Relationship between Na₂SO₄ concentration and thermal response of reconstituted stone in the laboratory and on site

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.