Hydration mechanism of Portland cement – calcium sulphoaluminate clinker – calcium sulphate binders: characterization of the X-ray amorphous hydrate assemblage

摘要

Two different binders composed of Portland cement, calcium sulphoaluminate clinker and calcium sulphate were studied, one containing gypsum and one containing anhydrite. To obtain sufficient time to cast the samples, an organic retarder was used. A study combining various analytical techniques on solids and extracted pore solutions coupled to thermodynamic modelling was needed to fully understand the hydration process and to determine the composition of the crystalline and of the X-ray amorphous hydrates present in the binders. On that purpose, X-ray diffraction, Rietveld refinement, thermogravimetry, ion chromatography and thermodynamic modelling were applied. Results show that the overall hydration mechanisms are similar in presence of gypsum or anhydrite, the main difference being in the kinetics of reactions due to the slower dissolution of anhydrite compared to gypsum in the presence of retarder. The hydration starts with the formation of ettringite and X-ray amorphous hydrates. In the anhydrite-bearing system, the ettringiteforming reaction is stronger delayed by the addition of the retarder. Therefore, a significant amount of X-ray amorphous hydrates form before ettringite starts crystallizing. The volume of the hydrates amorphous fraction in the two ternary binders constantly increases. It is composed of different phases and its chemical composition is changing over time. During early hydration, it is mainly composed of aluminium hydroxide and strtlingite, while in the anhydrite-bearing system it can additionally contain monosulphoaluminate. At later ages, the aluminium hydroxide content decreases and additional monosulphoaluminate and C-S-H are formed.