This suggests that these two entities of gypsum have different crystal size distributions and (meso)structures, as we infer from other trends observed in Fig. . The changes in exponent D (Fig. ) represent the evolution of interface roughness and the presence of surface fractals . These interfaces originate from large crystals "outside" our q-range. Smooth Porod interfaces exhibit D = 4, while rougher surfaces have 3 < D < 4, with lower D values indicating higher roughness . Additionally, this parameter tracks the anisotropic evolution of crystals, where the roughness of the gypsum surface is direction-dependent before ~4000 s (T = 35 °C). This phenomenon further supports the mesocrystalline nature of freshly precipitated gypsum crystals (Fig. , stage I).