Influence of salt contamination on consolidation of slaked lime mortar by ammonium phosphate and nanolimes

This study aimed at evaluating the influence of salt contamination on the performance of two consolidants (diammonium hydrogen phosphate, DAP, and nanolimes, NL), applied onto slaked lime-based mortars contaminated with sodium sulfate before and/or after consolidation. The strengthening ability of the two treatments was assessed in terms of dynamic elastic modulus and compressive strength by double punch test, while the alterations in the pore system were evaluated by MIP and the features of the new consolidating phases were assessed by XRD and FEG-SEM. To test the durability of the consolidated mortars, salt weathering cycles were performed by alternating immersion into a 14 wt% aqueous solution of Na2SO4·10H2O and drying in oven, as recommended by the European Standard EN 12370. The effects of the salt weathering cycles were assessed by measuring the changes in weight and dynamic elastic modulus after each cycle and the residual compressive strength after 10 cycles and desalination. The results of the study show that, right after the consolidant application, with or without salts initially present in the mortar, the strengthening ability of DAP was higher than that of NL. The DAP treatment induced formation of carbonate hydroxyapatite, as assessed by XRD, and notably the initial presence of salt inside the mortar pores did not negatively affect the outcome of the treatment. However, the formation of carbonate hydroxyapatite was responsible for a shift of the pore size distribution towards smaller pores, which may increase the stress generated by salt crystallization. When the consolidated mortar specimens were subjected to salt weathering cycles, both consolidants provided some benefit against salt weathering, but the high aggressiveness of the salt weathering test might have reduced the performance of the consolidants.