Thermal response of building stones contaminated with salts

The accumulation of salts in building stones can lead to weathering features such as scaling, sanding or even slab detachment. To detect salts in walls at the early stages and to predict their evolution, the non-destructive technique (NDT) Infrared Thermography (IRT) was tested in a stimulated way. The main objective was to trial this technique in order to obtain more information such as the type of salt, its precise location and its concentration in building stones. Three building materials were studied, including two natural limestones and a reconstituted stone. Samples were submitted to immersion/drying tests in water and in salt solutions of Na2SO4, NaCl and MgSO4 at 7%, 14% and 26% in weight during 20 cycles of 24 h. The weight of the samples and the surface colour were recorded after each cycle. After 1, 5, 10 and 15 cycles, 3 samples were taken out for active IRT measurements. The results showed that when the porosity and the capillarity are low, the intake of salts into the porous network is low and the thermal response of the stone to heating (ΔTmax) corresponds to the salt thermal signature and has no relation with concentration. On the other hand, when the initial porosity is high, salts crystallize inside the porous network and porosity decreases. Hence, the thermal response of the porous stone is related to salt concentration and the ΔTmax increases until the stone starts to disaggregate. Active IRT is a promising NDT adapted to identify and quantify salts within building stones during the first states of accumulation.