Consolidation works on sandstone monuments: A new approach

Abstract

Portugal is the home of some very important buildings and monuments where natural stone is the predominant material, for example, in castles, churches, and palaces. Sandstones use is common in traditional buildings in western regions of Portugal (Peniche and Lourinhã) and in Algarve (Silves). The International Council on Monuments and Sites-International Scientific Committee for Stone (ICOMOS-ISCS) [1] describes stone degradation patterns on monuments and ancient buildings. All of five families of ICOMOS-ISCS [1] patterns were found in the facades of the medieval monuments of Atouguia da Baleia, in Peniche [2]: crack and deformation; detachment; features induced by material loss, discoloration, and deposits; biological colonization. All porous building materials are reached by salt weathering effects near the shoreline and in desert environments. The seawater is the source of sodium chloride salt, absorbed by capillarity from the water table below foundations and by salt spray that reaches building facades. Salt effect on porous building materials is visible even at a distance of about 3-5 km from the coast, besides the decrease of circa 30%-50% on chlorides concentration in total deposition [3]. The oldest known reference about salt effects on buildings in desert environments belongs to Herodotus and was written in fifth century before common era (BCE). The Greek historian reported that salts injured the pyramids, cited in Ref. [4]. Lucas cited in Ref. [5] referred to sodium chloride as playing the most important leading role in the majority of conservation and restoration works on sandstones and limestones of dry environments-namely, in Egypt-followed by sulfates, carbonates, and nitrates. A recent report indicates general salt damage occurrence on the buildings of the Jazirat Al Hamra area in United Arab Emirates [6], a town located near the shoreline and in a desert environment.

Onset of Alveolization Processes in Sandstones Exposed to Salt Weathering

“…This first stage of consolidation works intended to assess the viability of future TEOS applications on these stone walls. Ludovico-Marques and Chastre (2014, 2016) [26,27] carried out experimental research on sandstone specimens similar to the stones of variety M in the monument, with circa 3-5 times the porosity values of varieties B and A. These products were applied by an initial immersion and capillarity procedure, followed by full immersion.…”

“…As it is visible, alveolization is in an early stage in this lithotype. 2002) [27,28], showed detailed data of consolidating treatments through the application of a MTMOS product, Brethane, on cultural heritage sandstones (e.g., in castles and abbeys sites) with higher values of porosity. The study reported that Brethane did not perform well when the durability was assessed through the salt effect.…”

“…Representative alveolization forms on a sandstone block of the B variety located at the south portal of St. Leonard's Church. Wheeler (2005), citing Butlin et al (2002)[27,28], showed detailed data of consolidating treatments through the application of a MTMOS product, Brethane, on cultural heritage sandstones (e.g., in castles and abbeys sites) with higher values of porosity. The study reported that Brethane did not perform well when the durability was assessed through the salt effect.Prior works of conservation based on the visual inspection of facades and monitoring the weathering forms on sandstone blocks of variety B has been carried out by the authors.The next sections will show the characterization of these stones, the experimental methodology followed and the corresponding theoretical background, the tests results, its interpretation and discussion regarding salt weathering follow-up during artificial ageing performed on heritage sandstones of lithotype A+B, and our conclusions regarding the effectiveness of the different saline solutions used in terms of triggering the onset of alveolization.…”