Effectiveness of hydroxyapatite as a consolidating treatment for lithotypes with varying carbonate content and porosity

A new inorganic consolidant for limestone, based on the reaction between diammonium hydrogen phosphate (DAP) and calcite in the stone to form hydroxyapatite (HAP), has recently been proposed. This consolidant was found to cause a significant increase in the mechanical properties of limestone, due to the bonding action between the stone grains, and, at the same time, a very slight alteration in the transport properties of treated stones, unlike many other consolidating products currently used. Moreover, HAP treatment is expected to exhibit a good acid resistance and low solubility, with corresponding benefits in terms of durability. The short reaction time (just two days) and the use of a solvent not hazardous for human health (water) are further advantages of this consolidant. Due to the good performance on limestone, its use for the consolidation of sandstone was investigated in the present paper, in comparison with a TEOS-based consolidant. Two lithotypes with varying carbonate content and porosity were used: a highly porous calcareous sandstone and a quartzitic sandstone with low porosity. After preliminary artificial weathering, each lithotype was treated: (i) with a 1 M aqueous solution of DAP; (ii) with a solution of TEOS oligomers in isopropyl alcohol, each applied by brushing until apparent refusal. The effects of the two consolidating treatments in terms of microstructural, physical and mechanical properties were investigated. HAP led to an improvement in dynamic elastic modulus and tensile strength of the stones substantially similar to that achieved by TEOS, but with smaller variations in porosity and pore size distribution, as well as in the rate of water sorption and in water vapor permeability.