MINERALOGY OF SYDNEY BUILDING SANDSTONES IN RELATION TO GEOTECHNICAL PROPERTIES-2: RELATION OF QUANTITATIVE X-RAY DIFFRACTION DATA AND CATION EXCHANGE CAPACITY TO GEOTECHNICAL INDICATORS OF ROCK DURABILITY

摘要

The percentages of quartz, feldspar, carbonates and clay minerals in a suite of sandstone samples, evaluated using X-ray powder diffraction techniques and the SIROQUANT data processing system have been compared to a range of geotechnical properties used to evaluate the materials as dimension stones. The total percentage of clay minerals in the sandstones was found to have a relatively strong positive correlation to the dry and wet density of the rock materials, and an inverse or negative correlation to porosity and water absorption, consistent with infilling of otherwise empty pores in a silica-cemented quartz framework by detrital and/or authigenic clay accumulations. Similar relationships to porosity and density are shown by the cation exchange capacity, which has been found in previous studies to be related to the total clay mineral content. The total proportions of quartz and, where present, feldspar and carbonate minerals, are inversely related to the dry compressive strength of the sandstones, whereas the total proportion of clay minerals shows a positive relationship to dry strength. Quartz, feldspar and carbonate, on the other hand, show a slight positive correlation to the wet compressive strength; the total clay content is negatively related, although again only slightly, to the wet strength values. These relationships are interpreted as representing a contribution to the overall compressive strength by the interstitial clays when the sandstone and its clay minerals are in a dry state, but no contribution by the clays to the compressive strength in wet conditions. The ratio of wet to dry compressive strength, used in some instances as an indicator of sandstone durability, is positively related to the quartz, feldspar and carbonate content, and negatively related to the total proportion of clay minerals determined from either quantitative XRD or from the sandstones' cation exchange capacity. Similar relationships exist for dry tensile strength, determined from the modulus of rupture, and hence this property of the sandstones also appears to be increased by cohesion from the interstitial clay minerals. However, unlike compressive strength, the tensile strength in the wet state also decreases slightly with the quartz, feldspar and carbonate content, and increases slightly with the total proportion of clay minerals, suggesting that, in the wet state, the clay minerals retain at least some cohesive effects to influence the tensile strength characteristics. The loss of weight experienced by the sandstones in the sodium sulphate soundness test, on the other hand, seems to show little relation to the mineralogy of the sandstone samples. These findings confirm that quantitative X-ray diffraction analysis of Sydney sandstones provides results that can be related to key geotechnical parameters of significance to dimension stone applications. Quantitative XRD analysis may therefore be a useful complement to thin section petrographic studies, and also be of value as a simple basis for quality control of geotechnical testing programs or for interpolation of the data from such programs over a wider sample range.