Alkaline Hydration of Tricalcium Aluminate

This study explored the behavior of laboratory-synthesized tricalcium aluminate (C3A) in alkaline media. In all the trials, the C3A was hydrated with water containing a certain percentage of alkali activators (4% Na2CO3, 4% Na2SO4, or 8% Na2SO4 by weight of C3A). Two- and 28-day mechanical strength values were determined and the products were characterized with XRD, DTA, and NMR. The compressive strength of the C3A pastes rose in the presence of the alkaline activators used in this study. The alkalis induced the formation of calcium monocarboaluminate (C4ACH11) as the main hydration product in all cases. In addition, when Na2SO4 was added, a sodium calcium sulfoaluminate, phase U (C4ASNH16), formed. The presence of this mineral, whose formation is associated with high pH values, furnished indirect information on the alkalinity in the medium.