Conservation of the weathered historic calcareous stones by apatite was studied. By mimicking the growth of bone, calcium and phosphorus were introduced into the weathered stone and then mineralized at room temperature. The conservation efficiency was investigated by SEM, EDX, XRD, “Scotch Tape Test” (STT), compressive strength test, capillary water uptake test, water vapor permeability test and weather resistance test. The results of SEM, EDX and XRD showed that the produced bone-like hydroxyapatite could bind the introduced calcium carbonate and the weathered stone blocks together. The test results also indicated that this conservation method could improve the surface strength, compressive strength and weather resistance of the treated samples significantly. Besides, due to the porous nature of the bone-like apatite, the water vapor permeability of the treated stone was not affected and its “breathe function” was retained. Therefore, the proposed method will be a good candidate for the in situ conservation of historic calcareous stones.

Water can trigger freeze–thaw cycles, acid rain corrosion, and microbial colonisation, all of which destroy stone. Water is one of the most influential factors in the destruction of outdoor stone heritage. Therefore, materials with excellent hydrophobic properties and durability are urgently required to effectively retard long-term stone weathering. In this study, two nanoparticles, TiO₂ and SiO₂, were used to modify dodecyltrimethoxysilane (DTMS), a waterproof coating commonly used for stone heritage protection, to fabricate nanocomposite superhydrophobic coatings. The micromorphology, water repellence (water contact angle and capillary water absorption), suitability to protect stone heritage (color change and water vapor permeability), and durability (thermal, light, and chemical stability) of DTMS and nanocomposite coatings were evaluated. The scanning electron microscope (SEM) images revealed that adding 0.5% (w/w) SiO₂ produced nanoscale roughness on the sandstone surface, leading to superhydrophobicity. The results of ultraviolet -visible (UV–Vis) spectrophotometer showed that adding 0.01% TiO₂ shielded more than 90% of UV light but accelerated the decrease in the contact angle under UVA irradiation. The addition of SiO₂ was able to avoid the detrimental effect of TiO₂ under UV light. The thermogravimetric analysis (TGA) results showed that both SiO₂ and TiO₂ nanoparticles improved the thermal stability of the coatings. In particular, the fabricated nanocomposite coating, SiO₂ and TiO₂ co-modified DTMS, had excellent water repellence, low color change and outstanding durability, and retained about 85% of the water vapor permeability of the stone, showing promise for stone protection.

The conservation of weathered historic sandstone with apatite was studied. Based on the growth mechanism of bone, calcium and phosphorus were introduced into weathered sandstone and then mineralized at room temperature. The conservation efficiency was investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD), and compressive strength, capillary water uptake, water vapor permeability and weather resistance tests. The SEM, EDX, TEM, SAED and XRD results showed that the produced hydroxyapatite could reunite the weathered stone blocks and provide sufficient reinforcement to hold them together. The results of the various tests indicated that the compressive strength and weather resistance of the treated samples was improved significantly. In addition, because of the porous nature of apatite, the water vapor permeability of the treated stone was not affected and its breathability was maintained.