Deposition of insoluble calcium phosphate layers by combining finely dispersed calcium hydroxide sols with soluble phosphates

Abstract

Carbonate containing materials are subject to severe weathering. Traditional formulations of stone strengtheners have low compatibility with the original material and further they contain VOCs (volatile organic compounds), which endanger human health and the environment. This study explores the high potential of novel treatments based on water-soluble phosphates used as an agent to react with calcium carbonate (CaCO3) to form an insoluble film of calcium phosphate in the pore space of the treated material. Pretreatments with nanolime suspensions ensure greater availability of calcium ions and reduce the consumption of the original material in the reactions. An alkaline environment is required to promote the conversion of the CaCO3 components to hydroxyapatite-like compounds. Based on experiments in aqueous solutions, different sources of phosphate ions could be examined and compared for the development of effective treatments to apply on different test specimens. To implement the treatments, barium phosphate solutions were investigated. Important aspects of this research are the use of green solvents and the search of components that avoid the formation of byproducts, to increase the efficiency of the chemical reactions and reduce possible negative effects on the operator, the environment and the very same built heritage material. The developed treatments are a valuable alternative to the traditional methods, as it follows an improvement in the material properties without affecting the moisture transport within it and allows the evenly reaction of the strengthened material to external physical and mechanical stresses without creating internal tension between the grains.