The influence of limestone powder (LSP) addition on the optimum gypsum (SO
3) content for Portland cement with different Al
2O
3 content was investigated. When setting time, compressive strength and size stability were considered, the optimum SO
3 content for cement having Al
2O
3 content of 3.5-7.2 mass% was varied from approximately 3 to 5 mass% positively depending on the Al
2O
3 content of cement. The influence of LSP addition on the optimum SO
3 content was negligible. By the analysis of combined water, porosity, and phase composition by XRD/Rietveld method, at less SO
3 content than the optimum content, with the increase of SO
3 content, the cement hydration was promoted mainly in Ca aluminate and the compressive strength increased. At the more SO
3 content, excess formation of expansive ettringite introduced more pores and compressive strength decreased. By the addition of LSP, the maximum compressive strength was obtained at lower SO
3 content. This is attributed to the hydration acceleration for silicate minerals and increase of carbo-aluminate hydrates content. This effect was significant for cement with high C
3A and C
4AF content.
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