Durability of natural hydraulic lime–pozzolan mortars for TRM strengthening systems under acidic aging: Linking microstructural degradation to mechanical performance via chemo-mechanical modeling

Lime-based mortars are widely used in sustainable construction and historic masonry rehabilitation, particularly in textile reinforced mortar and composite reinforced mortar systems. Despite their increasing application, their durability under aggressive environmental exposures remains poorly understood. This study investigates the aging behavior of a commercial binary natural hydraulic lime (NHL)–pozzolan mortar subjected to accelerated environmental exposure, including dry curing, water immersion, and immersion in sulfuric acid solutions at pH 3.0, 2.0, and 1.5, over three exposure durations: 1000, 3000, and 6000 h. More than 500 specimens were tested to evaluate compressive strength, flexural strength, and elastic modulus. These mechanical tests were complemented by physical measurements of porosity and bulk density, and by extensive mineralogical and microstructural analyses, including XRD, FTIR, SEM, TGA, optical microscopy, and leaching tests. The results indicated an initial improvement of mechanical properties under moderate acidic exposure, attributed to pore filling by secondary phases such as gypsum, followed by progressive deterioration due to decalcification, matrix softening, and microcracking at extended durations and lower pH levels. To quantitatively assess this degradation, a trilinear continuum damage mechanics model was developed and calibrated using experimental stress–strain data. The model was further extended by incorporating empirical degradation laws derived from Fe-based leaching indices, enabling predictive simulation of mortar performance under untested aging conditions. Supported by chemical and microstructural analyses that guided the selection of degradation indicators and informed the interpretation of results, the proposed chemo-mechanical framework provides a robust, experimentally grounded tool for forecasting the long-term behavior of NHL-based mortars in aggressive environments.