Drying of a white porous limestone monitored by NMR imaging

Staining of porous limestone is a frequent problem observed when this kind of stone is used as floor pavement. From a chemical point of view, this staining originates from a reaction between alkalies (NaOH and KOH) released by the fresh mortar and absorbed by the stone and the organic elements always present in this kind of stone. Up to now, no other explanation of this phenomena has been given. CSTC and UCL showed that, in addition of the chemical potentiality characterised by the organics content of the stone, another factor had to be taken into account: the drying process of the material. A French limestone (Tercé Sculpture) sample has been saturated by capillarity, and its drying has been monitored by NMR imaging. NMR results were compared with the weight loss curve. The results have clearly shown the existence of two different stages in the drying process. The first stage corresponds to a constant drying rate period, and the second is characterised by a falling drying rate. NMR measurements led us to a better understanding of the physical meaning of these two stages. During the first one, evaporation occurs on the external faces of the sample. During the second one, evaporation occurs inside the sample. We concluded that the physical key factor in the problem of staining was the amount of water evaporated during the first stage of drying.