Interaction of silicate and aluminate reaction in a synthetic cement system: Implications for the process of alite hydration

Synthetic cement with different sulfate additions is investigated by means of heat flow calorimetry and quantitative in-situ XRD. This study especially analyses different sulfate additions between a typical undersulfated and a properly sulfated white Portland cement. A series of sulfate additions was monitored, and the interference of aluminate and silicate reaction was observed. This interference has never received closer examination before. It is shown that while interference occurs at different times during hydration, the reaction turnover of all synthetic cements is the same after 48 h. The results indicate that negative effects on silicate hydration are likely due to the precipitation of AFm phases and not an Al-inhibition process. The accelerating effect of sulfate on alite hydration is most likely caused by a seeding effect of the very fine ettringite that has just precipitated, resulting in more substrate surface for C-S-H precipitation in comparison to pure alite paste.