Characterisation of weathering of Sydney sandstones in heritage buildings

Abstract

“Yellow block” sandstone, a colloquial expression used for a locally quarried variety of sandstone, has made an important contribution to the cultural and architectural heritage of Australia’s largest city, Sydney. The golden colour of this dimension stone adds to the attractive appearance of a number of significant landmarks of Sydney. After almost a century of exposure, the progressive decay of the natural consolidant, which is predominantly clay, is causing deterioration of many of these sandstone buildings. While in some cases partial and total replacements have been successful in the restoration work, a proper understanding of the cause of decay of the natural consolidant is necessary if a suitable consolidant is to be developed to preserve the original blocks. This paper reports the findings of a study of the weathering behaviour of the clay matrix in “yellow block” sandstones used in some of Sydney’s historic buildings. Sandstone samples were investigated by the techniques of FTIR spectroscopy, X-ray diffraction, scanning electron microscopy, inductively coupled plasma–atomic emission spectroscopy (ICP–AES) and thermal analysis. The analytical results support the theory that the changes to the original kaolinite clay structure upon prolonged exposure and weathering of the sandstone blocks investigated are at least partly due to the substitution by Fe 3+ for Al 3+ in the octahedral sheet.