Hydration of quaternary phase-gypsum system

Since the quaternary (Q) phase (Ca20Al26Mg3Si3O68) shows a potential to improve early strength and reduce the shrinkage of Portland cement paste, understanding the hydration of Q phase with and without sulfate is of great importance. In this work, the hydration of Q phase-gypsum systems were investigated in terms of the kinetics, phases evolution and microstructure development. The test results show that the hydration kinetics, microstructure and composition of hydration products depends on the gypsum addition. The observation of heat evolution indicates that the hydration rate is accelerated by gypsum, and formation of ettringite (AFt) and monosulfoaluminate (AFm) is probably the controlling factor at early age. The main hydration products are AFt, AFm, calcium aluminate hydrate and AH3 phase. And the calcium aluminate hydrate and AH3 phase will continue to form as the hydration proceeds. The compressive strength increases significantly with increasing gypsum content, which is due to the increasing hydration degree of Q phase-gypsum samples. Microstructural study shows that the formation of AFm phases occurs in the space between the Q phase grains but also within the “inner” regions around the Q phase grains. The AFm platelets become small and the “inner” regions around grains disappear in the samples with high gypsum. In addition, the AFt or AFm is closely intermixed with AH3 and calcium aluminate hydrate, microcracks occur in samples with high gypsum content due to the formation of AFt.