The overall aim is to understand how periods of prolonged, deep-seated wetness impact on the ‘greening’, deterioration and potential conservation of sandstone masonry, through an interdisciplinary study linking civil engineering, geomorphology, climatology and environmental microbiology with architects and conservators.
Specific objectives needed to meet the overall aim are listed below:
1. To understand the current greening of sandstone walls typical of that found across the NW UK today and its relationship to climate, air quality and micro-environmental conditions.
2. To monitor the moisture contents of sandstone test walls to quantify ‘time of wetness’ (both at the surface and at depth) in the west of Northern Ireland and its relationship to greening.
3. To relate wetness regimes in sandstone walls to existing climatic conditions.
4. To determine future wetness regimes in sandstone walls through statistically downscaling climate projections and utilising links between wetness and climatic parameters discovered in objective 3.
5. To investigate the relationship between moisture levels and ion diffusion of key anions and cations within sandstone blocks and their potential significance for sandstone deterioration.
6. To investigate the links between greening (algal colonisation) of sandstone blocks and moisture regimes and deterioration and to develop and test new conceptual models of sandstone deterioration under wetter, ‘greener’ conditions.
7. To devise practical advice for building with sandstone and managing the greening and deterioration of existing sandstone walls under wet conditions.
Predicting the impacts of future climate change on surface and deep-seated wetness of sandstone masonry is a highly difficult task, involving downscaling global predictions and consideration of probabilistic models. Initial work by the Noah’s Ark project produced model-based maps of changing sandstone moisture contents showing current very dry conditions across Europe and even drier trends in future (NOAH’s ARK, 2007). However, UK-specific studies predict increases in winter wetness in the northwest UK over the next 50 to 100 years (UKCIP) which will enhance the moisture contents of sandstone masonry. There is now a clear need to downscale such predictions to UK regions, build more probabilistic models, and produce firmer links between climatic parameters and observed sandstone moisture regimes. This research contributes to two developing EPSRC science areas, i.e. ‘Adaptation and resilience to a changing climate’ and ‘Science and heritage’
Recent observations show an increase in algal ‘greening’ of external sandstone walls in many places in the northwest of the United Kingdom (NW UK). We hypothesize that this is caused by a combination of increased moisture and decreased air pollution, and thus reflects recent changing environmental conditions. Research shows that moisture availability at the stone surface is the dominant control on the colonization and growth of algae and other organisms (Crispim et al., 2003; Urquhart et al., 1995; Young, 1997). Algae (especially green algae or Chlorophyceae) grow predominantly in cool/ wet conditions, whilst warm/ dry climates encourage the growth of cyanobacteria (Richardson, 1995, Ortego-Calva et al., 1992). Whatever the type of colonizer, wetter conditions favour more luxuriant growths. Algae, especially green algae such as Trentepohlia, have been identified as agents of biodeterioration (Wakefield et al., 1996; Welton et al., 2003), although their impacts are poorly studied compared to cyanobacteria, fungi and lichens. Algal impacts are biochemical (attack by acid exudates from algae and their extracellular polymeric substances) and biophysical (wetting and drying, expansion and contraction causing detachment of particles). Reductions in sulphur-based air pollution, combined with stable nitrogen-based air pollution, may also be contributing to increased ‘greening’ of sandstone walls through enhancing nutrient availability (Mitchell and Gu, 2000; Herrera and Videla, 2004; Thornbush and Viles, 2006). Previous work by the applicants showed rapid surface colonisation by algae of sandstones in exposure trials from April 1999 to March 2001 across Belfast and an associated decrease in surface porosity and permeability, thus providing some support for a potential ‘positive feedback’ role of algae in altering moisture regimes. These exposure results are very different from previous trials in the early 1990s which indicated little if any algal colonization, but extensive inorganic soiling and surface gypsum formation (Smith and Curran, 2000; Smith et al., 2004). Between the early 1990s and 2000 Belfast experienced a reduction in atmospheric SO2 levels and locally increased NOx concentrations; associated changes in domestic fuels and traffic; and over this period winter precipitation marginally increased while summer precipitation decreased. On the basis of these observations there is a clear need for detailed examination of the controls on algal ‘greening’ and the impacts of increased algal colonisation on the soiling and deterioration of stone, especially quartz sandstones prevalent across NW UK. Northern Ireland provides an ideal ‘natural laboratory’ for such studies.
Associated with the greening of stone, EPSRC-funded research also identified deep-seated wetting of sandstone blocks within walls. This could have major implications for both the decay of stone and moisture penetration into buildings, encouraging processes under saturated conditions during winter, such as ion diffusion. Turkington and Smith (2000), for example, found complete salt penetration of entire sandstone blocks from buildings in Belfast, together with inconsistent anion/cation ratios suggesting the importance of ion diffusion of salts during periods of saturation. This process is well recognised by those studying chloride corrosion of concrete where it can be measured using diffusion cells, in which reservoirs of salt solution and de-ionised water are separated by the concrete samples to be tested (Shaát et al. 1994). In low porosity concrete, diffusion rates are invariably very slow. However, exploratory diffusion cell measurements using 20mm thick discs of Permian quartz arenite (Dumfries sandstone) showed that diffusion can be comparatively rapid through porous building stone (Smith et al. 2005). These results emphasise the need to think beyond near-surface mechanisms of salt ingress and penetration in solution to consider, for example, seasonal climatic cycles, the effects of which penetrate deep into stone. Anions mobilised by these cycles may also enhance selective corrosion of quartz cements, increasing susceptibility to salt weathering in certain sandstones. During diffusion, ions may also be adsorbed and held on pore walls, producing an effective or ‘apparent’ diffusion coefficient (Andrade et al. 1994). Experimental research also highlighted the possibility of convective flow of ions in porous materials in response to differential pressure and density gradients in tests using diffusion cells (e.g. Poupeleer et al. 2003, Terheiden 2008). Such flows could be highly relevant to salt movement within masonry blocks with highly variable boundary conditions. Neither enhanced surface wetness and algal growths nor increased wetting at depth are taken into account by current models of sandstone deterioration. These models, which are also used as a basis for durability testing, are thus inadequate for making decisions over the use, management and restoration of sandstone masonry in the NW UK under present and future environmental conditions. There is a clear need to develop a better understanding of the role of deep-seated wetness on sandstone deterioration, and to test a more appropriate model of sandstone deterioration.
Previous explanations of sandstone deterioration tended to emphasise the importance of infrequent wetting followed by thorough drying. As indicated above, evidence suggests that such cycles are not the only ones found today in many parts of NW UK, but that climate change will make them less likely to occur in future – especially during winter. Our proposed new model reflects current and future conditions in NW UK more accurately. Research is now required to test the different components of this model, specifically how the dominance of saturated conditions will influence chemical deterioration processes at depth and how surface biofilms will influence near-surface drying regimes and deterioration processes.