Hydration of cubic tricalcium aluminate in the presence of calcined clays

Calcined clay blended cements play a major role in cement industry's strategy to reduce CO2 emissions. During their hydration, an accelerated aluminate reaction is often observed to affect the sulfate balance. The objective of this study was to provide insights into the influence of different calcined clays on the hydration of cubic tricalcium aluminate (C(3)A). A cementitious model system consisting of cubic C(3)A, quartz powder, calcium sulfate and a model pore solution was investigated. The influence of three different calcined clays and one nanolimestone was examined by a successive replacement of the quartz powder and variation of the calcium sulfate. Heat flow and hydrate phase development were followed by isothermal calorimetry and quantitative in-situ X-ray diffraction. Accelerated ettringite formation and sulfate depletion were observed for all calcined clays, while the nanolimestone exhibited the opposite effect. It was found that adsorption of SO4 ions and/or Ca-SO4-complexes at the surface of calcined clay particles is the main factor inhibiting retardation of the C(3)A hydration in absence of a silicate reaction. In the Al-rich systems a retardation through sulfate adjustment seems to be impeded by additional Al ions, which react with Ca adsorbed onto and leached from the C(3)A surface.