The durability of Tasmanian building sandstones
Creators: name: Sharples, Christopher
Publisher: name: University Of Tasmania
Year: 2023

description: Tasmanian building sandstones have been almost entlrely quarried from fluvial sandstones of the Early Triassic Quartz Sandstone Sequence and the Permian Lower Freshwater Sequence. These continue to be the only horizons considered prospective for new sources of high quality building sandstone. Technical data on all significant Tasmanian building sandstone sources is presented. There are three methods of assessing and predicting sandstone quality and durability in the built environment: 1 ) Inferring predicted stone behaviour from measurement of sandstone properties. 2 ) Accelerated decay tests. 3 ) Observed performance in existing buildings. Each method is subject to limitations. Optimum assessments are made by interpreting a combination of data from all three approaches, in the light of an understanding of the nature of sandstone properties and of the processes of sandstone decay. Investigation of the geological processes controlling the genesis of sandstone properties has led to the development of models to facilitate exploration for high quality building sandstones: Most jointing in Parmeener Supergroup sandstone is related to Mid-Mesozoic and Early Tertiary faulting. The areas most prospective for widely-jointed sandstones are predicted on the basis of known regional variations in fault densities. The bulk colour of sandstone is related to the content of iron-rich minerals in the sandstone, particularly vermiculite, smectite and chlorite, which oxidise to yield brown ferruginous colouring. Liesegang rings form in proximity to iron-rich basic igneous rocks, probably through weathering-related groundwater processes. For most building purposes, thick and massively-bedded sandstone is the ideal. There is no way of predicting the occurrence of such beds on a regional or local scale, except insofar as they are more prevalent in the stratigraphlcally lower parts of the Triassic Quartz Sandstone Sequence as a whole. Sandstone strength and porosity are functions of mineralogy and intergranular texture. The geological controls on these properties are discussed. The occurrence of superficial pachydermal fractures on natural outcrops is considered to be an indicator of sandstones having weak intergranular textures resulting in a high degree of dimensional instability. Smectite swelling clay is detrimental to sandstone durability. The proportion of smectite in sandstone varies markedly within individual outcrop areas, but on a larger scale there appear to be regional and possibly stratigraphic patterns in smectite occurrence. The smectite (together with vermiculite) is considered to have formed by alteration of volcanic dust deposited from ash clouds produced by contemporaneous volcanic sources to the southeast of the Tasmania Basin. Weathering of natural outcrops alters important sandstone properties, most importantly through near-surface kaolinisation. An exploration program for high quality building sandstones is proposed which takes account of this limitation, and of the exploration models developed in this work. The major contribution of this thesis is that an understanding has been achieved of the avenues of research which are necessary to further the development of models explaining the genesis of important quality and durability-related sandstone properties. These are listed.