Effect of Calcium Sulfate on the Formation of Ettringite in Calcium Aluminate and Sulfoaluminate Blended Systems

The formation of ettringite and the expansion associated with it were studied on paste made from Portland cement, two Al-bearing compounds such as calcium aluminate cement and calcium sulphoaluminate cement, and calcium sulfate with different reactivities (anhydrite, a-hemihydrate and gypsum). The hydration process and formation of ettringite in the binders with different content of calcium sulfate was investigated by free expansion testing, isothermal conduction calorimetry, X-ray diffraction (XRD) and environmental scanning electronic microscope (ESEM). Results showed that the reactivity and amount of the calcium sulfate determined the balance between the hydration products of ettringite and monosulfate, and also the early hydration kinetics including the formation content, size and location of ettringite. It was also found that all the ternary systems with higher addition of calcium sulfate expand significantly. When a higher content of a-hemihydrate was added, besides ettringite much secondary gypsum also forms in voids between cement granules, which exerted adverse effects on the properties of ternary systems. However, the formation content of ettringite appeared to have no connection with the expansion value.