A combination technique to improve natural low-grade illite as supplementary cementitious material for concrete

Kaolinite-rich clays are the most utilized clay type as a supplementary cementitious material (SCM) due to their higher pozzolanic reactivity. However, these higher-grade clays may not be available due to widespread usage in other industries and geographical locations. Hence, this study investigates the potential of combining low-grade illite clays, characterized by low clay mineral content (<30 %) and reactivity, with higher-grade kaolinite clays to enhance pozzolanic properties and mechanical performance in clay-cement blended cementitious mixes. An optimal clay mixing ratio of 1:1 was found to yield improved strength (3 % higher than the control mix) and reactivity particularly when clays were co-calcined(13 % higher bound water content). Analysis indicated that the co-calcination process increased amorphous content by around 18 %. The high Si/Ca ratio (24 % higher than separately calcined and mixed) in the C-A-S-H component of the co-calcined mix is also visible with more amorphous Si. A separate Fe phase was formed, which improved the pore structure of the co-calcined mix (reduced porosity by 41 %). This contributed to an 8 % increase in compressive strength compared to the clays separately calcined and subsequently mixed. The findings suggest that the combination technique effectively enhances the utility of low-grade clays, providing a viable strategy for mitigating reliance on high-grade resources. • Combining low-grade illite with kaolinite to 1:1 ratio provides 28-day SAI > 1. • Co-calcination of clays increased amorphous content by 18 %. • Co-calcined clays formed a separate Fe phase. • The separate Fe phase lowered the porosity by 41 %. • CASH in the co-calcined blend had a 24 % higher Si/Ca ratio.