Abstract

The expansion of cement paste can occur as a result of secondary ettringite formation due to attack by sulfates. It has been proposed that this expansion is affected by coexisting hydration products such as calcium-silicate-hydrates (C-S-H), although the mechanism has not yet been clarified. The present study experimentally examined the effects of coexisting materials on secondary ettringite formation. Synthesized monosulfate and coexisting materials such as C-S-H were mixed with a sodium sulfate solution. The secondary ettringite proportion was markedly increased by the presence of C-S-H, and calcium-aluminate-silicate-hydrates (C-A-S-H) containing four-coordinate Al were formed in these samples. It is evident that ettringite is likely to form from four-coordinate Al, and that C-S-H promotes secondary ettringite formation because it transitions six-coordinate Al derived from the monosulfate to four-coordinate Al by substituting Al in the Si chains.