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. Drying conditions may also influence secondary ettringite formation because the structure of the monosulfate is changed. The present study experimentally examined the effects of monosulfate pre-drying conditions and coexisting materials on secondary ettringite formation. Synthesized monosulfate samples dried under varying conditions and with coexisting materials such as C-S-H were mixed with a sodium sulfate solution, and the amount of secondary ettringite was found to increase following drying under 0% or 11% relative humidity. The secondary ettringite proportion was also markedly increased by the presence of C-S-H, regardless of the pre-drying conditions, 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.
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