Fig. 2: Key SAXS/WAXS results of all four dehydration experiments.

Upper row a, c, e, g: pairs of 2D SAXS and WAXS maps of unreacted Volterra alabaster for low- and high-stress experiments (see map titles) at T = 129 °C ± 2.5 °C (a, c) and T = 142 °C ± 2.5 °C (e, g). The colour scale maps the decadic logarithm of raw intensity in the interval [0; 6] where blue denotes low values and red high ones. Characteristic gypsum reflections are annotated and marked with white arrows, indexed according to space group C2/c³². Centre row b, d, f, h: pairs of 2D SAXS/WAXS maps for the reacted samples. Selected hemihydrate reflections are annotated and marked by white arrows, indexed in space group I2³¹. i, j Conversion curves as a function of time showing reaction progress for low- (white circles) and high-stress (grey squares) experiments at T = 129 °C ± 2.5 °C on the left (i), and at T = 142 °C ± 2.5 °C on the right (j). Error bars for time and conversion fraction are smaller than the symbol size and thus not shown. Orange markers indicate datapoints at induction time (α = 0.05) and green ones those at the characteristic time (α = 0.63). The orange and green double arrows above the plot boxes indicate the differences in induction and characteristic times, respectively. The induction time is often interpreted as the time required for the formation of critical nuclei and growth to detectable size while the period thereafter involves crystal growth from those nuclei^33,34. The characteristic time denotes the inverse of the kinetic rate constant for exponential conversion models^35,36.