Duthie, Laura Jane
The impact of climate change on blond sandstone decay in Glasgow.
PhD thesis, University of Glasgow.
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Glasgow contains many buildings made from local ‘blond’ sandstones that are showing the legacy of 150 years of urban weathering and this decay may accelerate as climate changes in the future. Most of the blond sandstones are Carboniferous in age and comprise of micaceous quartz arenites with varying amounts of diagenetic minerals including ankerite and kaolinite. Chemical decay can be visually identified in the carbonate minerals, but the combination of quantitative X-ray microanalysis (chemistry of mineral) and Raman spectroscopy (structure of mineral) also allows chemical decay to be observed in both kaolinite and muscovite. The Raman spectroscopy shows a variation in the muscovite minerals between the outermost surface and internal region (20 mm depth), demonstrating that structural decay is occurring within the micas and reduces them to an “ionic slurry state” in a relatively short time frame. The impact of microbial colonisation on the stones was also investigated. Using osmium stained polished blocks, complimented by scanning electron microscope imaging, microorganisms were found to be living cryptoendolithically in a few samples but at very shallow depths (<2 mm). Light penetration results revealed that a thin weathered crust (<1 mm) on the surface of sandstone will restrict the transmission of light into the rock, thereby preventing the colonisation of photosynthetic microbes. As a result, most microbial communities are restricted to the stone surface but only where they will not be subjected to photo-oxidative damage, which frequently occurs during summer months. Consequently, the most extensive microbial colonisation is restricted to the sandstone’s surface and during the winter. Using novel internal microclimate monitoring technologies it was found that stone temperature and humidity is decoupled from ambient conditions. During the summer the stone interior is considerably hotter than air temperature, whilst relative humidity is generally comparable to external conditions, whereas in the winter interior temperatures are closely related to ambient conditions and relative humidity is generally much higher. To understand how sandstone buildings will react to a change in climate, current conditions were extrapolated to 2080 using predictive models for the Glasgow region and the impact of these conditions was investigated by accelerated weathering experiments in a climate chamber. Results reveal rapid granular disintegration, the rates of which are independent of grain size. Overall, this study concludes that rates of chemical decay will increase as the climate warms and becomes wetter overall, primarily
through dissolution, decay and loss of diagenetic minerals, and the extent of microbial activity will change, but these effects will be strongly dependent on local microenvironment. Implications of these findings are that more work on conservation and preservation techniques will be very important to protect the stone-built heritage of Scotland.
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