Weathering assessment under X-ray tomography of building stones exposed to acid atmospheres at current pollution rate

Interactions between the environment and the building stones lead to their alteration, besides water and air pollution play an important role. The link between air pollutants such as SO₂ and stone deteriorations are well known. Last decades the SO₂ emissions have decreased much more than those of NO_x and consequently the SO₂/NO_x ratio reversed until reaching a current proportion of 1/3. In this new environment, we have to adapt our knowledge on ageing and weathering of building stones, particularly to make restorations of historical monuments as sustainable as possible. The aim of this study was to assess the effect of this new ratio of SO₂ and NO_x on the weathering kinetics of building stones. For this purpose, laboratory aging tests were performed simulating acid atmosphere exposure and acid rain runoff. Weathering was assessed by high resolution X-ray computed tomography (HRXCT) on two building materials frequently used as restoration material in cultural heritage buildings in a.o. northern France and Belgium: the oolitic limestone Savonnières and an artificial reconstituted stone made of debris of limestone mixed with a cement in variable proportion. Acid deposition effects were assessed by exposing one sample of each stone to a strong mixed acid atmosphere. Acid rain was simulated by immerging one other sample of each stone type in a mixed acid solution at pH = 5. Each sample was scanned before and after 1, 10 and 28 days of testing. HRXCT allowed to visualize the first steps of acid pollutant attack on the stones’ microstructure. The two tests led to different alterations, mainly salt crystallization for atmospheric exposure and dissolution for immersion. After only one day of test, HRXCT scans showed that exposure to mixed acid gas led to salt crystallization preferentially on sample surfaces for both types of stone. The thickness of this salt crust remained stable until the end of the test, after 28 days. The treatment of the reconstructed scans highlighted a dissolution stage which occurred before the crystallization stage. The dissolution produced by the immersion test happened immediately during the first day on the surfaces and the open porosity for both stones, although Savonnières dissolved homogeneously due to its pure calcitic composition, while the reconstituted stone showed granular disaggregation because of its mineralogical heterogeneity.