in a hardened matrix, the heterogeneous expansions associated with its formation can cause cracking and peeling. For this reason, it is important to record the evolution of length changes in these types of composites. The results of the monitoring according to the relevant ASTM standard (ASTM, 2011b) are shown in Figure 2. The presence of SF in these types of system inhibits the formation of ettringite, as has been established in different investigations comparing composites of HH–Portland cement with and without the addition of SF (Bentur et al., 1994) and HH– BFS systems with the addition of SF or fly ash (MagallanesRivera et al., 2012). In the HH–Portland cement mixtures, 3



Offprint provided courtesy of www.icevirtuallibrary.com Author copy for personal use, not for distribution 0·030

50-40-10 50-42·5-7·5 60-32-8 60-34-6

Length change: %

0·025

70-24-6 70-25·4-4·5 100-0-0

0·020 0·015 0·010 0·005 0 0

7

14

21

28

56

Curing time: d

Figure 2. Expansion of mortar bars cured under water at 23°C

those that did not contain SF showed a diminishing of mechanical properties to zero after 200 d of curing in water whereas those that contained SF continued to show an increase in compressive strength. The mix that showed the best mechanical behaviour was reported to be 75% HH, 20% Portland cement and 5% SF, reaching a value of 17 MPa after 200 d of curing. For the HH–BFS systems, those that did not contain a pozzolan addition showed a length change of more than 0·15% after 180 d of curing in water; the length changes were 0·09% for the system containing fly ash and 0·04% for that containing SF under similar conditions. The importance of the use of SF and its ability to decrease the loss in properties due to ettringite formation in these types of systems is thus established. However, it is important to know how much SF to add for this purpose, because it is an expensive material and its interaction with synthesised AH is scarcely reported. If these systems are to be used as construction materials, their dimensional stability in humid environments is the most important factor to consider.

As shown in Figure 2, the length changes of the studied mortar bars tend to stabilise after 28 d. In the systems with the same gypsum content, those containing more BFS and less SF showed less expansion at the end of 56 d. However, all systems remained below the acceptance limits of ASTM C150 (ASTM, 2007) of 0·02% expansion for mixed cements containing high amounts of sulfur trioxide or 0·03–0·04% for blended cements exposed to a sulfate solution. This seems to coincide with the results reported elsewhere (Magallanes-Rivera and EscalanteGarcia, 2014a; Magallanes-Rivera et al., 2012) where the expansion shown by similar systems containing SF remained stable and below 0·05% even after 180 d. Both systems containing 60% AH showed the largest length changes, with mix 60-32-8 showing the greatest length change of all the systems evaluated after 56 d. This coincides with the results of Figure 1, mix 60-32-8 being the system with the lowest compressive strength at the same age. Mix 100-0-0 (prepared without the addition of BFS or SF) ceased to be measurable at 56 d due to detachment of the measuring references. This was due to dissolution of the gypsum, again providing evidence of the decrease in dissolution degree of the gypsum due to the hydrated phases formed by BFS and SF reactions.

Microstructures Figure 3 shows micrographs (2000 magnification) of fracture surface samples of the 50-40-10 system: at 1 d (Figure 3(a)) it is possible to observe well-formed gypsum crystals (DH) of 5–10 μm length, as well as very fine ettringite crystals embedded in a still porous matrix. However, at 28 d (Figure 3(b)), areas of ettringite precipitation were no longer visible and a less porous microstructure was observed. This is due to the slow reaction of the BFS that filled the pores left by the AH reaction, as evidenced by C–S–H products that engulfed the gypsum crystals in Figure 3(b). A somewhat contrary situation occurred in system 70-25·5-4·5, which presented a greater formation of ettringite at 28 d than

DH

Ettringite

C–S–H C–S–H

(a)

DH

(b)

Figure 3. Scanning electron microscopy (secondary electron mode) images of fractured samples of mix 50-40-10 system at (a) 1 d and (b) 28 d

4



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DH C–S–H

Ettringite DH Ettringite

(a)

(b)

Figure 4. Scanning electron microscopy (secondary electron mode) images obtained from fractured samples of mix 70-25·5-4·5 at (a) 1 d and (b) 28 d

at early ages, as shown in Figure 4. In this system, there was not enough BFS to fill the space initially occupied by water and its reaction products and, because there was space available, the formation of ettringite continued. It is worth mentioning here that this system also showed the highest strength at 56 d (Figure 1). The increase in mechanical properties can thus be attributed to the densification of the cement matrix by the transformation of AH into gypsum and the formation of ettringite and C–S–H type products (Singh and Garg, 1992). It can then be said that the amount of AH is related to the ettringite still present at 28 d in this type of mixture. The higher the content of dense phases the less the appearance of ettringite. Chemical composition of products by EDS analysis The nature of the phases present in the composites was determined by performing EDS analyses. The results were arranged 50-40-10 60-34-6 70-25·5-4·5

AFm

1·2

DH

50-40-10 60-34-6 70-25·5-4·5 100-0-0

1·0

0·4

AFt AFt

BFS

0·3

0·2

Sulfur/calcium

Aluminium/calcium

0·5

in silicon/calcium against aluminium/calcium and aluminium/ calcium against sulfur/calcium diagrams derived from the spot microanalysis. EDS was performed on polished samples of mixes 50-40-10, 60-34-6, 70-25·5-4·5 and 100-0-0 at 28 d (Figure 5). Each point on the graphs represents an individual microanalysis. The theoretical compositions of some expected phases (e.g. as C–S–H, anhydrous BFS, gypsum (DH) and AFm and AFt phases) obtained from different references are also shown on the graphs (Magallanes-Rivera and Escalante-Garcia, 2014a; Matschei et al., 2005; Taylor, 1997; Taylor et al., 2001). When the analyses were carried out in different areas of interest, the formation of C–S–H type phases could be determined, but this C–S–H was different from that formed by Portland cement. Several investigators have observed that the C–S–H phases formed in composite cements containing BFS present silicon/calcium ratios from 0·5 to close to 1 (i.e. greater than those of Portland cement, which are

0·8 0·6 0·4

AFm

C–S–H 0·1

0·2

DH

0

0 0

0·5

1·0 Silicon/calcium (a)

1·5

0

0·1

0·2 0·3 0·4 Aluminium/calcium (b)

0·5

Figure 5. Phases in the mortar systems determined by EDS: atomic ratios of (a) silicon/calcium against aluminium/calcium and (b) aluminium/calcium against sulfur/calcium at 28 d

5



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Intensity: arbitrary units

= DH = Calcium sulfate = Calcium carbonate = Silicon dioxide

10

20

30 40 50 2θ : degrees

28 d

28 d

28 d

7d

7d

7d

1d

1d

1d

60

70

10

20

30 40 50 2θ : degrees

60

70

10

20

30 40 50 2θ : degrees

60

70

Figure 6. Mineralogical composition of the mortars as determined by XRD: (a) mix 50-40-10; (b) mix 60-34-6; (c) mix 70-25·5-4·5

typically 0·48–0·61) (Escalante-Garcia, et al., 2009b; Taylor, 1997). Taylor (1997) noted that this increase in silicon/calcium ratio is proportional to the increase of highly reactive silicon particles, in this case provided by SF. Figure 5(a) shows the zones (marked by ovals) corresponding to the expected compositions of the experimental composites: C–S–H formed from the reaction of the BFS and the SF, anhydrous BFS and, since it does not contain aluminium or silicon, DH at the origin. There was a higher dispersion in the points resulting from analysis of the 50-40-10 and 60-34-6 systems than that of the 70-25·5-4·5 system, due to the variation in the compositions. In the 70-25·5-4·5 system, containing the highest amount of AH (70%), the main hydrated product was DH while, in the other two systems, although AH was always maintained as the major constituent, it was diminished and the BFS content increased. For this reason, the results are scattered and tend to areas corresponding to C–S–H, anhydrous BFS (see Figure 5(a)) and AFm- and AFt-type phases (see Figure 5(b)). Figure 5(a) shows several points near the silicon/calcium axis in the range between the DH and C–S–H zones, making it difficult to differentiate between the points corresponding to one zone or the other. This suggests an intermixed matrix of DH and C–S–H products, as shown in Figures 3 and 4. On the other hand, in Figure 5(b), some points are aligned with the zones of AFm-type and C–S–H phases (towards the origin), indicating intermixing of these phases. It should be mentioned that, the AFm phases were considered stable and did not have a negative effect on the mechanical properties. For AFt-type phases like ettringite, their formation and crystal growth generates internal stresses in the hardened matrix, which results in the appearance of cracks (Mehta, 1983); according to the results shown in Figure 5, the formation of this phase was limited. Some authors attribute the enhancement in mechanical strength to ettringite formation, especially at early ages 6

(Fraire-Luna et al., 2006; Singh and Garg, 1995a). However, ettringite may present problems in later stages and therefore the presence of ettringite is not completely desirable. In Figure 5(b), most of the points appear on the vertical axis (sulfur/calcium ratio) because the main product was DH. Mineralogical composition by XRD analysis The XRD results in Figure 6 show similarities, independently of the system formulation or the SF/BFS ratio. Calcium sulfate was the most abundant raw material in the composites and thus all the systems showed well-defined reflections of DH from 1 d of curing. Also, AH (calcium sulfate) peaks were appreciable up to 28 d, with a main peak at 2θ = 25·5°, indicating relatively slow hydration. Calcium carbonate (CaCO3) was also identified, probably originating from the carbonation of the calcium hydroxide added as the AH activator, with a low-intensity peak that overlapped with one of the DH peaks at 2θ = 29·4°. It was not possible to identify peaks related to hydrated phases derived from the BFS and SF reactions, which suggests amorphousness in these products, as would be expected from C–S–H products. However, it was possible to observe them in the micrographs shown in Figures 3 and 4, in addition to their contribution to the development of strength previously discussed. No peaks indicative of ettringite formation were observed in any diffraction pattern, suggesting lower formation of this phase in relation to the other products formed in the cement matrix.

Conclusions &

The addition of BFS and SF to AH systems was found to be beneficial for both the development of mechanical properties and the prevention of dissolution in wet environments and consequent loss of properties at advanced ages.



Offprint provided courtesy of www.icevirtuallibrary.com Author copy for personal use, not for distribution &

&

&

&

&

The compressive strength obtained by the composites after 56 d was 20 MPa. This indicates that this material could be a feasible alternative for the manufacture of masonry elements or elements with low mechanical requirements, with appropriate behaviour in wet environments. The formation of ettringite had a densifying effect on the cement matrix. The system with a small amount of ettringite and another system with a considerable presence of ettringite were those that showed the highest compressive strength. Therefore, the formation of ettringite does not necessarily have a negative effect. All systems containing BFS and SF, regardless of the formulation, were below the acceptance limit of 0·02% expansion at 56 d of curing. Cracking and peeling were not visible. The use of BFS to form C–S–H, in addition to the use of SF, induced a silicon-rich C–S–H formation with a silicon/calcium ratio of 0·45–1·05. The mechanical properties of the studied systems can be attributed to the formation of an intermixed matrix of gypsum formed from AH, C–S–H, AFm and ettringite.

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Acknowledgements Thanks go to CONACYT and SEP for the scholarship granted to M. A. Guerra-Cossío to carry out this research and for the financing granted for projects CB 2012-183490-Y and DSA/103.5/16/9762.

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