Influence of Surface Finishes and a Calcium Phosphate-Based Consolidant on the Decay of Sedimentary Building Stones Due to Acid Attack

Abstract

Natural stone has long been used as a building material because of its physical-mechanical resistance and its esthetic appeal. However, over the last century increasing industrial activity has produced more acidic environments, such as polluted urban areas, that can cause serious damage to many buildings and historic monuments, and in particular those made with carbonate rocks, which are most prone to decay. In order to mitigate the physicochemical processes that degrade these buildings, a number of phosphate-based consolidants have been developed, which are highly compatible with the carbonated substrate. Research about the role of the surface and its different possible finishes in the transmission of the agents that damage or protect the stone would therefore be very useful, both when choosing the most suitable stone for new constructions and when restoring historic buildings. The main objective of this research was to determine whether the roughness of three types of surface finish (sawcut, honed and bush-hammered) influences the durability of four types of natural stone (two calcarenites, one travertine and one sandstone) widely used in Andalusia (Spain). The efficacy of a calcium phosphate-based consolidant as a mitigator of deterioration in polluted urban environments was also studied and to this end the physical properties of untreated and treated samples were measured and compared. The samples were exposed to artificial atmospheres with SO 2 pollution in order to assess the damage caused to each surface finish.The results indicate that all the surface finishes were vulnerable to the decay caused by acidic atmospheres, although the saw-cut finish was less affected, perhaps because it did not require additional industrial processing. The mineral composition and texture of the rocks were critical factors in terms of the amount and type of decay they suffered, and the travertine and sandstone were more resistant to deterioration than the calcarenites. Similarly, the pore system of each rock was decisive in the penetration of the consolidant. Application of the consolidant improved the behavior of the treated samples by making them more resistant to acid attack without significantly altering the water vapor permeability, the color or the roughness of the surface.

Investigation of Calcium and Magnesium Phosphate Crystals in Stones Treated with Diammonium Hydrogen Phosphate Conservation Product: Potential of Micro-Raman Spectroscopy

“…The crystallochemical composition, the crystallinity of these phases, as well as their spatial distribution within the treated stone materials was found to be strongly dependent on several variables mutually interacting during the DAP reaction. These factors include the physical features of the substrate (its mineralogy, grain size and microstructure), as well as the treatment modalities (i.e., DAP molarity, treatment duration, application methods, pH, ionic strength during the DAP reaction) [5][6][7][8].…”

Synchrotron X-ray diffraction computed tomography to non-destructively study inorganic treatments for stone conservation

“…These treatments lead to a partial transformation of the minerals of the original matrix into newly formed phases. In particular, diammonium hydrogenphosphate (DAP, (NH 4 ) 2 HPO 4 ) is gaining increasing attention nowadays as a reagent ( Celik et al., 2020 ; Menningen et al., 2021 ; Molina et al., 2020 ; Murru and Fort, 2020 ; Sassoni et al., 2021 ; Sena da Fonseca et al., 2021 ). DAP is a water-soluble salt that reacts with the carbonatic matrix of the stone substrate and forms new calcium phosphate phases (CaPs) as a result of dissolution-recrystallization reactions occurring at the interface between the DAP solution and calcite grains ( Matteini et al., 2011 ; Possenti et al., 2016 , 2019b , 2020 ).…”