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ISTerre is part of Labex OSUG@2020. Use of the Geochem- istry–Mineralogy platform at ISTerre is acknowledged. The authors wish to aknowledge the ESRF for provision of beam time (MA4498). We are also thankful to Dr Pierre-Olivier Autran for useful insights on tomographic reconstructions and Dr Marta Majkut for discussions on grain orientation calcu- lations. The authors also thank Dr Irina Snigireva and Dr Nathaniel Findling for SEM characterization of the samples and Dr Catherine Dejoie for complementary high-resolution powder diffraction data from the ID22 beamline of the ESRF., The mechanism of hydration of calcium sulfate hemihydrate (CaSO4 0.5H2O) to form gypsum (CaSO4 2H2O) was studied by combining scanning 3D X-ray diffraction (s3DXRD) and phase contrast tomography (PCT) to determine in situ the spatial and crystallographic relationship between these two phases. From s3DXRD measurements, the crystallographic structure, orientation and position of the crystalline grains in the sample during the hydration reaction were obtained, while the PCT reconstructions allowed visualization of the 3D shapes of the crystals during the reaction. This multi-scale study unfolds structural and morphological evidence of the dissolution–precipitation process of the gypsum plaster system, providing insights into the reactivity of specific crystallographic facets of the hemihydrate. In this work, epitaxial growth of gypsum crystals on the hemihydrate grains was not observed.