Cyclic wetting-drying ageing test and patina formation on tuffeau limestone.

This paper presents the results of an ageing test based on cycles of wetting-drying of a French building limestone named tuffeau. During 3 years, 50 complete wetting-drying cycles with pure distilled water were carried out on the side of a cylindrical sample. The characterization of the stone shows that the ageing test applied induces changes in the surface appearance, with the formation of a patina with decrease in porosity and mineralogical changes due to dissolution, transport, and recrystallization of the dissolved mineralogical component of the rock as calcite. The decrease in the imbibition kinetics confirms the alteration of the tested stone sample. After 50 wetting-drying cycles, imbibition coefficients are almost halved. This alteration is not homogeneous as the patina covers only some sections of the exposed surface of the stone. On the well-formed parts of the patina with a brownish and varnished aspect, which represent about 20 % of the total surface, measurements show a decrease in porosity and roughness of about 25 %. The physico-chemical characterization shows that the patina formed in the laboratory is fully comparable to natural patina observed on monuments built of tuffeau. [ABSTRACT FROM AUTHOR]