Irreversible modifications of the porous microstructure of soluble anhydrite β-CaSO4 induced by hydration-dehydration cycles

The internal surface area of hexagonal (soluble) primary anhydrite CaSO4 produced by dehydration of gypsum decreases by about 70 % when the anhydrite is successively subjected to rehydration and dehydration at room temperature in humid air and in vacuum respectively. In the rehydration step, the hemihydrate is formed; its dehydration yields secondary anhydrite. Additional hydration-dehydration cycles in the same conditions have a much smaller effect. The first cycle also brings about a modification of the t-plot, which reveals that the micropores of primary anhydrite are irreversibly healed. Mercury intrusion porosimetry shows that the primary dehydration of gypsum also generates an open porosity of 25-28% comprised of macro and mesopores with opening width ranging over 5 decades. The distribution of the finest mesopores only is slightly modified by subsequent hydrationdehydration cycles. The initial external volume of the gypsum crystals remains essentially unchanged throughout the successive reaction cycles (i.e. the pseudomorphs undergo neither swelling nor shrinkage).