Whereas Diamond (3) speculates that the nate glass present in fly ashes with more than 20% CaO is a calcium aluminate glass of 12 CaO•7 Al2O3 composition, the author believes (2) that, similar to the glass in blastfurnace slags which contain 35-40% CaO, the glass in high-calcium fly ashes is still an aluminosilicate glass with considerable calcium substitution. In any case, the glass in the high-calcium fly ashes is more reactive than the glass in low-calcium fly ashes.