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.