MgO in clinker promotes the stable growth of alite in favor of the occurrence ofM3. Mg ions go into solid solution in alite by simply replacing the Ca‐sites in the structure. While the concentration of Mg in alite increases in proportion to the amount of MgO in the clinker, the concentration of Al, Fe, and Ti in alite remains constant. It is thus the Mg ions occupying the Ca‐sites that give rise to the occurrence ofM3at ambient temperature. By contrast, SO3encourages the growth instability of alite. The concentration of impurity elements in alite tends to increase with increasing amount of SO3in clinker. This is most remarkable for Al and seems closely connected with the occurrence ofM1at ambient temperature. The presence of S was clearly recognized in al