Interaction of CaSO4 with water vapour at high relative humidity

16 pages. Prepublication en attente d'un complément expérimental; Soluble anhydrite β-CaSO4 reacts with water vapour at room temperature to form CaSO4-0.5H2O at 20-30% RH and CaSO4-0.6H2O at 70-80% RH. At higher RH, additional water uptake can only add-up to an adsorbed water layer. The present work provides evidence that a small amount of the adsorbent dissolves in the adsorbed layer and gypsum starts precipitating at 97-98 %RH even though condensation of the vapour phase does not take place. After several days the adsorbent is almost entirely and irreversibly converted. The kinetics of weight gain clearly discriminates between adsorption and dissolution-precipitation. If a minute amount o…