Hydration of calcium sulphoaluminate clinker with additions of different calcium sulphate sources

Abstract

In these last decades the complete comprehension of mechanisms that affect the hydration of calcium sulphoaluminate cement (CSA) has become crucial in the optic of an extensive use of CSA based systems as promising low-CO2 binders alternative to ordinary Portland cement. In this study the hydrated phases evolution of cementitious systems based on CSA clinker, mainly composed by ye’elimite (C4A3$) phase, blended with two calcium sulphate forms (i.e. gypsum and anhydrite) has been investigated. The adoption of 27Al magic-angle spinning NMR spectroscopy led to identify and follow the evolution of all the main CSA hydrated phases (i.e. ettringite, monosulfate, aluminium hydroxide) supporting the X-ray diffraction technique in the amorphous phase resolution. Thermogravimetric analysis has been adopted as complementary technique in the hydrated phases identification and the kinetic of the hydration process has been monitored by means of isothermal calorimetry. Experiments made on pastes hydrated at different ages (8, 16, 24 and 48 h) showed system reactivity strongly dependent on sulphate ion availability. In presence of gypsum, more soluble, a higher ettringite phase formation with a consequent greater C4A3$ consumption was detected in the first hours of hydration (8 h) slowing down the kinetic of phases formation only at later ages (48 h). Results showed how the different sulphate ion availability modifies the kinetics of CSA hydration process as well as the morphology of hydrated phases. SEM observations confirmed that the high solubility of gypsum led to formation of poorly-crystallized hydrated phases, with a consequent more dense matrix, while in the presence of anhydrite, less soluble, a well crystallized needle like ettringite has been detected as main hydrated phase.