Impact of sample preparation on the specific surface area of synthetic ettringite

The effect of sample preparation, including degassing conditions and method to stop cement hydration, on the specific surface area (SSABET) of synthetic ettringite (AFt) was studied. Dehydration of AFt occurs even at moderate temperatures under N2 flow (0% RH). However, partial dehydration associated to a loss of up to 12 water molecules does not imply a change in SSABET. A mild degassing 16 h at 40 °C under N2 flow is suggested mainly to preserve the chemical composition and morphology of calcium sulfate carrier. At higher temperatures, ettringite dehydration involves a significant modification of its crystalline structure and increase of the SSABET. Typical protocols to stop cement hydration, solvent exchange and freeze-drying, lead to larger water removal from the AFt and crystallinity reduction, being significantly higher when the latter is applied. However, only freeze-drying leads to a moderate increase of the synthetic AFt SSABET.