Carboxymethyl\nChitosan as a Reversible Template of\nCalcium Phosphate for Multifunctional Conservation of Carbonate Stone

The accelerated deterioration of\ncarbonate stone artifacts under\nclimate change has long been an urgent issue. Inspired by biomineralization,\nwe developed carboxymethyl chitosan-diammonium hydrogen phosphate\n(CD) composite and investigated the conservation effectiveness of\nthe CD composite compared to diammonium hydrogen phosphate (DAP) on\nlimestone. The morphologies and microstructures were characterized\n using scanning electron microscopy (SEM), X-ray diffraction (XRD),\nand Fourier-transform infrared spectroscopy (FT-IR). The consolidating\neffectiveness was investigated through a compressive strength test.\nThe protective ability was assessed by disintegration resistance\ntest, acid attack resistance test, salt attack resistance test, and\nfreeze–thaw aging cycle test. SEM observations revealed that\ncarboxymethyl chitosan (CMCS) served as an effective template, inducing\nthe in situ formation of a uniform and continuous calcium phosphate\ncoating on both the surface and interior of the stone. The tests indicated\nthat the CD composite further enhanced the consolidating effectiveness\nand improved resistance to disintegration and freeze–thaw cycles.\nNotably, as an amphiphilic polyelectrolyte, CMCS functioned as a pH\nbuffer and a protective barrier against sodium sulfate salts, which\nimproved the resistance to acid and salt attacks. Additionally, the\nCD composite did not cause significant variations in the esthetic\nappearance or water vapor permeability. We then applied the CD composite\nat an actual carbonate stone cultural heritage site, successfully\ndemonstrating the feasibility of CD application and the reversibility\nof CMCS in a real-world setting. Based on the study’s results,\nour approach provides a new perspective for developing multifunctional\nand sustainable conservation materials for carbonate stone artifacts.