Evaluation of mechanical properties and durability of nanolime synthesized by an anion-exchange method for the consolidation of Dazu Rock Carvings

Introduction: Weathered sandstone at the Dazu Rock Carvings is susceptible to progressive loss of intergranular cohesion under repeated moisture fluctuations and salt crystallization. This study evaluated a compatible nanolime treatment designed to enhance the mechanical resistance of weathered sandstone while preserving its visual appearance and pore connectivity. Methods: High-purity calcium hydroxide nanoparticles were synthesized via a one-step anion-exchange route. The synthesized nanolime and treated sandstone were characterized and evaluated using X-ray diffraction (XRD), transmission electron microscopy coupled with energy-dispersive spectroscopy (TEM/EDS), colorimetry, porosity and permeability measurements, uniaxial compressive strength (UCS) testing, nanoindentation mapping, accelerated wetting-drying and hygrothermal-salt aging tests, and in situ Leeb hardness monitoring. Results: The synthesized nanolime consisted predominantly of hexagonal portlandite platelets approximately 50–100 nm in size, with no detectable chloride-containing crystalline by-products under the applied characterization conditions. Nanolime treatment increased the mean UCS from 37.35 to 47.00 MPa while causing negligible changes in color, porosity, and permeability. Following accelerated aging, nanolime-treated specimens retained higher hardness and elastic modulus than both untreated specimens and those treated with traditional lime water. Field monitoring further showed rapid recovery of surface hardness within 24 h after treatment, followed by continued strengthening over 100 days in selected weathered zones. Discussion: These results demonstrate that anion-exchange-derived nanolime provides mineral-compatible reinforcement while maintaining the visual and hydro-physical characteristics of weathered sandstone. Longer-term field monitoring and direct assessment of carbonation depth are still required to establish its long-term durability and service-life performance.