The Proficiency of Sand Equivalent and Methylene Blue Test Methods for Determining the Deleterious Mineral Content of Manufactured Sands in New Zealand

Abstract

Weakly metamorphosed sedimentary rocks have the potential to contain levels of deleterious minerals that can have a negative impact on aggregate properties. Two of the most frequently employed methods to determine the cleanliness and therefore ensure suitability for use of fine aggregates and sands are the sand equivalent and methylene blue tests. Values relating to these test methods are often adopted in specifications as pass/fail criteria for establishing the suitability of an aggregate. This approach means many aggregate sources are precluded from use and a primary aim of this research is to highlight what potential effect this approach has on maximising the utilisation of non-renewable aggregate resources. The importance of aggregate fines quality and some of the specific deleterious minerals of concern are discussed in the context of New Zealand (NZ) mineralogy (although findings are relevant to countries with similar weakly metamorphosed sedimentary rocks) along with the methodology of both test methods and some of the limitations and concerns. Analysis of test data determined no strong correlation (R² = 0.14) between the current test methods and so further research and testing was undertaken to explore the possibility of modifying the existing methods to enhance their performance. Two modified methods were tested and proposed, normalised clay index (CIn) and steady state sand equivalent (SEss), on the basis that the correlation was much improved (R² = 0.65) and the modifications therefore appeared to better represent the actual product being tested. Investigations in to the influencing factors on the sand equivalent test were also conducted. These covered grading, settling rate and the influence of washing. The result showed that the sand equivalent test is heavily influenced by all these factors rather than the actual mineralogical clay content of the product being tested. The rapid clay test which is a newly developed test was trialled to see if it was an improvement on the current methods and therefore whether it had the potential to replace one ii or more of the current specified test methods. The initial results from the test programme, undertaken as part of this thesis, are encouraging and show that the rapid clay test definitely has good potential to become a key test for accurately determining the clay content of fine aggregates and manufactured sands. The test showed good correlation with the clay index test (R² = 0.68) and even better correlation with the normalised version of the test (R² = 0.78). The research presented shows that the current two main test methods of sand equivalent and clay index for qualifying whether fine aggregate or manufactured sand is suitable for use, are not allowing the full utilisation of non renewable quarry aggregate sources in New Zealand.