Turning sandstone clay into supplementary cementitious material: activation and pozzolanic reactivity evaluation

The present paper explores the methods of transforming low grade kaolinitic clay, sandstone clay, into supplementary cementitious material (SCM). The sandstone clay is the waste product in the sand manufacturing from sandstone mineral. Thermal calcination at 600-900 degrees C was applied to activate the initial sandstone clay. Microstructure characterization was then carried out on the obtained activated sandstone clays (ASCs) to study the microstructure and chemical changes during the thermal activation. The pozzolanic reactivity of ASCs were assessed by direct methods (isothermal calorimetry test and lime consumption test) and indirect method (strength measurement of ASC blended mortar). The compressive strength of the ASCs blended cement based mortar showed that ASCs possessed high pozzolanic reactivity through the dehydroxylation and the alternation of Al and Si coordination environments during thermal activation. Agglomeration occurred after the sandstone clay calcination, however the particle size was reduced. Even at a short time calcination for 1 h, the pozzolanic reactivity of ASCs improved as the calcination temperature increased from 600 to 900 degrees C. Mortar strength test results confirmed that the ASCs with a fineness similar to OPC, can be used as high quality SCMs. Further isothermal calorimetry results indicated that ASCs not only contributed to pozzolanic reaction but also had a good filler effect in the early hydration stage due to the existence of micro-quartz.