3.1. Compression strength

Figure 2 shows the results of all the systems analyzed in this study. In Figure 2 A, it can be observed that the use of additives SP1 and SP2 bettered the development of the RC for all treatment time periods, resulting in a RC at 1 and 60 days of approximately 49 and 76 MPa for the system manufactured with SP1 and of 44 and 76 MPa for the paste with SP2. The polycarboxylate-based superplasticizers are efficiently absorbed on the surface of the CP particles causing a highly observed dispersion in the functional ether groups, providing as a result an increase in workability and compression strength. The CP paste manufactured with the SP1 additive showed slightly higher resistances than that made with SP2. This result is possibly due to the presence of the functional ether group that appears in the FTIR spectrum of the SP1 additive. ^{Winnefeld et al. (2007)} reported a compression strength between 38.1 to 54.6 MPa at 7 days into the treatment, indicating that at early stages the RC increases with the increase of longitude and density of the lateral chain of the functional ether groups of the superplasticizer. Figure 2 B shows the results of the RC developed by the binary systems, where it is observed that at 60 days, 10CV pastes with SP1 and SP2 developed a compression strength above the binary systems of 45EGAH and 15 MK. ^{Golapan (1993)}, ^{Isaia et al. (2003)}, and ^{Slanicka S (1999)} have reported that the RC increase of cement paste partially replaced with CV is due to the size and morphology of the fly ash particles. Small and spherical particles fill the gaps and produce a dense matrix, further increasing the pozzolanic activity of the material which causes an increase in the RC. It is worth mentioning that this pozzolanic activity is not immediate, rather it can take up to 3 to 7 days; thus, improvement in the RC is seen at intermediate treatment stages. This effect is also seen in cement pastes with 10 and 15% CV [^{Gutteridge et al. (1990)}]. The 15MK pastes with both additives showed superior RC to CP and other binary systems at a period of 1 day of treatment. This behavior is attributed to the fine size of MK. It has been published [^{Mansour et al. (2010)}; ^{Caldarone et al. (1994)}; ^{Wild et al. (1996)}] that the use of MK contributes to the development of mechanical resistances at early stages due to the fine particle size that densifies the microstructure of the pastes, and due to its strong pozzolanic activity. ^{Khatib et al. (1996)} analyzed the development of resistances for cement pastes replaced with 10% MK at different points in time during the treatment and obtained the maximum RC at 14 days, which indicated that the pozzolanic activity of MK reaches its maximum reaction point in this timeframe. It has also been reported [^{Mansour et al. (2010)}] that in order to disperse the MK in a cement paste the use of superplasticizers is needed, which improves the workability, rheology and mechanical properties of the pastes. The CP paste replaced with MK using SP2 showed superior RC, approximately 10% when compared to 15MK paste with SP1. The factors that could have affected this behavior were the a/s ratio, the SP quality and the dispersion effect caused by the additive.

Figure 2 Compression strength of the systems with SP1 and SP2 additives: A) CP pastes, B) Binary systems, C) CP pastes 50% replaced with CV, EGAH and MK, D) CP pastes 60% replaced with EGAH, CV and MK, E) CP pastes 50% replaced with CV, EGAH, MK and its HS comparative using SP1 additive. 

The development of the RC for the 45EGAH binary systems, at 1 and 60 days of treatment, was found to be below the RC of 100CP pastes and 10CV and 15MK systems; this behavior was attributed to the fact that under these conditions the a/s ratio, level of EGAH replacement and SP dosage, the fluidity of the paste of the 45EGAH binary systems was increased causing an excess of water in the mixture and consequently a change in the hydration process and on the mechanical properties of the system.

The results for the RC tests in the quaternary cements with both superplasticizers are shown in Figures 2 C and D. It is understood that the 10CV-25EGAH-15MK system was the one that developed the greatest RC at 1 day of treatment (approximately 37 and 36 MPa for both SP). However, these resistances were found to be below the RC of CP pastes at the same stage of treatment. The 10CV-35EGAH-5MK paste for the two superplasticizer additives developed the best RC at 60 days of treatment, being greater for SP1 (73 MPa). This behavior was related to the one presented by the binary systems with 10CV and 15Mk, given that the effect of the pozzolanic activity of MK in the early development of RC was evident. On the other hand, the pozzolanic activity of the CV and the larger ratio of EGAH benefitted the development of RC at 60 days of treatment. Furthermore, it can be observed that the pastes with SP1 developed RC above those manufactured with SP2. The factors that could have had an impact on this behavior were the a/s ratio, the dosage of the superplasticizer, and the dispersion caused by the additives. On the other hand, the development of RC in the CP pastes replaced 60% with CV, EGAH, and MK show a similar tendency to the RC results of the CP pastes with 50% replacement; however, the RC development at early stages was smaller. Moreover, the 10CV-35EGAH-15MK with both additives developed a better RC at 1 day of treatment in comparison with the 10CV-45EGAH-5MK paste, which showed the best RC at 60 days of treatment. Due to the fact that the 10CV-35EGAH-5MK paste manufactured with SP1 showed the highest RC values at 60 days of treatment, the effect of replacing the 5% MK for 5% HS was compared. In addition, there was consideration to study the same effect on the 10CV-30EGAH-10MK paste.

The tendency of the results in Figure 2 E indicates that pastes 10CV-30EGAH-10MK and 10CV-35EGAH-5MK showed a greater RC than the pastes with 10CV-30EGAH-10HS and 10CV-35EGAH-5HS at 60 days of treatment, with the resistance being greater in the pastes where MK ((73MPa) was used. ^{Roy (2001)} indicated that HS only increases resistance at early stages, while RC at later stages decreases with the increase of CV replacement. However, in this investigation, the CV replacement was maintained at a set point. Furthermore, the best resistances were in the pastes with a greater replacement of EGAH and a treatment time of 60 days, which corresponds to the study presented by Gesoglu et al. (2003), who concluded that the addition of 30% EGAH and 10% HS betters the RC at 28 days of treatment.

3.2. X-ray Diffraction

Representative samples of the cement pastes replaced at 50% and 60%, manufactured with the two SP and treated for 28 days, were analyzed, observing the following phases:

* - Portlandite (CH). C - Calcite (CaCO₃), α - Quartz (α-SiO₂), * - Alite (C₃S), ▲- Ettringite (Ca₆Al₂(SO₄)₃ (OH)₁₂.26H₂O), S - Stratlingite (Ca₂Al₂SiO₇.8H₂O,), β - Belite (Ca₂SiO₄- C₂S,), ● - Hydrotalcite ((Mg₀.₆₆₇Al_0.33)(OH)₂(CO₃)_0.167(H₂O)_0.5).

In Figure 3 A, the results of the DRX analysis for the CP paste manufactured with SP1 (CP-SP1) are shown and in Figure 3 B those manufactured with SP2 (CP-SP2) at 28 days of hydration, where it was possible to observe that for both the portlandite (CH) reflection characteristics were shown in an angular position 2θ of 18.08º and 34.3º. Also in the CP-SP1 system, ettringite was identified with characteristic reflections in angular positions 2θ of 9.147º, 23º and 51.784º. The ettringite is directly related to the quantity of C₃A and the availability of SO₄ ^2- ions in the liquid phase (^{Meredith et al., 2004)}. A study on the effect of the polycarboxylate-based superplasticizer additives in the hydration process of C₃A showed that the molecules of the superplasticizer additive are preferably absorbed in the surface of the C₃A phase [^{Plank et al. (2006)}]. However, during initial hydration of the C₃A, the molecules of the superplasticizer additive can form complex organometallics by interlocking between the layers of the hydrated phases of the C₃A. The interlocking of the polycarboxylate molecules is not a desired process, as it decreases the dispersion effect of the cement particles. ^{Plank et al. (2010)} carried out a study on the interlocking mechanism of polycarboxylate between the layers of hydrated C₃A phases and the function of the sulfate ions present in the cement. They reported that a high concentration of sulfate ions in the aqueous solution favors the interlocking mechanism of the sulfate ions between the layers of the hydrated C₃A phases giving rise to the formation of AF_t and AF_m• phases.

Figure 3 Diffraction pattern of the binary systems at 28 days of treatment, manufactured with (A) SP1 additive, and (B) SP2 additive. 

Figure 3 also shows the DRX patterns for the binary systems 10CV, 15MK, and 45EGAH manufactured with additives SP1 and SP2. In this figure one can observe that the 15MK paste showed a greater pozzolanic activity due to the reflection intensity of the CH at 34.3º from an angular position 2θ decreasing in a larger proportion with regard to the other pastes; this was associated with a greater CH consumption. It was also possible to identify the stratlingite-Ca₂Al₂SiO₇.8H₂O (angular position 2θ at 7.045º and 31.08º), phase from the AF_m phase family, a characteristic of the hydration products of cement pastes replaced with MK (^{Janotka et al., 2010)} and the hydration of EGAH (^{Martinez Alvarado, 2009)}. The ettringite phase was also identified, whose intensity was slightly greater in the 15MK paste with SP2 additive. The formation of ettringite is evidently due to the preferential reaction of the MK aluminates with the SO₄ ^2- ions of CP [^{Talero, (2005)}]. In the same patterns, the presence of C₃S and β-C₂S was also observed, indicating the incomplete reaction of the phases of Portland cement, at least up to the first 28 days of treatment. Another of the identified phases was hydrotalcite (angular position 2θ at 11.81º), which was observed in the 45EGAH pastes with both additives. The reflective intensity of the characteristic peak of this phase was slightly greater in the 45EGAH paste with additive SP1. The formation of this phase is attributed to the hydration of EGAH through the activation caused by the presence of CH. ^{Haha et al. (2011)} reported that for an alkaline activated slag, hydrotalcite is observed intermingled with the C-S-H and MgO that contained the slag as one of the products of the reaction.

Figure 4A shows the diffraction patterns of the CP pastes and the CP pastes replaced 50% with CV, MK, and EGAH and manufactured with SP1 at 28 days of treatment. Based on the results, it is possible to consider a high consumption of calcium hydroxide caused by the pozzolanic reaction of CV and MK and by the hydration of the EGAH. Crystalline phases were also identified, which are characteristics of the hydration products of MK and EGAH, such as stratlingite and hydrotalcite, respectively. Furthermore, the analysis of the CP pastes replaced with CV, MK, and EGAH indicated that paste 10CV-35EGAH-5MK showed reflections characteristic of stratlingite (angular position 2θ at 11.65º and 34.88º) and hydrotalcite, and in paste 10CV-30EGAH-10MK only stratlingite was present. Based on these results, it is possible to consider that with a replacement level of 15% MK in the Portland cement pastes replaced with CV, EGAH and MK, the formation of stratlingite is favored due to the pozzolanic activity of MK at early stages. It is also suggested that the formation of hydrotalcite is due to the activation of EGAH with CH and is favored at 28 days of treatment in the system with greater EGAH replacement.

Figure 4 Diffraction pattern for the CP pastes replaced 50% with CV, EGAH, and MK using additive a) SP1 and b) SP2 at 28 days of treatment. 

Figure 4 B shows the diffraction patterns for CP pastes and CP pastes with 50% replacement with CV, MK, and EGAH manufactured with SP2 at 28 days of treatment. According to the analysis of this information, SP2 had a slightly different effect in the hydration process of these cement pastes when compared to SP1. Whereas in the diffraction patterns of the CP pastes, reflections characteristic of weak intensity ettringite were identified, which was not observed in the CP paste manufactured with SP2. In pastes 10CV-25EGAH-15K, 10CV-30EGAH-10MK, and 10CV-35EGAH-5MK, the reflections characteristic of stratlingite were identified, but not those for ettringite and hydrotalcite. These results suggest that the SP2 was preferably absorbed in the C₃A phase and the reaction of the sulfate, calcium, and aluminum ions in the aqueous solution promote the formation of ettringite during the initial hydration. Moreover, the interaction of the SP2 polycarboxylate molecules with the amorphous silica-alumina fraction favors the formation of stratlingite during the initial hydration of the cement pastes. In pastes 10CV-30EGAH-1OMK and 10CV-35EGAH-15MK, the presence of hydrotalcite was observed. According to these results, the SP2 additive favors the formation of the stratlingite phase and a greater consumption of CH in the pastes with a larger MK content at 28 days of treatment.

Figure 5 shows the diffraction patterns of the CP pastes 60% replaced with CV, EGAH, and MK at 28 days of treatment. In general, the tendency of the X-ray diffraction results for the system with 60% CV, EGAH, and MK manufactured with SP1 can be described as follows: The reflective intensity of the peak characteristic of the CH phase in an angular position 2θ at 34.3º decreased in greater proportion in pastes 10CV-35EGAH-15MK and 10CV-40EGAH-10MK at 1 and 28 days of treatment, which contained a greater percentage of MK replacement, demonstrating the strong pozzolanic activity of MK. Stratlingite, ettringite, and hydrotalcite were identified at 1 and 28 days of treatment. Stratlingite showed a weaker intensity in paste 10CV-45EGAH-5MK. By using the SP1 additive and comparing the results of 50 and 60% replacement, the presence of the hydrotalcite phase could be observed in the latter. Carrying out the same comparison, but for pastes with SP2, it can be observed that ettringite was not identified at 28 days of treatment and no stratlingite reflections were present in pastes 10CV-40EGAH-10MK and 10CV-45EGAH-5MK at 28 days of treatment.

Figure 5 Diffraction pattern for the CP pastes 60% replaced with CV, EGAH, and MK, using additive a) SP1 and b) SP2 at 28 days of treatment 

3.3. Conduction Isothermal Calorimetry

Initially, superplasticizer additives were tested in the CP pastes. Subsequently, based on the replacement percentage and the RC results, systems 10CV-35EGAH-5MK and 10CV-45EGAH-5MK were selected and manufactured with both additives. According to the RC results, the pastes with SP1 additive developed the best RC, therefore another paste was selected for this additive, paste 10CV-30EGAH-10MK. Finally, a comparison between the effects of adding HS instead of MK in the pastes manufactured with the SP1 additive was carried out. Figure 6 shows the results of the isothermal calorimetry analysis of the CP pastes with and without additives at 25ºC.

Figure 6 Progression curves for hydration heat and total heat released from the CP pastes without additives and those with additives SP1 and SP2. 

It can observed that the main peak of the evolution curve of the hydration heat of the CP paste without additives, due to the hydration of the C₃S, corresponded to 9 hours of hydration time. By comparing this curve with those obtained for the CP with both additives, it is possible to observe a displacement of the same at the main hydration peak, which corresponded to 11 hours, showing a delay in the hydration process. It has been reported that the superplasticizer additives delay the hydration of silicates (especially C₃S). ^{Lothenbach et al. (2007)} found that the polycarboxylate based superplasticizers delay the dissolution of C₃S, and therefore the formation of portlandite and C-S-H. Furthermore, a difference was observed in the start of the acceleration period between the CP pastes without additives and those that contained SP1 and SP2. The initial hydration time for the acceleration period for the CP pastes was 2.5 hours, whereas for pastes with SP1 it was 4 hours and for those with SP2 3 hours. This difference was attributed to the fact that the introduction period in the presence of both additives was longer, providing more time for the hydration reaction of C₃S in order to form C-S-H. It is also observed that the progression of the hydration heat (J/Kg-h) and the release of total heat (Q/Kg) of the CP paste with SP2 was greater than that of the CP paste without additive and that with SP1. The acceleration period due to the nucleation and growth of the C-S-H gel and CH for the pastes with SP2 was more exothermic.

From these results it is possible to infer that the superplasticizer additives change the hydration mechanism of the CP pastes; however, each additive also showed a different behavior in pure cement. In this way, additive SP1 prolonged the induction period, which can be related to the dispersion effect caused by the observed aversion to the presence of the ether functional groups of the polycarboxylate-ether based superplasticizers, associated with the presence of the vinyl ether functional group shown by SP1. Additive SP2 caused the induction period to be shorter than that of the CP paste with SP1, which can be attributed to the lesser absorption of the polycarboxylate molecules on the surface of the cement particles. The SP2 additive showed two additional absorption bands characteristic of carboxylate salts. It has been reported that high density and a long lateral chain polycarboxylate-based superplasticizer additives decrease the delay of the induction period due to the polymer molecules being absorbed in a smaller ratio in the cement particle. It is possible that the rapid progression of the hydration heat during the acceleration period of the hydration process of the CP paste with SP2 is related to the low absorption of the polymer molecules on the surface of the cement particles, due to a larger spread of Ca⁺ ions and water from the liquid phase to the solid phase, which could have generated more sites of nucleation and growth of the C-S-H gel. ^{Mollah et al. (2000)} proposed three different mechanisms in order to explain the retardant effect of the polycarboxylate-based superplasticizers on the hydration of the cement, of which it is possible to associate the effect of SP1 and SP2 to two of them: (a) the molecules of the superplasticizer are absorbed on the surface of the cement particle and hinder the spread of water and calcium ions on the cement-solution interphase; however, the absorption in C₃S is less than in C₃A; (b) the spreading action of the superplasticizer changes the growth kinetic and morphology of the hydrated phases. According to the DRX results, one of the hydration products formed in the CP paste with SP1 at 28 days of treatment was ettringite; this phase could have been formed during the induction period, due to the fact that the absorption of the polycarboxylate-ether molecules of SP1 reduced the spread of Ca⁺ ions and water from the liquid phase to the hydrated calcium silicate phase, and it is possible that the hydrated calcium aluminate phase and the SO42- ions of the aqueous solution were favored.

Figure 7 compares the progression curves of the hydration heat for pastes 10CV-35EGAH-5MK and 10CV-45EGAH-5MK manufactured with SP1. The progression of the hydration heat for pastes 10CV-35EGAH-5MK and 10CV-45EGAH-5MK was less than that for the CP pastes without additives and those with SP1. It has been reported [^{Langan et al. (2002)}; ^{Snelson et al. (2008)}] that the progression of the hydration heat of cement pastes with EGAH, CV, and MK was less than that of the CP paste and are related to the higher water requirement of the pozzolanic materials and to the lesser availability of Ca⁺ ions for the growth of CH and C-S-H.

Figure 7 Progression curves for hydration heat and total heat released from the CP pastes without additives, with SP1, and pastes replaced with CV, EGAH, and MK manufactured with additive SP1. 

Another difference of the pastes 10CV-35EGAH-5MK and 10CV-45EGAH-5MK with regard to the CP paste is the formation of an additional peak in the hydration heat progression curve during the acceleration period. ^{Talero and Rahhal (2009)} reported a third peak in the hydration heat progression curves of the CP pastes replaced with 20% MK, to which they associated the pozzolanic activity of Al₂O₃ ^r- of MK. ^{Escalante (1996)} also reported the appearance of a third peak in the hydration heat progression curve due to the contribution of the EGAH to the hydration process at temperatures of 10 to 60ºC. It is also possible to relate this increase in progression of the hydration heat with the formation of hydration products such as stratlingite and hydrotalcite, favored by the activation of the EGAH the formation of hydrotalcite was favored, whereas the pozzolanic activity of MK favored the formation of stratlingite and based on the RC results, it was also higher for paste 10CV-35EGAH-5M manufactured with SP1.

The delay in the hydration process of pastes 10CV-35EGAH-5MK and 10CV-45EGAH-5MK with regard to the CP pastes without additives and those with SP1, can be due to a slow dissolution of the C₃S for the production of C-S-H and CH. However, this effect was more significant for the paste 10CV-45EGAH-5MK, given that its main peak was observed at 12.5 hours, in comparison with the paste 10CV-35EGAH-5MK that occurred at 8 hours. This difference is likely due to the greater EGAH content and the retardant effect of the CV. According to ^{Hwang and Shen (1991)}, the increase in the quantity of EGAH reduces the hydration heat produced by C₃S and C₃A. ^{Langan et al. (2002)} found that during the first minutes of hydration for CP pastes with 10 and 20% CV, the ion concentration in the solution for the first couple of hours decreases, which is why the nucleation and growth of the C-S-H- and CH is delayed.

The results of the isothermal calorimetry at 25ºC for pastes 10CV-35EGAH-5MK and 10CV-45EGAH-5MK with SP2 as an additive are shown in Figure 8. The progress of the hydration heat for these pastes was less (60 and 50 J/Kg-h, respectively) than that of the CP pastes with SP1 (190 J/Kg-h) and without it (115 J/Kg-h), due to the aforementioned. These curves also showed a third peak due to the MK pozzolanic activity and the hydration of the EGAH. Furthermore, the delay of the hydration process during the induction period was less for these systems, which can be related to the lesser absorption of the SP2 molecules on the surface of the cement particles.

Figure 8 Progression curves for hydration heat and total heat released by the CP paste without additives, with SP2, and pastes replaced with CV, EGAH, and MK manufactured with SP2 

The results of the isothermal calorimetry analysis for the CP paste with SP1 are shown in Figure 9, comparing the behavior of an additional system manufactured with 5 and 10% HS. It can be observed that the hydration reactions of pastes 10CV-35EGAH-5HS and 10CV-30EGAH-10HS develop less hydration heat than paste 10CV-35EGAH-5MKS. This relates to the RC results in the presence of MK it generated the best values at early stages. The decrease in the hydration heat can be due to: (1) the precipitation of the reaction products in the CP pastes with MK was less exothermal than in the CP pastes, and (2) due to the smaller quantity of cement utilized it is possible for there to be a smaller quantity of hydration products formed in the initial stages [^{Janotka et al. (2010)}]. The total heat released by the paste 10CV-30EGAH-5HS was greater than that of the paste 10CV-35EGAH-5MK, and it can be associated to the fact that during the hydration of paste 10CV-30EGAH-5HS the deceleration period was prolonged favoring the hydration process of the CP and that both the pozzolanic reaction of the CV and HS as well as the activation of the EGAH were more exothermal.

Figure 9 Progression curves for hydration heat and total heat released heat by the CP paste with SP1 and the pastes replaced with CV, EGAH, MK, and HS manufactured with SP1.