The ternary system Portland cement–calcium sulphoaluminate clinker–anhydrite: Hydration mechanism and mortar properties

Binders composed of ordinary Portland cement (OPC), calcium sulphoaluminate clinker (CSA) and anhydrite (CS¯) were examined in order to study the impact of variations of the OPC:CSA:CS¯ ratio on the hydration process and related mortar properties. A first sample series had various anhydrite contents and fixed OPC to CSA ratio, and a second various OPC contents and fixed CSA to CS¯ ratio. Experiments made on pastes and thermodynamic modelling showed that the phase assemblage formed during the hydration of the binders was not very sensitive to changes in modal composition, while the ettringite to monosulphoaluminate volume ratio was influenced. All mixes started to hydrate with the formation of ettringite during a reaction involving C4A3S¯ and calcium sulphate. This generated high early strength. Until about 7 d, mainly the CSA clinker reacted, and 15–20% of the dry binder was converted to ettringite. From about 7 d on, the OPC clinker phase alite reacted significantly, strätlingite, C–S–H and monosulphoaluminate formed, while the ettringite content decreased. According to the laboratory experiments, the CSA clinker was mainly responsible for the early mechanical properties, while OPC played an important role at later ages.