In situ neutron powder diffraction investigation of the hydration of tricalcium aluminate in the presence of gypsum

Abstract

The hydration of a 1:3 molar ratio of tricalcium aluminate, Ca 3Al 2O 6, to gypsum, CaSO 4·2D 2O, was investigated at temperatures of 25, 50, and 80 °C using time-of-flight powder neutron diffraction combined with multiphase Rietveld structural refinement. It was shown that ettringite, Ca 6[Al(OD) 6] 2(SO 4) 3·∼26D 2O, was the first and only hydration product of the system, in contrast to a prior investigation which suggested the occurrence of a precursor phase prior to the formation of ettringite. Kinetics data showed that the hydration reaction is very sensitive to temperature: hydration at 25 °C was characterized by a single kinetic regime while hydration at higher temperatures consisted of two distinct kinetic regimes. The presence of two kinetic regimes was attributed to a change in either the dimensionality of the growth process or a change in the rate controlling mechanism in the hydration reaction.