The soundness of cement, the expansion rate and expansion stress of hardened cement paste(HCP) were tested. The effects of various mineral additives such as fly ash, blast furnace slag and silica fume on the action of free lime in cement, and the mechanism of their effects on the cement soundness and expansion behaviors of HCP have been studied and discussed. The results show that the additives such as fly ash with lower hydration reactivity can not improve the soundness effectively, it has only a dilution effect of additives. When the expansion stress is larger than cohesive force, the apparent volume of HCP will expand. If the fly ash with high free lime is used as mineral additive, the use level must be strictly controlled.