Potential of Calcined Recycling Kaolin from Silica Sand Processing as Supplementary Cementitious Material

Abstract

Suitability of calcined kaolinitic filter cake arising from the production of high-quality silica sand as SCM was tested. The investigation comprises two different grades of materials. The first one represents a cross section from one week of sand production. The other sample has been subsequently prepared by sedimentation on a laboratory scale in order to investigate the impact of lower sand content. Chemical and mineralogical compositions of both samples were determined by means of ICP-OES, XRD and FTIR. The laboratory sample yielded higher kaolinite and a lower quartz content in comparison with the industrial product. The dehydroxylation behavior was determined using TG/DTG. After thermal activation, the reactivity was investigated by measuring the solubility of Al- and Si-ions in alkaline solution. It turned out that a calcination temperature of at least 650 °C is required for a complete dehydroxylation. Heat of hydration was studied by isothermal calorimetry using a substitution of 20 wt% of cement by the calcined product. The same substitution was chosen for the determination of strength activity index on mortar bars. Both materials provided a significant acceleration of the early hydration by promoting the aluminate reaction. After 28 days, the higher kaolinite content of the laboratory sample leads to a higher activity index of 121% in comparison with 102% of the industrial product.