Preparation of Highly Crystalline Nano Ca(OH)2 and Its Comparative Assessment with Commonly Used Materials for the Protection of Wall Paintings

Due to the ecocompatibility with carbonate-based substrates, Ca(OH)₂ nanoparticles are currently used for cultural heritage conservation such as wall paintings. However, the nano Ca(OH)₂ still suffers from different forms and poor uniformity, limiting its application potential. Also, there is a lack of systematic comparative studies between nano Ca(OH)₂ and the commonly used wall painting reinforcement materials. In this study, homogeneous hexagonal nano Ca(OH)₂ particles with a size of ≈100 nm are successfully prepared through the convenient chemical liquid phase method and by utilizing surfactants to control the growth. The resulting nano Ca(OH)₂ is less agglomerated and has superior crystalline morphology, prolonged suspension time, and more suitable carbonation time in comparison to commercial Ca(OH)₂ materials. Additionally, the reinforcement effect of the resulting nano-Ca(OH)₂ with that of the commonly used pigment layer reinforcement materials such as AC33, B72, Tetraethyl orthosilicate, WPU (Waterborne polyurethane) and commercial Ca(OH)₂ is systematically compared. The synthesized nano Ca(OH)₂ penetrated wall painting blocks to a depth of 683 µm, three times deeper than commercial Ca(OH)₂, achieving moderate color deviation, higher flexural strength (0.529 MPa), and bond strength (1.105 mg cm⁻²), thus highlighting its potential in wall painting reinforcement and expanding its application scope.