Marine clay in ultra-high performance concrete for filler substitution

This study demonstrated the utilization of marine clay as a substitute to quartz filler in ultra-high performance concrete (UHPC). The clay used in the study, obtained as a waste from excavation works in Singapore, was a low-grade kaolinite clay with 29% kaolinite content. It was dried, ground, and calcined at 700 degrees C to activate the aluminates through de-hydroxylation. Studies were conducted wherein the calcined marine clay was used to replace quartz powder by 30%, 50%, 70% and 100% by weight (wt.) and investigated for compressive strength, hydration kinetics, phase composition and portlandite consumption. Comparable strength as that of the reference mix was achieved at 30% quartz filler substitution, while higher level of substitutions led to decrease in strength. The results from iso-thermal calorimetry indicate strong nucleation effect due to clay showing substantial acceleration in hydration peaks. The long-term effect on hydration degree was comparable for all mixes as determined from bound water calculated using thermo-gravimetric analysis (TGA). In fact, higher level of substitution affected the amount of portlandite consumption. Overall, this study shows waste marine clay as a viable alternative to costly and carcinogenic quartz powder and hence, provide a new approach for sustainable UHPC. (C) 2020 Elsevier Ltd. All rights reserved.