Effect of Water–Solid Mixing Sequence and Crystallization Water of Calcium Sulphate on the Hydration of C(3)A

Tricalcium aluminate (Ca(3)Al(2)O(6): C(3)A) is the most reactive clinker phase in Portland cement. In this study, the effect of the sequence of mixing of C(3)A with gypsum and water on the hydration kinetics and phase assemblage is investigated. Three mixing sequences were employed: (i) Turbula mixing of C(3)A first with gypsum and then with water (T-mix); (ii) Hand mixing of C(3)A with gypsum before mixing with water (H-mix); (iii) Pre-mixing gypsum with water and then with C(3)A (P-mix). The results suggest that there is a considerable difference in the hydration kinetics and hydrate phase assemblage, particularly during the initial stages of hydration. P-mix promotes a higher degree of hydration in the initial minutes and considerably influences the main peak in the calorimetry curve of C(3)A hydration. Effects of calcium sulphate with different amounts of crystallisation water (anhydrite, hemihydrate and gypsum) on C(3)A hydration are also investigated, and it is found that the water of crystallisation does not have a significant impact on the kinetics of reaction or the formed hydrate phase assemblage.