Proposal of New Natural Hydraulic Lime-Based Mortars for the Conservation of Historical Buildings

NHL mortars are known to be compatible materials for the conservation of architectural heritage. To improve their properties with regard to salt resistance and lower their carbon footprint, NHL-based mortars with salt inhibitor agents were studied and different formulations were produced: NHL-based mortars (MSs), composed of natural hydraulic lime; and sand and cocciopesto mortars (MSCs), in which NHL, sand and brick powder were admixed with two different products, diethylenetriaminapenta and chitosan, in different concentrations. The mortar performance was tested against freeze–thaw and salt crystallization through immersion–drying cycles in a 14% sodium sulfate solution. The results highlighted that the addition of cocciopesto was effective in increasing the salt resistance, but increased the water intake during the freeze–thaw tests. The use of DTPMP produced less thixotropic mortars and decreased the water uptake, but worsened the salt resistance of hardened mortars. Chitosan allowed a good workability of fresh mortar; its water uptake was similar to the reference mortar and slightly increased the salt resistance. In the cocciopesto samples, both additives reduced the weight variation during freeze–thaw tests; meanwhile, for the lime samples, the additives increased the weight variation during the final cycles.