Properties Of Sulphoaluminate Cement Modified By Portland Cement Clinker

Sulphoaluminate cement(SAC) is a special cement invented by Chinese in the last century. It has many excellent properties such as high early-strength, good volume stability and low emission of carbon dioxide. However, the application of SAC is restricted by the ultra-high price with respect to Portland cement(OPC) due to the demand of high quality of aluminum raw material in the cement clinker production. Besides, the pH of the main hydration products of SAC(AFt) is lower than that of OPC. The strength of SAC concrete decreased seriously when the concrete is carbonized. Therefore, it is essential to improve the properties of SAC. Introducing OPC clinker to SAC is an effective approach.Phosphogypsum(PG), which consists primarily of calcium sulfate dehydrate with minor amount of some impurities, is a harmful solid waste from the production of phosphoric acid by the wet process. Because of the presence of the harmful impurities, the utilization ratio of PG is extremely low. Some researchers have shown that PG has tiny influence on the properties of SAC, so it is chosen to be used in SAC and composite cement for the study of utilizing it in producing building materials.In this paper, ternary composite cement was prepared by SAC clinker, OPC clinker and PG. The influence of the content of OPC clinker on the normal consistency, setting time, compressive strength and drying shrinkage of composite cement was investigated. The hydration process was studied by the heat flow of cement. The results show that the adding of OPC clinker can accelerate the hydration of SAC with the same PG content. Besides, the water demand for the normal consistency increased and the setting time decreased. The initial early-strength was improved while the later strength decreased.The pH of hydration products of SAC is lower than OPC. It can be increased by adding some OPC clinker because the composition of the hydration products of SAC are changed. X-ray diffraction(XRD) patterns of the hydration products of SAC before and after the carbonization process indicates that the carbonization process of SAC is the process of the decomposition of AFt. Aragonite and gypsum formed after the decomposition of AFt. The strength of SAC decreased after the carbonization process. The ability of anti-carbonization of SAC is enhanced by introducing some OPC clinker to SAC. With 10 wt.% of OPC and 20 wt.% of PG. The carbonization coefficient of SAC concrete at 90 d increased from 73.12% to 85.45%, which is close to that of OPC. The load capacity of SAC after carbonization process is enhanced by the adding of OPC clinker.In order to study the influence of impurities of PG on the composite cement, PG was pretreated by water washed, lime neutralization and fast-fired. Compared with the original PG, SAC composited with washed PG and lime neutralized PG have a relatively higher early compressive strength, which indicates that the impurities in PG adversely influence on the early strength development of SAC. But when PG is pretreated with fast-fired process, the CaSO4·2H2O was decomposed and CaSO4·1/2H2 O with CaSO4 were formed. The water demand for the normal consistency of SAC and composite cement increased while the setting time decreased with the adding of fast-fired PG, at the same time the compressive strength also increased.