New insights into the role of highly reactive pozzolans in the early hydration process of C3S and C3A monitored by conductometry, calorimetry, phase composition and microstructure analyses

The presented research aimed at understanding the effect of a highly reactive pozzolan (HRP) on the early age hydration of alite (C₃S) and tricalcium aluminate (C₃A). Five supplementary cementitious materials of varying activity were used throughout the investigation. The hydration process was continuously monitored by examining the heat release rate of hydrating pastes (calorimetry) and the conductivity changes of alite and tricalcium aluminate hydrating suspensions (conductometry). Moreover, at specific time intervals, phase composition was analyzed using X-ray diffraction and thermal analysis, along with the liquid phase composition of hydrating suspensions (ICP-OES). The microstructure of hydration products was also observed using TEM and SEM. The results showed that the position of the ‘aluminate peak’ observed in the calorimetric curves is most likely the result of a synergy of two interactions: the effect of acceleration of C₃S hydration process and the effect of HRP on the hydration of C₃A. The lack of a linear relationship between the amount of heat released and the specific surface area indicated, that despite the significance of physical presence of SCMs, the chemical interactions within the system are more important. The C₃A reaction is associated with the release of a large amount of heat. In a hydrating C₃A system, the saturation of the solution with calcium hydroxide has a significant effect on the course of the reaction. During hydration in water, the degree of C₃A reaction is greater in the presence of HRP, whereas when the system reacts in saturated calcium hydroxide, the degree of C₃A reaction is lower. It seems that the factor accelerating the C₃A hydration is the nucleation of products on the surface of HRP grains, leading to the formation of a larger amount of hydrated calcium sulfoaluminates, with simultaneous acceleration of C₃A dissolution. In saturated Ca(OH)₂, HRP promotes the formation of ettringite and, most importantly, affects its stability in the solution. Silicate ions originating from the HRP are likely incorporated into the crystallizing phases. In both environments, HRP affects the formation of hydrogarnets (C₃AH₆).