Conservation of cultural stones: relationship among texture, thermal ageing and consolidation strategies in white Apuan marbles

A major problem in the conservation of stone materials concerns durability and the use of compatible and efficacy conservation treatments. Among cultural stones, white Apuan marble stands out for its wide diffusion in built environment; thus, the study of its decay and the recognition of smart solution for its conservation is a quite relevant issue. In spite of an aesthetical homogeneity, white marbles exhibit a range of textural features able to influence their durability. The aims of this study are therefore to evidence the relationship between decay and textural features in marbles, and to test the efficacy of some innovative nanostructured products in comparison to ethyl-silicate. Ten white marbles from different Apuan Alps quarrying areas, characterized by a range of textural features, were analyzed. Preliminary, textural, mineralogical and chemical analysis have been performed by petrographic, XRD, and XRF analyses; physical properties were determined by both capillarity and water absorption by total immersion tests on specimens. After an initial drying at 60±5°C, specimens were artificially aged by thermal stresses (cycles at 110°C, 110°C and cooling in water, 250°C, and 550°C). The comparison of physical parameters before and after thermal stresses allows evaluating the influence of some textural factors (especially grain boundaries and mean grain size) on the decay. Artificially thermal damaged samples were subsequently treated with two nanostructured products, nano-lime and nano-silica, and with ethyl-silicate, to compare the efficacy of innovative consolidants in respect to a traditional one. The efficacy was evaluated on the basis of water absorption behavior of marbles before and after treatments. The study evidenced better performances of nano-silica in respect to nano-lime, even ethyl-silicate confirmed its high effectiveness in consolidating marbles regardless their textural features.