Biogenic Hydroxyapatite: A New Material for the Preservation and Restoration of the Built Environment.

Ordinary Portland Cement (OPC) is by weight the world's most produced man-made material, and consequently is used in a variety of applications in environments ranging from buildings, to nuclear wasteforms, and within the human body. In this paper, we present for the first time the direct deposition of biogenic hydroxyapatite onto the surface of OPC, in a synergistic process which relies upon the unique properties of the cement substrate. The synthesised hydroxyapatite is very similar to that found in nature; having a crystallite size, iron and carbonate substitution, and a semi-crystalline structure which identify that this biogenic hydroxyapatite is akin to that found in natural bone and tooth enamel. Hydroxyapatites with such a structure are known to be mechanically stronger and more biocompatible than synthetic or biomimetic hydroxyapatites. The formation of this biogenic hydroxyapatite coating therefore has significance in a range of contexts. In medicine, hydroxyapatite coatings are linked to improved biocompatibility of ceramic implant materials. In the built environment, hydroxyapatite coatings have been proposed for the consolidation and protection of sculptural materials such as marble and limestone, with biogenic hydroxyapatites having reduced solubility in comparison with synthetic apatites. Hydroxyapatites have also been established as effective for the adsorption and remediation of environmental contaminants such as radionuclides and metals. We further identify that in addition to providing a biofilm scaffold for nucleation, the metabolic activity of \textit{Pseudomonas fluorescens} increases the pH of the growth medium to a suitable level for hydroxyapatite formation. The generated ammonia reacts with phosphate in the growth medium, producing ammonium phosphates, which are a precursor to the formation of hydroxyapatite under conditions of ambient temperature and pressure. Subsequently, this biogenic deposition process takes place in a simple reaction system under mild chemical conditions, which are cheap and easy to apply to fragile biological or architectural surfaces.