Calorimetric study on the influence of calcium sulfate on the hydration of Portland cement–calcium aluminate cement mixtures

The influence of calcium sulfate with different reactivities (anhydrite, α-hemihydrate, and gypsum) on the Portland cement–calcium aluminate cement (PC/CAC) mixtures was presented in the paper. The hydration process and main hydration products (ettringite) of the binders with different content of calcium sulfate was investigated by isothermal conduction calorimetry, setting times, compressive strength, X-ray diffraction (XRD) analysis, and environmental scanning electronic microscope (ESEM) analysis. It is found that the pure PC/CAC mixture without any calcium sulfate addition exhibits very slow hydration kinetics during the first 2 days. By adding calcium sulfate, the setting of the PC/CAC mixture is delayed, but the hydration can be accelerated. The results also show that the reactivity and the amount of the calcium sulfate determine the balance between the hydration products of ettringite and monosulphoaluminate, and also the early hydration kinetics not only in the formation content but in the location of ettringite. In general, when a high content of reactive α-hemihydrate is added, much secondary gypsum forms in voids between cement granules which exert adverse effects on the properties of PC/CAC mixtures. Additionally, ettringite can be formed stably and good binders having good physical properties can be obtained when low reactive anhydrite is added.