Influence of the long term curing temperature on the hydration of alkaline binders of blast furnace slag-metakaolin

Construction and Building Materials 113 (2016) 917–926

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Influence of the long term curing temperature on the hydration of alkaline binders of blast furnace slag-metakaolin Oswaldo Burciaga-Díaz a,⇑, Lauren Y. Gómez-Zamorano b, José Iván Escalante-García a a b

Cinvestav Saltillo, Ceramics Engineering Group, Av. Industria Metalúrgica No. 1062, Parque Industrial, Ramos Arizpe, Coahuila, MX, C.P. 25900 Saltillo, Coahuila, Mexico Centro de Investigación y Desarrollo Tecnológico, Universidad Autónoma de Nuevo León Pedro de Alba s/n, Ciudad Universitaria, San Nicolás, N.L., Mexico

h i g h l i g h t s  Alkali activated binders based on metallurgical slag and metakaolin were studied.  Analysis of variance provided insights on the significance of the parameters of activation.  Curing temperature plays a major role on the properties; it improved early strength.  In the long term higher temperatures did not favor strength and limited the reactions.  The reaction products were amorphous and included C-N-(A)-S-H, N-A-S-H and silica gel.

a r t i c l e

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Article history: Received 1 November 2015 Received in revised form 5 March 2016 Accepted 20 March 2016

Keywords: Alkali-activated slag Metakaolin Long-term curing Compressive strength Microstructural evolution

a b s t r a c t This study analyzed the effect of the curing temperature on the long term properties and reactions of silicate-activated slag-metakaolin binders. Cubic specimens were permanently cured at 20, 60 and 70 °C for up to 520 days evaluating the compressive strength and microstructural evolution. The treatment of fresh pastes at elevated temperatures accelerates the strength development at early ages but in the long run, curing at 20 °C is more advantageous. For 100% slag pastes, increasing the curing temperature from 20 to 60 °C favored an intense dissolution of the slag particles and the formation of a strong microstructure that reached 100 MPa, this contrasted with the pastes of 100% metakaolin and composites of 50% metakaolin. X-ray diffraction and Scanning Electron Microscopy revealed that higher temperatures resulted in higher incorporation of Al in the outer products of slag pastes, while in pastes with 50% metakaolin silica rich phases condensed from the unreacted activating solution forming a matrix of intermixed products of C-(A)-S-H and N-A-S-H type gel. The total heat released by the silicate-activated binders was lower than that reported for the hydration of Portland cement. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction The current global trends in scientific research on cementitious materials focus on the reuse of industrial by products to preserve natural resources and to reduce the environmental footprint of the Portland cement (PC) industry [1,2]. Granulated blast furnace slag (GBFS), a glassy by-product formed after the production of iron, has been used in the construction industry as a partial or total replacement of PC for the last decades [3–5]. As a total replacement in alkali activated cements, the GBFS has resulted in concretes with very good mechanical performance, durability and reduced environmental footprint relative to PC-based materials [1,6,7]. ⇑ Corresponding author at: Instituto Tecnológico de Saltillo, Blvd. Venustiano Carranza # 2400, Col. Tecnológico, C.P. 25280 Saltillo, Coahuila, Mexico. E-mail address: [email protected] (O. Burciaga-Díaz). http://dx.doi.org/10.1016/j.conbuildmat.2016.03.111 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.

However, as a response to improve some drawbacks exhibited by such binders, including rapid carbonation, efflorescence, drying shrinkage and cracking [8–10] new formulations of composite binders with less Ca content have been recently reported by the addition of aluminosilicate-rich materials, such as fly ash (FA) [11–15] and metakaolin (MK) [16–20]. FA is a byproduct from the combustion of pulverized coal in thermal power plants, while MK is an amorphous aluminosilicate that results from the calcination of kaolinite at 500–800 °C and is highly reactive under alkaline conditions [21,22]. In the case of alkali-activated GBFS-MK binders, it has been recognized that under the proper activation conditions, some drawbacks noted for the activation of the individual GBFS or MK, are counterbalanced, so the blends benefit from a synergy, resulting in cements and concretes with good properties in fresh and hardened state [4]. In general, the inclusion of GBFS in the blends

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reduces the water demand, which benefits the compressive strength of pastes and concretes; while increasing the MK content usually delays the setting times, enhances the workability, and although it reduces the strength, it improves the carbonation performance [17–19,23,24]. It is believed that such properties result from stable coexistence of complex and chemically different reaction products as the high Al-substituted calcium silicate hydrate (C-N-(A)-S-H)-type gel and the alkali aluminosilicate gel N-A-S-H [20,25]. The C-N-(A)-S-H is a mechanically strong gel composed by a combination of crystalline tobermorite-like structures and amorphous cross-linked products with a relatively high content of silicon in the form of Q1, Q2 and Q3 sites that promote the densification of the microstructures [26–29]. Additionally, Q3(1Al) and Q4 (4Al) units are also present suggesting that the C-N-(A)-S-H in alkali-activated slag cements may contain disordered nanoparticulate zeolite-like products [30,31]. On the other hand, the N-A-S-H gel is a highly amorphous and cross-linked binding phase with silicon appearing in a wide variety of Q4(nAl) (n = 0, 1, 2, 3 and 4) environments [32,33]. Among the factors affecting the properties of alkali-activated cements, numerous publications have shown that the curing temperature greatly affects the kinetics of reactions as well as the distribution, density, nature and properties of the reaction products [34–36]. While the effect of temperature on the microstructure and properties has been reported for alkali activated GBFS [5,34,37–39] and MK [36,40–43] cements, no reports were found about the influence of this parameter on the longterm mechanical properties and microstructural features of alkali-activated blended binders based on GBFS–MK. High curing temperature and its effects on the reactions of alkali-activated cements is relevant to understand the behavior of such systems under different environmental conditions and also to design or formulate modern cements with good stability and improved properties. Therefore, the aim this research was to analyze the evolution of the strength and the microstructure of composite alkali-activated binders of GBFS-MK, subjected to permanent curing temperatures of 20 °C, 60 °C and 70 °C up to 520 days.

2. Experimental procedure 2.1. Materials Granulated blast-furnace slag (GBFS) and a metakaolin (MK), were used as precursors, and their chemical composition determined by X-ray fluorescence is shown in Table 1. Laser diffraction indicated that the GBFS had a mean particle size of (d50) = 11.77 lm and 10% had a particle size smaller than 2 lm, the Blaine fineness was of 4653 cm2/g. The MK was obtained after calcination of commercially available high purity kaolin at 800 °C for 6 h; the Blaine fineness of the MK was of 9469 cm2/g with a mean particle size (d50) of 7.6 lm and also 10% had a particle size smaller than 2 lm. Both raw materials were predominantly amorphous to

Table 1 Chemical composition by XRF (mass%) and Blaine fineness of the solid GBFS and MK used. Oxide (mass%)

GBFS

MK

SiO2 Al2O3 Fe2O3 CaO TiO2 Na2O K2O MgO MnO SO3 Blaine (cm2/g)

33.4 11.3 0.5 37.8 1.4 0.5 0.9 8.9 0.5 3.4 4653

51.05 45.26 0.34 0.10 1.76 0.06 0.15 — — — 9469

X-ray diffraction, nevertheless the GBFS showed traces of crystalline phases as akermanite (Ca2MgSi2O7), gehlenite (Ca2Al2SiO7), and merwinite (Ca3MgSi2O8), while MK showed quartz (SiO2) and anatase (TiO2). The alkaline chemicals used as activators were blends of sodium silicate (SiO2 = 29.5%, Na2O = 14.7% and H2O = 55.8%) modulus SiO2/Na2O = 2 and sodium hydroxide flakes, both of industrial grade. 2.2. Sample synthesis and test procedures In order to elaborate the pastes for the investigation, the starting powders were mixed in GBFS-MK proportions (mass%) of: 100-0, 80-20, 50-50, 20-80 and 0-100; this nomenclature was used throughout the paper. The alkaline solutions had a modulus Ms = 1 and the Na2O was added in concentrations of 5, 10 and 15 mass% with respect to the sum of the GBFS + MK. This range of activation conditions has been identified as resulting in GBFS-MK binders with high compressive strength and acceptable workability [18,19]. The water/solids ratio was adjusted to obtain flowable pastes of similar workability suitable for casting into the molds. The pastes were cast in cubic molds with sides of 50 mm, which were vibrated for 45 s to eliminate air bubbles. The samples were left covered with a thin plastic film for 24 h at 20 °C. Afterwards, the demolded cubes were covered with plastic bags and then stored in plastic boxes to continue with a process of permanent curing in isothermal chambers at 20, 60 and 75 °C for up to 520 days. The compressive strength (CS) was statistically analyzed as described later, testing four samples cured for 1, 3, 7, 14, 28, 90, 180, 360, and 520 days using an automatic hydraulic machine (Controls Sercomp 7) with a constant loading rate of 500 N/s. The results were statistically analyzed as described later. For the characterization of selected samples, the reaction process was stopped at various ages by submerging fragments of pastes in acetone for 2 days, and then drying them in a vacuum oven (VWR 1430M) at 40 °C for 48 h. The samples were then ground in a planetary mill (PM 400/2; Restch, Newtown, PA), using agate media, to pass the #100 sieve. The ground powders were characterized by X-ray diffraction (XRD, Philips D-Expert, Netherlands) in a range of 7–60°(2h) with a step size of 0.03° and a count time of 2 s per step, using CuKa (1.542 Å) radiation. For scanning electron microscopy (SEM) analysis, pieces of dried samples were mounted in resin and polished using diamond pastes down to 1/4 lm. Further carbon coating was necessary to make the samples conductive under the microscope (ESEM Philips XL30, Eindhoven, the Netherlands) accessorized with energydispersive X-ray spectroscopy (EDS). Representative backscattered electron images of microstructures were taken at 500 magnifications in high vacuum mode using an accelerating voltage of 20 kV, spot size of 4–5 and a working distance of 9.8–10 mm. EDS spot analyses were collected throughout the microstructure at 5000 magnifications, the microscope was operated at 20 kV and the time of analysis was of 50 s. For selected formulations, the early progress of the chemical reactions was investigated by following the evolution of the rate of heat of reaction, by means of an isothermal conduction calorimeter. For the test, 15 g of raw GBFS + MK were placed in an isolated cell tests accessorized with a propeller for manual agitation, the powder was contained by a cling plastic film, and placed on a thin electrolytic copper foil that was in contact with the Peltier device used as heat flow sensor. The required amounts of activating solutions for each studied formulation were weighed in syringes. The insulated containers bearing the samples were then submerged in water in an isothermal bath (ISOTEMP 3028 at 20 °C and 60 °C ± 0.01 °C) altogether with the syringes. Once the thermal equilibrium was achieved, the solution was injected to the powders and the paste was mixed for 40 s sufficient to achieve proper homogenization of the pastes, as determined from a series of preliminary tests). Data logging (National Instruments Model SCXI-1303) was started along with the injection of the solution and continued for up to 40 h. Calibration was performed afterwards to convert the signals from mV to kJ/kg.h. 2.3. Statistical analysis of the compressive results In order to investigate the effect of the variables and their interactions on the development of the compressive strength in composite GBFS-MK pastes, a complete factorial experimental design (52  32) was conducted for each age studied following the methodology explained in a previous work [18]. A total of 45 compositions were evaluated by modifying various factors or independent variables. The nomenclature used to identify the factors was A – GBFS/MK ratio; B – %Na2O; C – Temperature, and D – curing time. The factorial experimental design (52  32) means that two factors (GBFS/MK ratio and Curing time) were evaluated at five levels and two (%Na2O and the Temperature) at three levels, Table 2 summarizes the factors and

Table 2 Factors and levels considered for the ANOVA. Factors

Levels

A = GBFS/MK ratio B = % Na2O C = temperature (°C) D = curing time (Days)

100/0; 80/20; 50/50; 20/80; 0/100 5%; 10%; 15% 20; 60; 75 1; 28; 90; 180; 520

O. Burciaga-Díaz et al. / Construction and Building Materials 113 (2016) 917–926 their levels. For the experimentation, the response variable (dependent) was the compressive strength at 1, 28, 90, 180 and 560 days, and a total of 225 data of strength were acquired and statistically treated. The statistical significance of each factor and their binary interactions, over the strength development was quantitatively determined using an analysis of variance (F-ANOVA) considering a confidence level of 95%.

3. Results and discussion 3.1. Compressive strength Fig. 1 presents the generalized effects that resulted from the analysis of variance (F-ANOVA) after 520 days of curing. From Fig. 1(a) it is noteworthy that binders with 100%GBFS (100-0) with approximately 38% Ca, developed the highest compressive strength of about 70 MPa, similar to that previously reported for samples cured only at 20 °C [18]. Also, the strength was reduced with the addition of MK [23]; the pastes with 50%MK (50-50) or 100%MK (0–100) showed average strengths of around 45 and 20 MPa respectively, which are acceptable for different applications. The results indicated that the gradual incorporation of aluminosilicates in the binders demands higher concentrations of alkalis in the solution in order to promote the dissolution of the Si-O-Si and Si-O-Al bonds, and the subsequent precipitation of cementitious reaction products [18,24]. This also means that, the lower demand of activator by the 100%GBFS pastes, which is the more expensive component in these compositions, makes them of lower costs of production and of a smaller CO2 footprint relative to binders with additions of MK. Nonetheless, it is possible that although the incorporation of MK reduces the strength in the composites, other properties like the durability can be improved making these compositions suitable for special applications where a combina-

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tion of good durability and optimum strength can be required [24] and unpublished data from the authors [44]. Fig. 1(b) confirmed that regardless of the curing temperature and ratio BFS-MK, the strength development is highly influenced by the %Na2O; i.e., 10% of Na2O in the activating solution resulted in pastes with the higher mean strength of 50 MPa after 520 days. On the other hand, Fig. 1(c) shows that temperature and curing time had a strong influence on the development of compressive strength. After 1 day of curing at 75 and 60 °C the mean strengths were of 50 and 45 MPa, respectively, while curing at 20 °C resulted in less than 20 MPa. After 28 days, the specimens cured at 20 °C showed a higher rate of strength gain, that the pastes cured at 75 °C but less than those cured at 60 °C (55 MPa). The samples cured at 20 °C gained strength after 90 days, whereas the opposite was noted for the pastes cured at 60 and 75 °C; the latter showed slight strength losses that stabilized at 47 and 43 MPa, after 520 days respectively; this behavior could be related to the continuous dehydration of microstructures and formation of microcracks. This could be explained by considering that during the early ages, higher curing temperatures enhanced the dissolution of GBFS and/ or MK and the heterogeneous condensation of reaction products mechanically strong in the matrix. Furthermore, the high curing temperatures may have caused a rapid evaporation of water, which, together with a relatively rapid densification of the microstructure, inhibited the further diffusion of species, limiting the progress of the reactions and consequently the development of strength. In contrast, the samples cured at low temperature (20 °C) underwent the opposite effect, in which a slow dissolution of the reactive species took place, resulting in a more homogeneous condensation of reaction products due to the increased water retention within the microstructures; at 20 °C the diffusion processes and the formation of products continued over time to a

Fig. 1. Influence of the factors A (a); B (b); and CD (c) on the development of compressive strength.

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point in which the densification of the matrix and the consumption of reactants became the limiting factors for the advance of the reaction processes. Table 3 indicates that from the four factors considered, there is a strong influence of the factors A (BFS/MK ratio) and B (%Na2O) and their interaction on the compressive strength since early ages of curing. This is coherent, because the demand of Na2O depends strongly on the physical and chemical characteristics of the raw materials used; generally, higher MK contents will demand higher %Na2O. Additionally, as the reaction time increases, from 28 to 520 days, the CD (temperature-time) interaction became more important while the individual factor D (Curing time) became statistically less important; this implies that the binary interactions AB and CD are statistically the most important variables on the development of strength for the studied formulations.

3.2. Hydration products Fig. 2 shows XRD results of binders 100-0, 0-100 and 50-50 activated with Ms = 1, 10%Na2O and cured for 180 days at 20, 60 and 75 °C; the values of compressive strength were included. Fig. 2(a) shows the GBFS (100-0) spectra for samples cured at 20 °C, where the main crystalline phases were gehlenite (Ca2Al2SiO7; PDF# 01089-5917) merwinite (Ca3MgSi2O8; PDF# 035-0591) and akermanite (Ca2MgSi2O7; PDF# 079-2424), contained in the unreacted slag; other crystalline reaction products were calcite (CaCO3 at 29.07° 2h; PDF# 01-083-0577) and calcium silicate hydrate (C-S-H at 29.57° 2h) with a riversideite 9 Å type structure (Ca5Si6O17
H2O; PDF# 029-0329), previously reported for slags with different chemical compositions [23,45]. The latter phases are commonly observed when the GBFS is activated with sodium silicate solutions [46] and the calcite indicates the carbonation of the samples during the preparation or storage of the samples. Hydrotalcite (Mg6Al2CO3(OH)16
4H2O; PDF#014-0191) was also observed as secondary reaction product [46,47]. After curing at 60 and 75 °C, sharpening and intensification of hydrotalcite and C-S-H (29.57° 2h) can be identified, indicating that higher temperatures favored the formation of more crystalline C-S-H or that the amount of such hydrate was greater than in samples cured at 20 °C. The latter is more feasible as higher temperatures resulted in higher strength, this is consistent with previous reports indicating that high curing temperatures enhance the reactivity of the GBFS, at early ages, as the energy of activation of the GBFS is relatively high [34]. However, when a binder that has free water is subjected to elevated temperatures during the first hours, the reactions are effectively enhanced, but the water will also tend to evaporate promoting the cracking and limiting the reactivity and thus restricting strength the strength gain as observed in the samples cured at 75 °C. On the other hand, Fig. 2(b) shows the spectra of geopolymer binders containing 100% MK (0-100) with only traces of quartz (SiO2; PDF# 00-074-0764) and anatase (TiO2; PDF# 01-083-2243) even after the activation at 20, 60 and 75 °C, suggesting that such phases remained inert. The patterns evidenced the presence of a large halo in the 15–35° 2h range, originated from the formation of alkali aluminosilicate N-A-S-H type gel [43,48]. In this case,

Table 3 Order of importance of the factors at different age of curing. Curing time (days)

Statistical significance of the factors

28 90 180 520

C < AB < B < D < A CD < D < AB < B < A D < CD < AB < B < A D < CD < AB < B < A

the increase in the temperature did not cause significative changes in the XRD patterns, and the N-A-S-H reaction products preserved an amorphous nature. However in contrast with binders 100-0, the increase in the temperature reduced the compressive strength, which was due mostly to unfinished reactions of the MK, the continuous dehydration and possibly the formation of microcracks produced by the evaporation of water. It is interesting that in the XRD patterns of the binder 50-50 in Fig. 2(c), the shape of the amorphous hump around 20–40° 2h was different to that observed in binders 100-0 and 0-100, but it also appears as the superposition of the 100-0 and 0-100 gels, this suggests the condensation of reaction products based on a intermixture of C-S-H and N-A-S-H gels with a more complex atomic structure than those of the products formed by the individual activation of GBFS or MK. Regardless of the curing temperature, quartz, anatase, akermanite and gehlenite reflections remained in all the patterns. The C-S-H peak at 29.57° 2h did not show any variation with the curing temperature but it was sharper and more intense than that detected in the binder 100-0, denoting a higher crystallinity promoted by the inclusion of Al. It was noted that the presence of 50% MK inhibited the formation of CaCO3 previously observed in 100-0 binders, which is in agreement with results reported by Bernal et al. [24]; however, similar to that observed in the formulation 0-100, an increased temperature limited the gain of compressive strength. 3.3. Microstructures and chemical composition of reaction products Fig. 3 presents the microstructures, obtained by scanning electron microscopy in backscattered electron imaging mode, of binders 100-0, 0-100 and 50-50 activated with sodium silicate Ms = 1, 10%Na2O and cured for 180 days at 20 and 60 °C. The microstructure of the binder with 100%GBFS (100-0) cured at 20 °C, showed cracks and unreacted GBFS particles distributed in a reaction products matrix of darker gray tone (outer products, OP) for which EDS showed high contents of Si and Ca. The formation of cracks in silicate activated GBFS binders could be related to the evaporation of uncombined water as the samples tend to dry under the vacuum of the microscope column that causes substantial shrinkage and therefore microcracking [49]. The content of unreacted GBFS grains correlates well with the lower strength shown at 20 °C, and the higher temperature resulted in a matrix apparently more compact, with less unreacted slag (more fully reacted slag grains) and mechanically stronger than at 20 °C. At 60 °C many slag grains were completely reacted and some appeared partially reacted with rims of reaction products, such reaction products appeared of a darker shade; these features were not noted after curing at 20 °C, which suggest different mechanisms of reaction of the slag. At 20 °C the predominant mechanism of the reaction was the dissolution of the glassy structure and subsequent precipitation of the hydration products in the spaces initially occupied by the activating solution; while at 60 °C the reactions occurred also via a solid state mechanism in which the reactants diffused through the reaction products and formed more products, consuming the BFS grains inwards. This indicates that although a high temperature may contribute to a rapid loss of moisture, it is also favorable for the reactivity of the GBFS. For the 100%MK binders (0-100) cured at 20 °C a dense microstructure was noted, which explains the relatively high strength. According to XRD and EDS results, the outer products (OP), consisted of an amorphous aluminosilicate gel (N-A-S-H) with high concentration of Si, Al and Na; such products are identified by the darker gray tone in the microstructures. Curing at 60 °C reduced the progress of the dissolution-condensation processes; which was evidenced with a higher content of unreacted MK particles. This indicates that in contrast to the binder 100-0, the

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Fig. 2. X-ray diffraction patterns of pastes with Ms = 1 and 10%Na2O; (a) 100-0, (b) 0-100 and (c) 50-50, at different temperature and 180 days of curing.

increasing in the temperature hindered the reaction processes [50], and it appears that the retention of free water in the microstructure seems to be more important for the dissolution of MK and condensation products of high strength [43]. On the other hand, the microstructure of the composite sample with 50%MK cured at 20 °C indicated the presence of unreacted GBFS and MK particles distributed in a matrix of reaction products. The OP matrix appeared dense regardless of the cracks present. Nevertheless the high curing temperature decreased the strength, as at 60 °C the condensed OP had greater porosity and were coarser. At 60 °C it was difficult to identify particles of unreacted MK, as these had a similar tone that the OP. This means that the high curing temperature probably produced the evaporation of water, which increased the average atomic number of the OP making them appear with a lighter gray tone in the micrographs. It is noteworthy that both microstructures showed dark zones rich in Si, which were more abundant at 60 °C. This suggests that a high temperature limited the dissolution reactions, resulting in the condensation of silica gel from the unreacted activating solution. In the microstructures the presence of two separate phases, indicating the formation of C-(A)-S-H and N-A-S-H gel was not observed, as reported by [25] for similar binders containing 60% GBFS-40% MK and activated with a sodium silicate solution of Ms = 1.2. However, the XRD results previously discussed showed the presence of crystalline C-S-H intermixed with amorphous compounds, probably N-A-S-H type gels and/or C-(N-)A-S-H. Fig. 4 presents microanalyses (SEM-EDS) from the sample 100-0 activated with 10%Na2O and cured for 180 days at 20 and 60 °C. The data are plotted in ternary diagrams of Ca-Si-Al and Ca-AlMg, normalized to 100 at.%. The results are classified in outer products (OP) and inner products within the GBFS particles (IP-BFS). In

the Ca-Si-Al diagram at 20 °C, the chemical composition of the OP the points gathered around a composition of 50% Ca, 10% Al and 50% Si, with an average ratio Ca/Si = 0.82 similar to that commonly reported for alkali activated slag cements [51,52]. At 60 °C, the analyses showed slight dispersion towards zones with a higher content of Ca and therefore less Si; this confirms that curing at 60 °C resulted in a higher dissolution of the GBFS particles with the consequent condensation of C-(A)-S-H gel (enriched in Ca) with Ca/Si = 0.87. The reduction in the Si/Al ratio (from 4.56 at 20 °C to 4.30 at 60 °C) suggest that increasing the curing temperature from 20 to 60 °C increased the Al uptake in the OP. On the other hand, the IP-BFS showed higher Ca/Si, Al/Ca and Mg/Al ratios than the OP, indicating differences among the hydration products formed in both areas. It has been reported that for sodium silicate activated slag pastes with content of MgO content >5%, as the one used in this research, high concentrations of Mg and Al were confined in the IP-BFS within the boundaries of the GBFS particles [28,45,47,53]. It appears that during the slag dissolution the Mg has a limited mobility and therefore it hinders the diffusion of Al, as the latter is necessary to form of hydrotalcite-type phases around the slag grains [46]. In the Ca-Al-Mg diagram, the data of the IP-BFS and OP followed a linear correlation; nonetheless they plotted separately, indicating hydrated products of different chemical composition. The IP-BFS had higher Ca/Si and Mg/Al ratios than the OP, in agreement with the previously described. The average Mg/Al ratios in the IP-BFS were of 1.76 and 1.86 for 20 and 60 °C, respectively, lower than those reported for the ideal formula of hydrotalcite of 2.4 [54]. As the temperature increased, a more intense dissolution of GBFS favored the dissolution of Mg to form hydrotalcite finely intermixed with the C-(A)-S-H on a nanometer length scale as noted by the XRD results [23].

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Fig. 3. Scanning electron microscopy in backscattered imaging mode of pastes 100-0, 0-100 and 50-50 cured for 180 days at 20 and 60 °C.

Fig. 4. Energy dispersive microanalyses of binder 100-0, cured 180 days at 20 and 60 °C.

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content of Si. This suggests that, as noted for binders 100-0, a high curing temperature dissolved faster the GBFS particles than those of the MK. This was also evidenced by the higher and lower concentration of Ca and Al respectively, incorporated in the reaction products which presented chemical compositions (at.%) of 13% Ca, 17% Na, 27% Al and 43% Si at 60 °C, and 5% Ca, 20% Na, 31% Al and 44% Si at 20 °C. On the other hand, the analyses taken from the dark zones (see Fig. 3) of samples cured at 60 °C for the binder 50-50, showed a high variability in Al content and a trend towards areas of Si greater than 70%. These dark zones had ratios Si/ Ca = 16.88, much higher than that observed in the OP (Si/ Ca = 3.47), indicating an incomplete chemical reaction processes that promoted the condensation of silica gel rich phases from the activator solution. The Na-Si-Al diagram in Fig. 6, showed a preferential incorporation of Na (probably as charge balancer of Al) [55] in the framework of the OP of samples cured at 20 °C compared with those at 60 °C, due probably to the acceleration of the reactions processes promoted by the retention of water in the pastes. However, in the dark areas, the concentration of Na had high variability and the average chemical composition (in at.%) of the condensed gel was 7% Ca, 27% Na, 15% Al and 20% Si, evidencing the formation of phases with high content of Si and Na and limited concentration of Ca and Al. Fig. 5. Energy dispersive microanalyses of binder 0-100, cured 180 days at 20 and 60 °C.

Fig. 5 presents the EDS results plotted in a ternary diagram Na-Si-Al for the 100%MK (0-100) binder. In general, the results from 20 and 60 °C gathered close to each other; however the tabulated results indicate that increasing the temperature modified the Si/Al ratio from 1.34 to 1.60. This could have been caused by a higher incorporation of Si or by a lower integration of Al in the OP. The latter is more likely, since the augment of temperature appears to have limited the dissolution of the MK particles, which are the main source of Al. The foregoing is evidenced by calculating the average composition (in at.%) of the condensed aluminosilicate gels, which was of 20% Na, 34% Al and 46% Si at 20 °C and 20% Na, 30% Al, and 50% Si at 60 °C. In the Ca-Si-Al diagram from Fig. 6, it is noteworthy that for the binder 50-50, the increase of the curing temperature changed the composition of the OP towards higher content of Ca and lower

3.4. Early reaction processes Fig. 7 shows the heat liberation rate from pastes of alkaliactivated 100%GBFS (100-0), 100%MK (0-100) and 50%MK (5050). In general, the curves are consistent with previous reports for silicate activated slag and MK binders [5,45,56–58], where during the first few minutes of contact of the activator solutions with the powders, an intense peak is observed, which corresponds to the initial heat release associated with the particle wetting and the start of dissolution of the raw materials. Afterwards a high intense acceleration-deceleration period (second peak) of complexation of species and condensation of reaction products was noted. For all the binders the initial peak appeared at similar time (<0.5 h), regardless the curing temperature and type of raw material; however, the peak was more intense at the higher temperature [5]. After the initial peak, a short period of induction of limited heat release appeared and its length was reduced as the temperature increased from 20 to 60 °C [23]. During the induction period occurs

Fig. 6. Energy dispersive microanalyses of binder 50-50, cured 180 days at 20 and 60 °C.

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Fig. 7. Heat release rate and cumulative heat of hydration at 20 °C and 60 °C from binders (a) 100-0, (b) 0-100 and (c) 50-50 with MS = 1 and 10%Na2O.

the gelation of species giving time to manipulate the fresh pastes before their solidification, which is consistent with the behavior of conventional Portland cements [59–61]. The end of the induction period determined the onset of a second peak of acceleration and then deceleration of the reactions, which seems to be highly dependent of the curing temperature and the composition of the binder [5,57,58,60]. For the 100% GBFS binder (100-0) in Fig. 7a, the second peak showed a maximum at 52 min for 20 °C, which shifted to 37 min and became more noticeable at 60 °C. These indicated that an increased temperature led to faster reactions and a more rapid dissolution of the slag particles, and consequently to faster precipitation reaction products like C-N-(A)-S-H type gel, which is noted because the curve of acceleration-deceleration is extended to longer times (from 50 min to 9 h) compared with the sample cured at 20 °C [23,28,59]. These results are coherent with those discussed in the SEM section, which showed a more effective dissolution of slag particles and a greater degree of precipitation of reaction products at 60 °C. Fig. 7b depicts the rate of heat release response for geopolymeric binders containing 100%MK cured at 20 and 60 °C. At 20 °C, the curves revealed a long induction period of 4.5 h followed by a broad and relatively weak exothermic period of acceleration-deceleration which extended from 5 to 35 h, evidencing the gradual advance of the reaction processes. During this period, OH anions begin the attack on Si-O and Al-O bonds resulting in an intense dissolution of MK particles and formation of complex hydroxide species of Al-Si which continue reacting to produce more complex species [50,62,63]. According with Provis et al. [4], the dissolved species gradually reduce the pH of the system, leading to the further condensation of a homogeneous microstructure formed by polymerized amorphous N-A-S-H gel. The results show that at 60 °C a more intense heat release peak was observed, reaching values even higher than those of the 100% GBFS sample, the second peak occurred right next to the first peak without any induction period (as shown by the insert in the graph); this sug-

gests the occurrence of a rapid dissolution of the MK at early ages; however, in contrast with the 100%GBFS, the degree of condensation of mechanically strong reaction products could be more limited by rapid evaporation of water produced by the high curing temperature [64]. This is in agreement with the microstructure of the binder that evidenced a high content of unreacted MK particles even after 180 days of curing at 60 °C. The results of Fig. 7(c) for pastes with 50%GBFS-50%MK (50-50) indicated that the heat release rate was affected by the ratio of raw materials GBFS-MK [23]. In this case the reaction of the sample cured at 20 °C was more intense than in the binder 0–100. It is possible that the alkalinity of the activating solutions promoted an early strong dissolution of the raw materials (GBFS-MK), to subsequently enter into a relatively short induction period similar to the observed for the 100-0 binder [65]. After 1.5 h, a weak second event of acceleration-deceleration occurred at times analogous to the binder 100-0 and at earlier times than in the geopolymer 0100. This could suggest that the main acceleration-deceleration peak is due mostly to the reaction of the GBFS, and the second overlapped weak peak is from the MK showing low reactivity in this period as noted in Fig. 7(b). On the other hand, an increase in the reaction temperature from 20 °C to 60 °C, accelerated the reactions resulting in a very short induction period weakly noted; the acceleration-deceleration peak was also intensified and appeared at earlier times similar to the alkali-activated slag binder 100-0, but with lower intensity [36,57,58]. Fig. 8 shows cumulative heat of hydration results versus time for samples cured at 20 and 60 °C. For the slag binder (100-0) the cumulative heat of hydration increased with the temperature; this effect was observed since the first hour of hydration and after 12 h the difference of released heat between both curing temperatures was of 200 kJ/Kg. For binder 0-100 the cumulative heat of hydration increased during the first 10 h with the curing temperature; the curve at 20 °C reached a steady state of 200 kJ/kg after 30 h and the curve at 60 °C stabilized at about 225 kJ/kg after 10 h, indicating that

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Fig. 8. Cumulative heat of hydration at 20 °C (a), and 60 °C (b) from binders 100-0, 0-100 and (c) 50-50 with MS = 1 and 10%Na2O.

for both curing temperatures, similar amounts of heat are released, but more rapidly at high temperatures. In contrast, the composite binder 50-50 showed the lowest heat of hydration of the three formulations; nonetheless, the increase of temperature accelerated the reactions and the values of heat released were higher at 60 °C (65 kJ/kg) than those of the binder cured at 20 °C (35 kJ/kg). Clearly, the total heat released by the 3 formulations is less than that reported for the hydration of the PC at 50 °C (325 kJ/kg) [5,60], and it is of special interest the low heat released by the composite 50-50, considering the compressive strength and reduced carbonation shown by such formulation. 4. Conclusions The temperature strongly influenced the gain of compressive strength of all the alkali-activated binders studied. The analysis of variance shows that curing at high temperature was favorable for a rapid strength gain, however, at later ages, the highest compressive strengths were observed after curing at 20 °C. Considering the individual formulations, curing at 60 °C was beneficial for slag binders forming dense microstructures, while the opposite occurred in metakaolin geopolymers and in the composite binders with 50% metakaolin. Higher temperature favored the initial reactions but caused water evaporation, which affected the reaction processes. Although after 90 days the trend of samples cured at 60 and 75 °C was to reduce their strength, all the samples assessed showed mechanical strength higher than 43 MPa after 520 days of curing, which can lead to produce mortars and concretes with suitable properties for many applications. The 100% BFS binders cured at 60 °C showed outer products (OP) with higher Al content than those formed in samples at 20 °C, indicating the formation of highly cross-linked C-(N-)A-S-H gel. The geopolymeric pastes of 100% MK exhibited the formation of N-A-S-H gel, with an increase in the Si/Al ratio with the curing temperature. Curing at 20 °C was advantageous for the compres-

sive strength, since at 60 °C the dissolution of MK was limited resulting in coarse microstructures with lower strength. Composite binders with 50% metakaolin were also affected by the temperature, at 60 °C the concentration of Na had a high variability and the average chemical composition (in at.%) of the condensed gel was 7% Ca, 27% Na, 15% Al and 52% Si, evidencing the formation of phases with high content of Si and Na and lower concentration of Ca and Al, being more favorable the curing at 20 °C for the advance of the reactions and development of compressive strength. The results are useful to understand and compare the behavior of different alkali-activated binders exposed to different curing temperatures, and will help as a basis to select the most adequate binder as a function of the environment and work conditions. Acknowledgements This research was funded by Conacyt Mexico project 53563. Burciaga-Díaz also acknowledges the scholarship from Conacyt (203549). References [1] F. Pacheco-Torgal, J. Labrincha, C. Leonelli, A. Palomo, P. Chindaprasit (Eds.), Handbook of Alkali-Activated Cements, Mortars and Concretes, 1st ed., Woodhead Publishing, 2014, p. 852. ISBN:9781782422761. [2] J.L. Provis, Geopolymers and other alkali activated materials: why, how, and what?, Mater Struct. 47 (2014) 11–25. [3] J. Newman, B.S. Choo (Eds.), Advanced Concrete Technology, Elsevier, Oxford UK, 2003. [4] J.L. Provis, J.S.J. van Deventer (Eds.), Alkali activated materials: State-of-the-art report, RILEM TC 224-AAM, Springer/RILEM, Berlin, 2013, p. 387. [5] C. Shi, P.V. Krivenko, D.M. Roy, Alkali Activated Cements and Concretes, Taylor and Francis, 2006. p. 371. [6] M.C.G. Juenger, F. Winnefeld, J.L. Provis, J.H. Ideker, Advances in alternative cementitious binders, Cem. Concr. Res. 41 (2011) 1232–1243. [7] S.A. Bernal, J.L. Provis, Durability of alkali-activated materials: progress and perspectives, J. Am. Ceram. Soc. 97 (4) (2014) 997–1008.

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