The invention provides an expansive material that allows for large expansion of concrete at a material age of 2 to 7 days after pouring and permits concrete to have higher initial compressive strength and makes sure good storage stability, and a process for preparing that expansive material.The invention provides an expansive material and a process for preparing that expansive material, characterized in that clinker or pulverized clinker containing free lime, a hydraulic compound and calcium sulfate anhydrite is heat treated in a carbon dioxide gas atmosphere to form calcium carbonate in it. Preferably, the expansive material contains a particle in which the free lime, hydraulic compound, calcium sulfate anhydrite and calcium carbonate are all present; the content of calcium carbonate is 0.5 to 10% by mass; and the expansive material has a Blaine's specific surface area of 1,500 to 90,000 cm2/g. The expansive material is blended with cement into a cement composition. The preparation conditions are preferably a carbon dioxide gas flow rate of 0.01 to 0.1 L/min. and a temperature of 200 to 800° C., under which conditions there is calcium carbonate formed.