Insight into the influences of β-hemihydrate and dihydrate gypsum on the properties and phase conversion of white calcium aluminate cement

Abstract

It is still uncertain whether β-hemihydrate and dihydrate gypsum influence the properties, especially the phase conversion, of white calcium aluminate cement (CAC). Therefore, in this study, we prepared CAC pastes/mortars containing β-hemihydrate and dihydrate and comparatively assessed their setting times and compressive strengths. Simultaneously, the influence of β-hemihydrate and dihydrate on their chemical shrinkage, hydration-heat, pore size distributions, crystalline, and thermogravimetric characteristics was investigated. It is interesting that 0.5% β-hemihydrate and dihydrate significantly postponed the setting, whereas more than 0.5% β-hemihydrate and dihydrate slightly prolonged the initial setting and significantly shortened the final setting. Both β-hemihydrate and dihydrate decreased the compressive strength of CAC pastes hardened for 1 and 3 days to different extents; however, CAC pastes with a β-hemihydrate or dihydrate content less than 1% exhibited a high compressive strength at 28 days. Beyond this dosage, the compressive strength of the specimens was found to reduce. In addition, CAC pastes containing more than 0.5% β-hemihydrate or dihydrate exhibited high chemical shrinkage over 3–12 h. Furthermore, β-hemihydrate and dihydrate enhanced the gel micro and meso pores. They accelerated or decreased CAC hydration in the fast reaction and first deceleration periods, prolonged or shortened the induction period, enhanced or reduced the hydration heat before or after 12 h. These cases are closely associated to their contents. Their inclusion restricted CAH10 formation while promoting the generation of C2AH8, AFm, and AH3 and conversion of CAH10 to C3AH6 after 3 days. Besides, there are some significant differences in their influence on the properties and phase conversion of CAC pastes. For these differences, CAC pastes/mortars with β-hemihydrate and dihydrate presented different setting times and compressive strengths.