Aging performance of nano AlPO₄ composite hydroxyapatite protective coatings on stone relic substrates

The natural aging and laboratory-accelerated aging performance of nano AlPO₄ composite hydroxyapatite protective materials on stone relic substrates were investigated by using multiple characterization techniques. Weathered stone samples (dolomite, calcite and mortar) were consolidated with the phosphate composite materials, and subjected to 60 days of natural aging in an open-air summer environment at the Beijing Stone Carving Art Museum. Raman spectroscopy, microscopy, SEM-EDS, X-CT and chromaticity analyses demonstrate that after consolidation and aging, dense and continuous protective coatings form on the mortar and calcite substrates, whereas discontinuous calcium phosphate deposits are observed on the dolomite surface. The phosphate composite material effectively reinforces fragile areas without inducing noticeable color changes. Results indicate that hydroxyapatite formation persists during the natural aging, with performance variations likely attributable to differences in Ca²⁺ release capacity among substrates. Damp-heat and freeze-thaw aging tests were conducted on simulated calcite specimens in laboratory. SEM, Leeb hardness, three-point bending strength, and colorimetry results indicate that after accelerated aging, uniform protective film coatings form on the stone, with enhanced mechanical strength and surface hydrophobicity, while no significant color change occur. Since both damp-heat and freeze-thaw aging involve cyclic moisture replenishment, they provide favorable conditions for the conversion of amorphous calcium phosphate in the protective film into reinforced hydroxyapatite. The nano AlPO₄ composite hydroxyapatite protective material demonstrates excellent long-term protective performance under accelerated aging conditions.