Smectite clay identification and quantification as an indicator of basic igneous rock durability

Abstract

In terms of aerial extent, basic igneous rocks are one of the most used road building materials in South Africa. This is mainly due to the fact that approximately 57% of the surface lithology in South Africa is of poor road construction quality, rendering the localized Karoo dolerite intrusions the best construction material available for many road construction projects. Historically, examples of such non-durable materials have been reported and investigated by many authors and current South African specifications specifically require consideration of this potential phenomenon. There is, however, still the need for an accurate, reasonably quick and inexpensive method of determining material durability in order to identify variations within any one aggregate source. Smectite clay content has been proposed for this task but quantification of the clay mineral content of typical Karoo basic igneous rock aggregates has been problematic in recent studies. Published works have commonly discussed reasons why quantitative X-ray diffraction analysis of clays and some of the techniques developed to qualify and quantify clays using X-ray diffraction are problematic. Recent attempts to quantify clay mineralogy as an indication of basic igneous rock aggregate durability proved inconclusive and lead to a comparative study on X-ray diffraction results obtained using different techniques and by different laboratories. The findings confirmed what that literature suggested and also revealed that proposed specifications based on smectite content are not feasible due to quantification accuracy exceeding some of the proposed limits.