The preparation of continuous hydroxyapatite film on stone is a promising method of protecting marble from erosion. However, many methods negatively affect the calcium in the substrate and forming of struvite on the dolomite surface, leading to a heterogeneous coating and low efficiency. In this study, a continuous hydroxyapatite coating on dolomitic marble was achieved from graphene enhanced Ca(OH)₂ nanoparticles as the calcium precursor using the sol-gel method. The morphology and the structure of the film was evaluated by a field emission scanning electron microscope coupled with energy dispersive spectroscopy (FESEM-EDS), an optical microscope, Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and analytical techniques. Moreover, the color and the contact angle measurements, as well as the simulated acid rain test and freeze–thaw treatment, were performed to assess the chromatic aberration, hydrophilicity, reliability, and durability of the coating. A suppositional combination model among hydroxyapatite, graphene quantum dots, and dolomite were suggested based on structural similarities between the support material and components of the functional coating. The integrality and efficiency of the hydroxyapatite film was improved by compositing with graphene quantum dots.

The deposition of a calcium oxalate layer on dolomite demonstrates potential application in stone culture heritage conservation. However, due to insufficient coverage and the presence of cracks, the film’s usefulness is restricted. In this investigation, we used a simple two-step procedure to create a cohesive and uncracked film. The findings show that the protective layer provides better coverage of the dolomite surface without causing cracks and significantly improves acid resistance. Furthermore, after the simple two-step treatment, the color and adhesive strength of dolomite substrates remained nearly unchanged.

Ca(OH)₂ nanoparticles are effective materials for cultural heritage restoration, hazardous substance absorption and photocatalyst. However, many methods are complex, and the particle sizes are usually above 80–100 nm, involving mediocre efficacy for application in the stone restoration field. In this work, Nano Ca(OH)₂ with diameters less than 70 nm and composited with Graphene Quantum Dots (GQDs) were successfully synthesized in aqueous media. The morphology and structure of the nanoparticles were investigated with TEM, HRTEM, XRD, Raman and FTIR. The particle size distribution and relative kinetic stability of the Ca(OH)₂ in ethanol were performed using a laser particle size analyzer and spectrophotometer. Firstprinciple calculations based on the spin-polarized density functional theory (DFT) were carried out to study the reaction process and combination model. The nanoparticles, as prepared, are composed of primary hexagonal crystals and high ammoniated precursors, which have a positive effect on reducing the grain size, and interacted with the GQDs hybrid process. According to the First-principle calculations results, the energy variation of the whole reaction process and the bonding mode between Ca(OH)₂ and GQDs can be understood better.

Being inspired by nature, a series of experiments was carried out to deposit a calcium oxalate layer on the surface of the stone by the reaction between carbonate rock and oxalate salt. To increase the anti-dissolution properties of the calcium oxalate layer, the use of mixed oxalate solution has been proposed in the literature by two main routes: (1) adding acid agent to ammonium oxalate, which has the advantage of changing the particle structure and reducing layer porosity, and (2) using neutral methyl oxalate solution, which has the advantage of surface coverage due to slowly hydrolysis. In this study, we investigated the sequential application of ammonium oxalate, methyl oxalate, neutral mixed, and calcium acetate acid mixed solution. With this method, calcium carbonate and calcium oxalate solution can react inside the stone to reinforce it. The protective film’s coverage area can then be increased using dimethyl oxalate neutral mixed solution, and the crystal morphology can be modified with calcium oxalate acid mixed solution. The anti-dissolution properties of the coating were investigated using both a custom-designed apparatus and a selective outdoor environment. The coating displayed good acid resistance properties at pH 2–4. After one year of exposure, the coating is firmly bonded with the stone.