1. PROCESS FOR MAKING SOREL CEMENT WHICH COMPRISES CALCINING A PORTION OF MAGNESIUM HYDROXIDE RECOVERED BY PRECIPITATION FROM A MAGNESIUM SALT-CONTAINING SOLUTION, TO MAGNESIA HAVING AN IODINE NUMBER OF FROM 4 TO 8, CALCINING ANOTHER PORTION OF SAID MAGNESIUM HYDROXIDE TO MAGNESIA HAVING AN IODINE NUMBER OF FROM 15 TO 40, ADMIXING SAID MAGNESIAS IN THE PROPORTIONS OF FROM ABOUT 30% TO ABOUT 70% OF MAGNESIA OF IODINE NUMBER 4 TO 8 AND FROM 70% TO 30% OF MAGNESIA OF IODINE NUMBER FROM 15 TO 40, AND GAUGING SAID ADMIXTURE WITH A WATER SOLUTION OF AT LEAST ONE SUBSTANCE OF THE GROUP CONSISTING OF MAGNESIUM CHLORIDE AND MAGNESIUM SULFATE.