AbstractAmorphous silica influences tricalcium aluminate (C3A) hydration both in pastes and in suspensions.Two heat peaks are found by isothermal calorimetry during the paste hydration of C3A. The addition of amorphous silica causes the second heat peak to shift towards shorter reaction times and become more pronounced.In suspensions, the change in ion concentration in the water phase is not influenced by the presence of amorphous silica except that the change in concentration occurs more quickly. QuantitativeX‐ray analysis shows that more 3CaO.Al2O3.6H2O is present in suspensions containing amorphous silica than in silica‐free suspensions at equal hydration ti