Algal degradation of natural stone masonry : implications for conservation and construction
The objective of this research was to determine the impact of algal colonisation on
natural stone masonry.
Experimental work was carried out to determine the physical and chemical damage
caused by micro-algae using reflected light microscopy, Scanning Electron Microscopy
and Inductively Coupled Plasma - Atomic Emission Spectroscopy. Colonisation
experiments were performed on individual mineral chips of quartz, calcite, dolomite,
siderite, labradorite, orthoclase, a perthitic albite, muscovite and montmorillonite; as
well as the Giffnock sandstone, a traditional building stone of the Glasgow area. Work
was also carried out to determine the effect of algal colonisation on the absorption of
water into a masonry surface.
The research determined that algae create an alkaline environment in the areas they
colonise. Algal mediated damage to the mineral substrates includes the dissolution and
pitting of carbonate surfaces as well as the etching of plagioclase feldspar surfaces.
Algal colonisation preferences were noted throughout the experiment with algae
preferentially colonising kinks and steps in the topography of mineral surfaces as well
as grain edges. Preferences were also seen in the colonisation of the Giffnock sandstone
with micas showing heavy colonisation compared to other minerals in the lithology.
Algal swelling and contraction cycles were examined in Environmental Scanning
Electron Microscopy experiments and the impact that this physical swelling may have
on the stone is modelled.
Algal biofilms at the surface of the stone lead to an acceleration in the rate at which
water enters the surface of the stone, this is important as water is the main weathering
catalyst for masonry weathering.
The findings of this project implicate algae in the weathering of natural stone masonry
through enhanced mineral dissolution, mineral etching and pitting, patina formation,
physical weathering through swelling cycles and the alteration of the surface physical
properties in relation to water absorption.