Recording of the thermal evolution of limestones undergoing experimental accelerated ageing tests
Résumé
Sodium sulfates are widely regarded as the most destructive salts for porous stone, concrete and brick. Thenardite (Na2SO4), mirabilite (Na2SO4, 10H2O) and heptahydrate (Na2SO4, 7H2O) are the common phases that occur under surface conditions. The heptahydrate phase has been largely neglected in most modern work about salt weathering. However, several recent publications suggest that it could play an important role in geochemical and planetary processes. Therefore its role in the Na2SO4- H2O system should be clarified. We present here results of accelerated ageing experiments performed on samples of a micritic limestone under two different ambient temperatures: 20 C (i.e. below the upper limit of metastability of the heptahydrate), and 30 C (i.e. above this limit and also below the upper limit of stability of mirabilite). Thermocouples were placed into the samples in order to follow the evolution of the temperature of the stones during the weathering tests. This method allows the recording of exo and endo-thermic reactions that are linked to physicochemical processes, such as crystallization, wetting, evaporative cooling, etc. The experiments show significant differences in the pattern and degree of damage. In the experiment at 20 C, the loss of material is noticeable, and occurs as crumbling and scaling. In the experiment at 30 C, we observe just efflorescence with no visible loss of material. The recorded thermal evolution is also different for the two experiments, which provides some clues as to the different physicochemical processes occurring in each situation, depending on the "ambient" temperature.
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