The Proficiency of Sand Equivalent and Methylene Blue (Clay Index) Test Methods for Determining the Deleterious Mineral Content of Weakly Metamorphosed Sedimentary Rock

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 so a primary aim of this research is to highlight what potential effect this approach has on maximising the utilisation of nonrenewable aggregate resources. The importance of the quality of aggregate fines and some of the specific deleterious minerals of concern are discussed in the context of New Zealand mineralogy (although the findings are relevant to countries with similar weakly metamorphosed sedimentary rocks). The methodology of both test methods and some of the limitations and concerns are also discussed. The paper outlines the experimental program and testing that took place over a period of three months. The results presented and discussed for 380 samples from seven quarries aimed at trying to establish a correlation between the sand equivalent and the clay index. In agreement with previous research it was concluded that no correlation between the two test methods exists (R2 = 0.08). Possible reasons for a lack of correlation are presented, along with the concepts of a new ‘normalised’ clay index and a ‘steady state’ sand equivalent. Early results from an initial testing program are presented which demonstrate that the adapted testing methodology gives a stronger correlation (R2 = 0.65) between the two test methods and has the potential to give a better indication of an aggregate’s suitability for use.