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.