In this study, we targeted a calcium hydroxide and ettringite composite-type expansive additive (EA) and obtained the basic characteristics of the EA mixed cement paste in order to create a method for calculating volume expansion based on the hydration reaction of the PCEA system. Expansion strain, setting time, and compressive strength were measured. In addition, we observed the state of EA after hydration using a micro-CT scanner and scanning electron microscope, and elucidated the expansion mechanism. Our results show that strain rapidly increased and the compressive strength decreased when the replacement ratio of the EA (Crep) exceeds 6.4%. At the same time, radial cracks were generated from the hydrated EA particles, suggesting that the EA itself grew to expand the cement paste. Although no ettringite was confirmed inside the EA when the Crep was 5.1%, when the Crep was 10.3%, ettringite was formed vertically from the surface of C3A with a large amount of pores. These results show that simultaneous formation of ettringite and pores causes rapid expansion
