Consolidating efficacy of diammonium hydrogen phosphate on artificially aged and naturally weathered coarse-grained marble

Abstract

The effects induced by diammonium hydrogen phosphate (DAP) on coarse marble varieties susceptible to granular disintegration as consolidation material is still poorly addressed. The work aims at investigating the behavior and efficacy of diammonium hydrogen phosphate(DAP)-based consolidant prepared in labo- ratory onto weathered coarse-grained marbles and, in the process, to unveil if the treatment investigated is a possible option to consolidate Trigaches marble elements from Roman Villa of Pisões. Sound, artificially and naturally weathered coarse marble samples were used for the purpose. The condition of naturally weathered marble slabs was assessed to reproduce artificially aged samples and used for testing the efficacy of DAP consolidant. Naturally weathered slab fragments showed a uniform damage in depth, an increased porosity and water absorption, and very similar conditions were success- fully achieved in laboratory by heating sound marbles up to 300 °C for 1 h. DAP consolidant has low viscosity and can easily penetrate within the porous structure of coarse mar- ble in granular disintegration. It promotes the formation of new calcium phosphate phases with binding capabilities that partially restore cohesion in-depth. Alteration of the color was negligible which is also a positive characteristic. Pilot tests in naturally weathered marbles revealed some interference of superficial bioorganisms residues in the absorption of DAP, however this was able to treat the entire thickness of the slab frag- ments, improving its cohesion state to an acceptable or good condition. Good correlations between the results of artificial aged samples with those of naturally weathered marble were achieved and the DAP-based treatment revealed to be an effective option to consolidate Trigaches marble elements from Roman Villa of Pisões.