Study On Preparation Of Alite-Strontium Calcium Sulphoaluminate Cement

More and more limitations of Portland cement is emerged out such as high sintering temperature, low early strength, poor durability, great pressure on the environment, etc. In order to improve the performance of conventional cement, strontium calcium sulphoaluminate (Ca_1.5Sr_2.5A₃(S|-) ) is introduced into Portland cement, which is named as alite-strontium calcium sulphoaluminate cement.The experiments were designed as follows. Experiments used chemical reagents as raw materials. Modulus value, KH, SM and IM were designed as 0.92, 2.5 and 1.5 respectively in this paper, and the content of strontium calcium sulphoaluminate were changed in order to find its optimal composition. Then the sinter temperature was determined. At the base of the optimizing composition and sinter temperature, excessive content of SO₃ and SrO was mixed into the clinker system to compensate SO₃ and BaO volatilization and solid phase dissolution. According to the analysis of f-CaO and mechanics performance, appropriate content of CaF₂ in the clinker system was found out. Next we studied the effect of gypsum on hydration degree and structure of hardened paste of alite-strontium calcium sulphoaluminate cement. Finally, industrial raw materials were used to make the optimal constituent cement clinker.Composition and microstructure of the clinker and the hardened cement paste were analyzed by XRD, SEM-EDS, Lithofacies of cement etc. The results show that strontium calcium sulphoaluminate and the minerals of Portland cement clinker can coexist in the clinker system. The optimal composition of alite-strontium calcium sulphoaluminate clinker is as follows: Ca_1.5Sr_2.5A₃(S|-) 9% (by the mass), C₃S 59.35%, C₂S 14.15%, C₃A 7.50%, C₄AF 10.00%. Strontium calcium sulphoaluminate mineral is well-developed when the sinter temperature is 1350℃-1380℃. The optimal excessive mass fraction of SO₃ and SrO are 50% and 80%. By the means of SEM-EDS and Lithofacies analysis, it can be seen that the growth of mineral crystals of Portland cement is incomplete, and the dimension of them is relatively small. This is caused by relatively low sintering temperature. Strontium calcium sulphoaluminate exists between the silicate minerals with small size. The compressive strength of the cement prepared under the testing conditions reaches to 32.8MPa, 67.7MPa and 120.4MPa at 1d, 3d and 28d curing age, respectively. The hydration products of alite-strontium calcium sulphoaluminate cement are mainly AFt, Ca(OH)₂, C-S-H gel, and the optimum content of gypsum is 9%. Industrial raw materials were used to prepare the cement later in the design process, and the compressive strength of the cement reaches to 22.5MPa, 43.0MPa and 98.0MPa at 1d, 3d and 28d curing age, respectively. Limestone of low-calcium and high-silicon (low-grade raw material) was used to prepare the cement and the compressive strength of the cement reaches to 24.5MPa, 56.1MPa and 102.2MPa at 1d, 3d and 28d curing age, respectively, which are little higher than these of cement with ordinary raw materials synthesis. The performance of alite-strontium calcium sulphoaluminate cement is better than that of Portland cement.