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RESEARCH PRODUCT

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

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26 pages; 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 hydration-dehydration 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)