The model explaining the expansion mechanism of the cement added with an expansive additive is proposed on the basis of the measurement of autogenous volume change and the observation result of hydration products. It is inferred that the calcium hydroxide and ettringite generated from the expansive additive grow by entangling pores because the cement added with an expansive additive shows autogenous expansion while being accompanied by hydration shrinkage. It is suggests that calcium hydroxide forms on the surface of the expansive additive particles and ettringite on the surface of C3A and the expansive additive particles from the observation result of the hydration products. Based on these phenomena, the thickness of the hydration product layer was obtained by calculation. When a low addition-type expansive additive excellent in expansion property is used, chemical press effect is expected large compared with conventional expansive additives.
