Simulation of acid weathering on natural and artificial building stones according to the current atmospheric SO₂/NO _x rate

The building stones are affected by pollution. Since 1980s, the actions to reduce the greenhouse gas emissions led to the inversion of the SO₂/NO _x proportions in the atmosphere. This study aims at estimating the effects of nitrogen and sulfur compounds on stones by assessing the changes of three building limestones and one reconstituted stone submitted to acid attacks. Two of these stones were already contaminated with sulfates, while the two others were fresh quarried. Two different types of accelerated aging tests were used: (1) the exposition to two mixed acid and saturated atmospheres (HNO₃ and H₂SO₃) to simulate the ancient and current pollutants ratio and (2) the immersion in a mixed acid solution (HNO₃ and H₂SO₄) and in rainwater (pH 5 and 5.9), with and without agitation to simulate stagnant water and storm runoff water. Macroscopic, binocular and SEM observations, variations of color, weight, porosity, salt content and dissolved calcium were assessed over time. The sulfur amount influences the esthetic alterations such as color changes due to the salt precipitation and the oxidation of metallic compounds. During the immersion tests, the dissolution in the acid solution was more efficient than in the rainwater, due to the combination of the acidity and the karst effects. In the mixed acid atmospheres, the behavior of the porous network depends on the pore size distribution while in the immersion tests it is the open porosity. The high initial sulfur content of the contaminated stones increases the dissolution rate and limits the crystallization.