From atom level to macroscopic scale: Structural mechanism of gypsum dehydration

Plaster, made of calcium sulfate hemihydrate (CaSO4•½H2O) and/or γ-anhydrite (CaSO4) is obtained by dehydration gypsum (CaSO4·2H2O) at 100–200 °C. When mixed with water, it dissolves while new crystals precipitate. Plaster's microstructures affect its reactivity and setting properties, but are poorly understood due to confusion between polycrystalline structures in the literature. Through a multi-scale approach combining X-ray microtomography, optical scanning electron microscopy, we revealed size, morphology orientation pores formed dehydrating single T = 105–130 °C pH2O 0–40 hPa. The expulsion water from (010) interlayers results three-level fragmentation: cracks form parallel (010), then solid divided into [101] needles (5–10 x 30–300 μm) micron-sized arranged fishbone pattern. same behavior observed whatever pH2O, however crystallites size increases pH2O.