Study On The Hydration And Hardening Process And Mechanism Of Belite-Barium Calcium Sulphoaluminate Cement

Belite-barium calcium sulphoaluminate cement is a new kind of binding materials. In this paper, mechanical performance and the influence of curing temperature on mechanical performance of the cement are studied. The phase compositions, microstructure and morphology of cement paste are analyzed mainly by means of x-ray diffraction and scanning electron microscopy. The expansion and dry shrinkage performance and pore structure of hardened paste of the cement have been measured by using the length instument and mercury porosimeter. Its heat liberation rate and durability such as impermeability, sulphate resistance, and so on are discussed.The mineral phases of the cement clinker are mainly C3S, C2S, C3A, C4AF and C2.75B1.25A3S . The C3S amount of the cement is less than that of Portland cement, and the C2S amount is more than that of belite cement. The quantity and the rate of heat liberation are studied, and the results showed that the hydration process of the cement can be divided into five stages: the pre-induction period, the induction period, the acceleratory period, the deceleratory period and the stabilization period. The rate of heat liberation is high in pre-induction period, and the induction period is lasted for a long time. The curve of the heat liberation rate is sharp and the rate is high in the acceleratory period. With increasing of gypsum content, the rate of heat liberation is increased in the pre-induction period, and the induction period is lasted longer, but the quantity of heat liberation is basically the same, which is between Portland cement and belite cement.The mechanical performance of the cement is studied. Results show that: the compressive strength achieves maximum with 10% gypsum addition. It reaches 45.0, 61.9, 82.1 and 85.6 MPa at 3d, 7d, 28d and 90d curing ages, respectively. The cement has a high early and long-term strength. The influence of curing temperature on the mechanical performance is studied. The result shows that curing temperature influences greatly the early compressive strength and slightly the long-term compressive strength of the cement. The hydration products are mainly AFt contained barium, Ca(OH)2 and C-S-H gels. Its types are basically the same with Portland cement and belite cement. With increasing of gypsum addition, the AFt and Ca(OH)2 amount grows and the hydration degree grows. Compared with Portland cement, the Ca(OH)2 amount is less and the performance of the cement is better. With heightening of curing temperature, the amount of AFt grows, the hydration degree grows and the structure of the hardened cement paste is dense at early hydration age, and the amount of C-S-H gel grows at late hydration age. At 1 day curing age, the boundary of the clinker grain is clear. At the same time some fine AFt crystal grows from the surface of grain. The boundary of clinker grain becomes misty and AFt crystals grows up and interweaves each other to build a cement skeleton when the cement hydrates 3 days. A large number of flower-like C-S-H gels is produced and covers the crystals such as AFt and Ca(OH)2, and cements with each other when the cement hydrates 28 days. The hydration level at early age is slightly lower than Portland cement, but its growth rate is high and it exceeds Portland cement at 15 days. The hydration level and hydration rate far surpass belite cement at any hydration age.Belite-barium calcium sulphoaluminate cement has certain volume expansion at the process of hydration and hardening, and the rate of expansion is higher than Portland cement. The rate of expansion increases fast at early hydration age (within 14 days), and becomes steady at late hydration age. The expansion rate is increased gradually with increasing of gypsum content. The dry shrinkage rate of the cement mortar is lower than that of Portland cement and it is decreased with adding gypsum content.Belite-barium calcium sulphoaluminate cement has excellent impermeability, particularly when gypsum content is 10%. The cement has excellent resistance to sulphate attack, and the ratio of sulphate resistance is higher than 1.00 and it is increased with adding gypsum content. The volum soundness of the cement is good under the high gypsum addition condition.