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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.

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The reconstituted stone used in the Orval Abbey is highly susceptible to salt weathering. It was produced from debris of natural stones (which were at that time in short supply) by craftsmen working in the Abbey and its composition is highly variable. The major salt involved in weathering is sodium sulfate (Na2SO4). This study focused on the thermal behavior of a reconstituted stone with various degrees of weathering observed on-site and it was studied by passive and active infrared thermography (IRT), and on the evolution of reconstituted-stone samples in a salt uptake experiment in the laboratory. On-site, the thermal responses of salt-weathered and unweathered zones were different. IRT allowed to detect unweathered though highly salt-laden zones, although correlations between thermal response and salt concentration are difficult to establish due to the influence of surface properties (color and roughness). In the laboratory, samples of the reconstituted stone were submitted to successive immersion-drying cycles in solutions of Na2SO4 with 7, 14 and 26 % concentration and characterized by weighing, surface colorimetry and active IRT. Results showed that the thermal response (increase in surface temperature due to flash excitation: T max) increased with salt concentration and with the number of cycles, as long as sample disintegration had not started. Then, it decreased and stabilized, whatever the salt concentration. Those results indicated that an increase in the thermal response of building materials (in active IRT) can be interpreted as an indicator of salt accumulation, possibly leading to damage. IRT proved to be an interesting non-destructive technique in this respect.