Influence of the calcium sulphate source on the hydration mechanism of Portland cement–calcium sulphoaluminate clinker–calcium sulphate binders

Two different binders composed of Portland cement, calcium sulphoaluminate clinker and calcium sulphate were studied from early hydration to 28 days, one containing gypsum and one containing anhydrite at equimolar CaSO4 amount. Sodium gluconate was used as retarder to obtain a sufficient fluidity to cast the samples. Solids were analyzed by X-ray diffraction, scanning electron microscopy and thermogravimetric analysis and quantified by Rietveld refinement, while pore solutions were measured by ion chromatography. Thermodynamic modelling was used to model the hydration process of the ternary binders. This combined study allowed a precise understanding of the hydration process over time and the determination of the composition of the crystalline and of the X-ray amorphous hydrates present in the binders, which cannot be determined by conventional methods. Results show that the hydration mechanisms are similar in presence of gypsum or anhydrite, the difference being in the kinetics of reactions due to the slower dissolution of anhydrite compared to gypsum in the presence of sodium gluconate. The hydration starts with the formation of ettringite and of some X-ray amorphous hydrates. In the anhydrite-bearing system, the ettringite-forming reaction is stronger delayed by the addition of the retarder compared to the gypsum-bearing system. This stronger delay results in the formation of a significant amount of X-ray amorphous hydrates. The hydrates amorphous fraction is composed of different phases and its chemical composition is changing over time. During early hydration, it is mainly composed of aluminium hydroxide and strätlingite, while in the anhydrite-bearing system it can additionally contain some monosulphoaluminate. At later ages, the aluminium hydroxide content decreases and additional monosulphoaluminate and a C-S-H type phase are formed.