Phosphate-based treatments for conservation of stone

To overcome the limitations of currently available protectives and consolidants for carbonate stones (such as marble and limestone), in 2011 the use of calcium phosphate was proposed. The idea is forming calcium phosphates (ideally hydroxyapatite) as the reaction product between the substrate and an aqueous solution of a phosphate salt that the stone is treated with. In this paper, the studies aimed at identifying the best treatment conditions (in terms of nature and concentration of the phosphate precursor, solution pH, reaction time, ionic and organic additions) are first briefly summarized. Then, the efficacy of the phosphate treatment in protecting marble from dissolution in rain and restoring cohesion of weathered marble and limestone is discussed. Some recent studies on the use of the phosphate treatment on alternative substrates and some future steps for research on the topic are finally outlined.