Very early age detection of ettringite from pozzolan origin

Abstract

The mineralogical composition of special purpose Portland cements must conform to certain requirements. Sulphate-resistant Portland cements, for instance, the tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF) phase’s contents must be very low to minimize the consequences of the hydration of large amounts of sulphates and the wide fluctuations in volume that would entail. At the same time, for economic, technical and environmental reasons, primarily pozzolanic additions, some of which are aluminic in chemical character, are included in Portland cements. When thermally activated clays such as metakaolin (aluminic pozzolan) are added to Portland cements with low (to nil) C3A content, an AFt phase of pozzolan origin may also form. The present study corroborated previous reports of the formation, but at very early ages (48h), of such an AFt phase of pozzolan origin, mainly, in a Portland cement blend containing minimal C3A content, an aluminic pozzolan in chemical character (metakaolin, M pozzolan) and excess gypsum, 15.05% (=7.0% SO3); and if C3A content in OPC is high, the AFt phase originated is of pozzolan origin as well, and in addition, of such C3A content, generating so greater or lesser Expansive Synergic Effect, ESE, according to the greater or lesser pozzolanic activity amount originated previously by M pozzolan in this case, and for this reason, being the ettringite from M pozzolan its chief direct and indirect cause. On the other hand, it also proved that such excess gypsum acted as a setting regulator in blended cements with metakaolin and a retarder in all the other cements tested, whether they were pure Portland cements or blends containing a crystalline mineral addition, Q, and also some blended cements with metakaolin can be regarded as being “hydraulic expansive cements” according to ASTM C 845-90 Standard.