New treatment for corrosion-resistant coatings for marble and consolidation of limestone

The goal of our work is to develop two classes of treatment for marble and limestone: 1) corrosion-resistant coatings and 2) consolidants for weathered stone. HAP has the same crystal structure as calcite and a small lattice mismatch (~5%), but is orders of magnitude less soluble. A layer of HAP can be deposited on the internal surface area of damaged limestone by exposing it to an aqueous solution of diammonium hydrogen phosphate (DAP) at room temperature and neutral pH. The treatment was applied to samples that were artificially “wea-thered” by heating in air, which is shown to produce a highly reproducible degree of damage for both limestone and marble. Limestone samples were treated with a 1 molar DAP solution by capillary absorption or by brushing until apparent refusal and sealing for 48 hours; they were then washed in deionized water and dried at room temperature. The dynamic elastic modulus was restored to its original value and the tensile strength (measured by the Brazilian test) rose about 25%. To protect marble from corrosion, the HAP deposit should ideally be nonporous, so it is necessary to achieve control over the nucleation and growth of the phos-phate layer. Preliminary results indicate that dense layers can be produced at room tempera-ture and neutral pH by controlling the calcium concentration in solution.