Preliminary results on the use of ammonium phosphate solutions for the consolidation of lime-based mortars

In this study, a preliminary evaluation of the effectiveness of aqueous solutions of diammonium hydrogen phosphate (DAP) for the consolidation of lime-based mortars is reported. Four types of mortars were considered: 2 lime-based mortars (containing either siliceous or calcareous aggregates), 1 lime-based mortar also containing brick powder as a pozzolanic addition (so-called “cocciopesto” mortar) and 1 mortar based on natural hydraulic lime. Two different formulations of the DAP solution were considered, differing in terms of DAP concentration, CaCl2 addition as a calcium source and ethanol addition to boost formation of calcium phosphate (CaP) phases. First, for the various types of mortar, the nature and the morphology of the new CaP phases were investigated by FT-IR and SEM. Then, the mechanical benefit resulting from the new CaP was evaluated, in terms of increase in dynamic elastic modulus and compressive strength. The microstructural compatibility of the treatments was also evaluated, by determining the alterations in pore size distribution. The results of the study indicate that new CaP phases are formed after 24 hours for both DAP concentrations, the amount of new phases and the mechanical improvement being higher for the more concentrated solution. In no case did dramatic pore occlusion and alteration in the pore size distribution take place, which allows expecting no dramatic alteration in the mortar ability to exchange water and water vapour with the environment. The more concentrated solution caused some visible colour change in the case of mortars not originally white (“cocciopesto” and NHL mortars), which makes the use of less concentrated solutions advisable on those substrates.