Artificial weathering of stone by heating

Abstract

Since the effectiveness of stone consolidants significantly depends on the weathering level of the stone samples on which they are tested, in this study the suitability of heating stone to high temperature, as an artificial weathering method to induce controllable microstructural, physical and mechanical alterations, was investigated. Three lithotypes with different characteristics were used: Giallo Terra di Siena (GS, a highly porous calcareous sandstone), Globigerina limestone (GL, a highly porous limestone) and Pietra Serena (PS, a porous quartzitic sandstone with low porosity). The lithotypes were characterized in terms of mineralogical composition, pore size distribution and water absorption, as well as dynamic modulus, static modulus, compressive and tensile strength. They were then heated for 1 hour, in different conditions: (i) dry samples were heated to 100, 200, 300 and 400 °C; (ii) water-saturated samples were heated to 200 °C; (iii) water-saturated samples were heated to 200 °C and, after cooling to room temperature, re-heated to 400 °C. After heating, all the lithotypes experienced an increase in open porosity and water absorption, as a consequence of the anisotropic thermal deformation of calcite crystals. Correspondingly, GS and GL exhibited an increasing reduction in mechanical properties for increasing heating temperature. PS, on the contrary, exhibited an increase in compressive and tensile strength, which was attributed to chemical-physical transformations undergone by secondary mineralogical fractions (clay minerals, etc.) at high temperature. All things considered, heating proved to be a fairly effective and reproducible method to cause artificial weathering in stone samples for the testing of consolidants. However, depending on the microstructural characteristics of the lithotypes, the effectiveness of heating may vary significantly, which requires a case-by-case adjustment of the most suitable heating procedure and the development of complementary methods for artificial weathering.