Implementation of a non‑destructive method to assess weathering deterioration of sandstones in cultural heritage

This paper proposes a non-destructive approach based on the Equotip hardness tester to assess weathering deterioration in a protected sandstone monument located in the historic centre of Camerino (Italy). The approach is tested on one sandstone column, where various forms of weathering, such as discolouration, scaling and loss of stone volume, are observed. The mechanical characterisation with Equotip was performed on 24 measuring points, systematically distributed in the column. Innovatively, the two probes available from Proceq (Proceq© 2010) were used to assess differences among surface and in-depth hardness values of the column. In addition, an un-weathered rock core from the original extraction site was also analysed and compared with the rock matrix of the column. The obtained results show a 15% hardness reduction from depth to the surface of the column and a 25% overall hardness reduction with respect to the fresh sandstone core samples. Equotip results were coupled with grain size analyses, mercury intrusion porosimetry, scanning electron microscopy and X-ray diffractometry results, and a correlation between hardness and grain size was evaluated. By combining these approaches, it was possible to identify the processes that occurred during weathering: (a) freeze-thaw cycles that caused a decrease in micropore volume and an increase in macropores connected with low Equotip values; (b) iron oxide and sulphuric acid released from pyrite oxidation contribute to the dissolution and precipitation of calcium carbonate, which can be rearranged in the outer and surface macroporosity. The quantitative approach proposed in this study may be a valid low-cost and quick tool to assess weathering heterogeneities on building stone materials and to provide insights for effective preservation strategies of historical monuments.