Correlation between microstructural characteristics and weight loss of natural stones exposed to simulated acid rain

The correlation between stone microstructural characteristics and material degradation (in terms of weight loss), in given environmental conditions, was investigated. Seven lithotypes, having very different microstructural characteristics, were used. Four acidic aqueous solutions were prepared to simulate acid rain (two adding H2SO4 and two adding HNO3 to deionized water, in order to reach, for each acid, pH values of 5.0 and 4.0), and deionized water at pH = 5.6 was used to simulate clean rain. Stone samples were then immersed in such aqueous solutions, the surface alteration being periodically inspected and the weight loss periodically measured. After 14 days of immersion, a good correlation was found between weight loss and the product of carbonate content and specific surface area in the starting materials. This was explained considering that this product accounts for the weight loss owing to the sample's fraction actually composed of calcite (the most soluble fraction) and the effective surface area exposed to dissolving solution (which depends on stone porosity and pore size distribution). Such correlation between stone microstructure and degradation may be useful for comparing the durability of different lithotypes, in given environmental conditions, and quantitatively predicting the weight loss of a lithotype, compared to another one. Hence, the correlation found in this study may be used to specifically tailor to various stone types, with different microstructural characteristics, some results that have been calculated in literature for specific stone types and then proposed as possibly representative for a broad category of stones with similar characteristics.

► We measured the weight loss of stones with different composition and porosity immersed in clean water and acidic solutions.
► A vulnerability index (VSSA, product of calcium carbonate content and specific surface area of the stone) was introduced.
► VSSA takes into account stone’s most soluble fraction and amount of material actually in contact with dissolving solution.
► A good correlation between VSSA and weight losses was found.
► The correlation we found may be useful for comparing the durability of different lithotypes in given environmental conditions.