Natural stone is an important component of historical heritage (buildings and art objects such as sculptures or rock engravings), and it is still widely used in contemporary works. Soluble salts are the main erosive agent in the built environment, and we review here comparative studies that subject the same rock type to testing with different salt solutions. The results mostly support the accepted notion of the major impact of sodium sulphate, although there are some exceptions. The effects of sodium chloride and calcium sulphate deserve specific discussion given field information on the relevance of these specific salts in the built environment. We relate the information collected to the issues of risk assessment (considering both geochemical conditions and salt effects) and conservation interventions (highlighting the interest of tests that do not produce damage to susceptible materials) and present some methodological suggestions to avoid a case study culture.

The present work reviews studies with information on the effects of water by itself on stones of the built environment both to assess the impact of this substance and to discuss possible implications for conservation. The analysis concerns empirical results from previous publications dealing with the effects, on several rock types, of freeze–thaw, wetting, erosion by running water and substances resulting from the water–stone interaction. Laboratory studies have shown that water freezing can cause physical damage even in low porosity rocks. As far as we know, this is the first review that considers comparative laboratory studies of freeze–thaw and salt crystallization on the same rock specimens, and these point to lower erosive effects than salt weathering, as freeze–thaw can provoke catastrophic cracking. Wetting has shown strong damaging effects on some fine-grained clastic rocks. Erosive features have been reported for rain exposition and for some fountain settings albeit, in these field studies, it could be difficult to assess the contribution of pollutants transported by water (this assessment could have meaningful implications for stone conservation, especially in fountain settings). Water also interacts with stone constituents, namely sulfides and soluble salts, releasing substances that could impact those stones. Sulfides are a relatively frequent issue for slates and granites, and our observations suggest that for this last rock type, this issue is mostly associated with the presence of enclaves and, hence, avoiding the surface exposition of such enclaves could solve the problem.

In Galicia (NW Spain), granite was the most used stone material in historical buildings. Despite the good properties of granite as a construction material, it overcomes several physical and chemical weathering processes that cause decay, resulting in a loss of value in building materials, architectural elements and details. This is caused by a range of processes, from aesthetic damage to stone erosion. The causes of such decay are well known from case studies of historical buildings, being correlated with atmospheric agents, air pollution and aggregate materials, among others. In this work, we studied 15 historical monuments built with granite blocks of the architectural Heritage of the Barbanza Peninsula (Galicia). Because of the geographic features of this area, there is a steep rainfall and sea spray gradient that allows us to study historical buildings exposed to different environmental conditions in a short distance from the shoreline to inland. We used geochemical, petrological and microscopic tools to assess the decay of the granites and compared the results with environmental factors to assess the role of humidity (rainfall) and sea spray on the decay. Both the observation of coatings and the assessment of weathering have shown that buildings close to the shoreline are more affected by sea salts, while buildings far from the coast are mostly affected by biological weathering. Surprisingly, chemical weathering is higher in a strip area some hundreds of meters away from the sea shore and at lower altitudes (between 10 and 30 m). Indeed, very good correlation is observed for weathering indices, such as CIA, MWPI, VR, Si-Ti index, Kr, CAN and AKN, with a distance to the shoreline from 0.5 km, with linear correlation values ranging from −0.91 to 0.80.

This work presents a review of recent publications, with publication date between 2017 and 2019, with information on the relation between rock characteristics and the effects of diverse agents associated with alteration of stone materials in the built environment. It considers information obtained from ageing tests performed under laboratory conditions and by exposure to outdoor agents. Several lithological groups were considered, with sedimentary carbonate rocks being the most frequently studied lithotypes and silicate metamorphic rocks being the group with scarcer information. In terms of ageing tests, salt weathering was the most frequent one while there was a noticeable lesser amount of information from tests with biological colonization. The collected data showed the influence of diverse features, from specific minerals to whole-rock properties and the presence of heterogeneities. These information are discussed in the context of formulating a general framework for stone decay.