Investigation into clay-based consolidants for conservation of 'yellow block sandstones' in Sydney's heritage buildings
- Publication Type:
- Thesis
- Issue Date:
- 2007
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Many of the 19th century heritage buildings, located in Sydney, were built from
locally quarried sandstone. After more than a century of natural weathering, a number
of the sandstone buildings are showing signs of deterioration. In order to ascertain the
appropriate preservation techniques of such buildings, an understanding of the
mechanisms of degradation of these buildings stones must first be sought before
consolidation treatment is carried out. The objectives of the thesis are to first
characterise the degradation processes of selected heritage yellow block sandstone,
followed by the synthesis and characterisation of potential polymer-clay
nanocomposites as stone consolidating systems. In order to target particular
degradation problems in heritage sandstones, a thorough understanding of the
degradation mechanisms of the sandstone is essential before suitable materials are
synthesised to prevent or slow down further damages to the stones. The novel
approach of this thesis is to use a large range of analytical techniques for the
characterisation of degraded yellow block sandstone samples. The methods of
preparation of a series of novel polymer-clay nanocomposite consolidating systems can
then be optimised according to the characteristics of each stone, and potential
consolidant systems can be identified. Although various materials have been
employed as stone consolidants in the past, the proposed use of polymer-clay
nanocomposites as potential stone consolidants is a novel approach.
A number of analytical methods including FTIR, NMR, XPS, XRD, SEM and
thermal analysis were used to characterise the sandstone and to determine the
degradation mechanisms of the sandstones in Sydney's heritage buildings. The yellow
block sandstones were found to be composed of sand grains (60 - 68%) bound together
by a kaolin-based cementing material (16 - 25%). As the silica sand is essentially
inert, the study focused on the clay component of the stone. An increase in iron
concentration on the stone surface contributed to the discolouration of the stone and
provided a source of Fe³⁺ for the isomorphous substitution of Al³⁺ in the octahedral sites
and possible Si⁴⁺ in the tetrahedral sites of the aluminosilicate layers in the cementing
clay. The substitution resulted in the brittleness of the stone, but preserved the layered
structure of the clay binder and retained the overall integrity of the sandstone. A
change in pore size distribution was observed on weathering of the sandstone, with an
increase in population of large pores providing greater access to atmospheric pollutants,
soluble salts and rainwater to the sandstone core, making the already weathered stones
more vulnerable to further degradation.
Based on the model of degradation, the physical properties of Sydney sandstones
and the aim to produce consolidants for easy application, hydrophilic polymer-clay
nanocomposite systems were prepared. Montmorillonite was used as the clay
component for its similar layer structure as the kaolinite presented in the cementing
materials in the yellow block sandstone samples, while poly(vinyl alcohol), poly(acrylic
acid) and poly(ethylene oxide) were used as the polymer component for their
hydrophilic nature. AFM and XRD analysis were used to investigate the polymer-clay
interactions in these composites. While the AFM analysis reveals the topography of
the synthesised polymer-clay film without melting the samples, XRD analysis indicates
the degree of separation of the montmorillonite clay platelets by the polymer chains
through the detection of the shift of the XRD peaks. The intercalation and partial
exfoliation of montmorillonite platelets in different hydrophilic polymer matrices was
observed in both the solution and melt intercalation methods. PAAMMTa samples
were found to be the best intercalated/exfoliated nanocomposites in the solution
intercalation method. Although better separation of clay platelets was demonstrated in
the XRD results using the melt intercalation method, it would not be considered a
preferred method at present time due to the impractical nature of using solid products as
stone consolidants. However, further research may provide solution for the dissolution
of such materials in suitable solvents without affecting its consolidating ability. The
hydrophilic nanocomposite materials investigated in this project show great potential as
a new class of sandstone consolidants for the binding of porous weathered sandstones in
Sydney's heritage buildings.
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