| The crystalline structure of ettringite, Ca6[Al(OH)6 ]2(SO4)3·26H2O, an important hydration product in the chemistry of Portland cement, was investigated using time-of-flight neutron diffraction combined with Rietveld crystal structure refinement. In addition, the changes in crystalline structure during the formation and thermal decomposition of ettringite were followed in situ, using specialized experimental assemblies. The diffusive motions of crystalline water within the ettringite structure were studied using incoherent quasi-elastic neutron scattering techniques.; The results of this investigation led to an improved model for the crystalline structure of ettringite, including a detailed understanding of the three-dimensional hydrogen bond network that extends throughout the structure. Structural changes were observed for ettringite specimens with thermal decomposition up to 30 wt. %. These studies showed that previous models of the dehydration process, based upon indirect experimental evidence, were incorrect. The formation of ettringite during the hydration of tricalcium aluminate (Ca3Al 2O6) in the presence of gypsum (CaSO4·2H 2O), for temperatures ranging from 25°C to 80°C, demonstrated that ettringite was the only crystalline hydration product, in contrast to a prior study which reported the presence of a precursor phase in the reaction pathway. An analysis of the kinetics of the hydration reaction showed that the rate law of the reaction changed markedly over the temperature range investigated. Approximately 12% of the hydrogen atoms present in a fully hydrated ettringite specimen were observed to perform a localized diffusive motion for temperatures between 200 K and 320 K, while an ettringite specimen dehydrated to a weight loss of 20 wt. % showed indications of translational diffusion for approximately 9% of the remaining hydrogen atoms. |
