Water as the Key to Expansion of Ettringite in Cementitious Materials

Damage in sulfate-bearing soils and aggregate systems stabilized with additives containing lime, including lime and portland cement, has drawn considerable attention over the past two decades. Researchers and practitioners have made considerable contributions to the understanding of the problem, including the mechanisms involved in the formation of the two minerals, ettringite and thaumasite, that are most often associated with this damage. This paper provides a case history analysis of the expansion history compared with the ettringite growth history of three controlled low-strength mixtures containing fly ash with relatively high sulfate contents. Samples were subjected to three curing conditions: a dry cure, in which only mixing water was available for curing; a moist cure, in which an external source of water was available for curing; and a sulfate cure, in which an external source of sulfate-bearing water was made available for the duration of cure. Ettringite was quantified by using both differential scanning calorimetry and X-ray diffraction; the resulting volume changes in the samples were measured. Results suggested that sorption of water by the ettringite molecule was at least part of the reason for expansion. The importance of sorption was based on the fact that although expansion increased in moist cure compared to dry cure systems, the quantities of ettringite formed under each regime were about the same and remained about the same throughout the experiments. As expected, samples exposed to sulfate cure responded with the greatest expansion, which was concomitant with continued ettringite crystal growth due to a supply of the limiting reagent, sulfate.