Hydroxyapatite conversion layer for the preservation of surface gypsification marble relics

The conversion of harmful gypsum crust on marble substrate by phosphate was studied. The conversion layer was characterized by SEM, XRD and EDS. The results showed that the conversion layer was composed of interlocking hydroxyapatite structure and it reunited the weathered fragments, particles and marble substrate together firmly. In addition, the properties of the hydroxyapatite conversion layer were evaluated by color difference, capillary water absorption and surface strength test. The results revealed that surface strength of the weathered layer was greatly improved. Meanwhile, the inherent physical properties of marble substrate such as pore structure and original appearance were almost unaffected. These highly positive results indicate that hydroxyapatite conversion layer is a good solution for the preservation of the surface gypsification marble relics.