Rapid and economical method for assessing metakaolin quality as SCM for low carbon binders

Abstract

While the assessment of the quality of clay calcination for use in cements is important to ensure optimal performance, the currently available test methods, such as lime-reactivity, R3, XRD, TGA, etc. do not meet the requirements for a quick and cost-effective quality control at industrial clay calcination units. This paper proposes the use of a combination of methylene blue and density measurements to obtain reliable assessment of the quality of calcination of kaolinite-rich clays. Seven different clays calcined within the temperature range of 400 °C to 1000 °C were studied using these techniques and the results were compared with traditional methods. The results clearly demonstrate that the degree of conversion of kaolinite to metakaolin can be effectively obtained by measuring the residual kaolinite-content using the methylene blue test. A 50% reduction in the methylene blue value indicates the presence of under-calcined clay. Additionally, the formation of the spinel phase, which is the first unreactive product that forms upon over-calcination, can be identified by density measurements, as it is associated with higher density values. The proposed test methods can be implemented at clay-calcination units without the need for expensive equipment and highly-skilled technicians. The methods are seen to be sufficiently quick and reliable for a continuous monitoring of the calcination quality at short intervals.