Early-Age Hydration Reaction of Cement Mortars with the Participation of Aluminosilicate Microsphere Fractions

The paper presents the influence of two fractions (M1 and M2) of aluminosilicate microspheres in the amount of 0.0%, 1.5%, 3.5%, and 5% as a cement substitute on the early heat of hydration of cement mortars assessed by isothermal calorimetry (IC). The study results showed a higher accumulated heat of hydration for cement mortars with 1.5% of the test material for both grain fractions. In turn, mortars with 3.5% and 5% of smaller grains (M1) released more heat of hydration than samples with larger grains (M2), which have a less reactive glass phase. The proportion of finer fraction results in higher consumption of calcium hydroxide after seven days, which reflects the pozzolanic reactivity and, consequently, a denser microstructure compared with mortar samples containing grains with a larger diameter of the tested material, as indicated by phase composition tests (XRD). SEM/EDX analyses of the hydrating particle surface (M1) in cement mortar reveal a lower Ca/Si atomic ratio and higher Al/Si in C─ S─ H than larger diameter particles (M2).