Optimization of gypsum and slag contents in blended cement containing slag

Abstract Increasing the gypsum and slag contents in the blended cements containing slag is conducive to promote its green degree. The blended cement was prepared by mixing slag, gypsum and Portland clinker. Contour maps were designed to determine the optimal proportion of slag, gypsum and clinker by comprehensive considerations of strength, expansion, hydration, microstructure and crystallization stress. The strength contour map shows that there are two separate regions with high cement strength in the ternary slag-gypsum-clinker system; and the expansion deformation of these two regions are relatively low. The Rietveld XRD and pore solution analysis reveal that the occurrence of the destructive expansion results from the high crystallization stress of ettringite. It is illustrated that the gypsum content could be increased up to 25% when the clinker content is less than 5%, while the gypsum content should be limited below about 10% when the clinker content is more than 20%. The [CaO][SO3]/[Al2O3] molar ratio is suggested to direct the optimization of the slag, gypsum and clinker in blended cement. As long as the [CaO][SO3]/[Al2O3] ratio is not up to 1.5, more gypsum and slag could be added into the blended cement to obtain relatively excellent performance.