In this study a comparative investigation for the effect of $K_2SO_4$ and $CaSO_4$ on the decomposition of $C_3S$ was made. When pure $C_3S$ which was synthesized in the laboratory was mixed with $K_2SO_4$ and oxides such as MgO $Al_2O_3$ and $Fe_2O_3$ and then reburned at the temperature range between 135$0^{circ}C$ and 145$0^{circ}C$ no decompo-sition occurred, But when $CaSO_4$ and $Fe_2O_3$ were added to $C_3S$ and then reburned at below 130$0^{circ}C$ $C_3S$ was partly decomposed to $C_2S$and CaO composing $2C_2S$.$CaSO_4$ When $CaSO_4$ and $Al_2O_3$were added $C_3S$ was entirely decomposed to $C_2S$ and CaO at 1300~140$0^{circ}C$ but it was not decomposed at 145$0^{circ}C$