6533b871fe1ef96bd12d0c78

RESEARCH PRODUCT

Interaction of CaSO4 with water vapour at high relative humidity

subject

description

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 of a deliquescent salt (NaCl) is added to the initial sample, the same phenomenon is observed when relative humidity reaches the deliquescent point of the salt. After return to ambient conditions, X-ray diffraction shows that in all cases the sample is essentially made of gypsum with only traces of not dissolved hemihydrate.