This paper studies the expansion laws of steel slag-cement composite cementitious material, and establishes the mathematical model of hydration expansion of steel slag-cement composite cementitious material. Xinyu converter steel slag was mixed into the cement to prepare 25 × 25 × 280 mm steel slag-cement pastes. After boiled and autoclaved tests, the volume changes before and after the steel slag-cement pastes were measured to verify the accuracy of the established mathematical model. The experimental results show that when the content of steel slag is lower than 30%, the hydration expansion of Xinyu converter steel slag conforms to the established mathematical model. When the content of steel slag is 40%, the measured expansion ratio is 10.4% larger than the calculated expansion ratio of the mathematical model; when the content is 50%, due to a large number of internal micro-cracks, the measured expansion ratio is 19.5% larger than the calculated expansion ratio of the mathematical model. When the steel slag content reaches 60%, the steel slag-cement pastes warps seriously, and the mathematical model is no longer applicable to this situation. The mathematical model can accurately predict the expansion ratio when the steel slag content is less than 30%. Therefore, it can be concluded that when the steel slag content is less than 30%, there are only a few cracks in the steel slag-cement pastes, which is of good stability. So the results show that the maximum content of steel slag to prepared the steel slag-cement pastes is 30%.
