PURPOSE: To provide a curing accelerator for concrete used for a tunnel, etc., and producing the effect even in a cold district when added by a small amt., by mixing 12CaO.7Al2O3 or 11CaO.7Al2O3.CaX2 (X is halogen) with 3CaOWSiO2.

CONSTITUTION: For example, 12CaO.7Al2O3 (C12A7), 11CaO.7Al2O3.CaF2 (C11A7 CaF2) and 3CaO.SiO2 (C3S) are synthesized with CaO, Al2O3, SiO2 and CaF2 commercially available first-class reagents. Starting materials for C12A7 are calcined at 1,650°C for 5hr, those for C11A7CaF2 at 1,400°C for 5hr and those for C3S at 1,700°C for 5hr. The calcined products are separately ground with ball mills and molded with 30t presses. Those operations are repeated three times to synthesize the minerals. C12A7 or C11A7CaF2 is mixed with C3S in a prescribed ratio to obtain a curing accelerator.