Waste Valorisation using biochar for cement replacement and internal curing in ultra-high performance concrete

The degree of hydration in ultra-high performance concrete (UHPC) is known to be low due to low water-cement ratio, leading to a substantial portion of cement merely being an expensive filler. The study presented herein demonstrates the potential of biochar as a mineral additive for cement replacement in UHPC, enhancing hydration due to its internal curing and nucleation effects. Biochar was prepared by pyrolysis of locally available mixed wood saw dust and segregated into three size ranges: coarse (C): 250-500 mu m, medium (M): 125-250 mu m, and fine (F): smaller than 125 mu m. These were then used to replace cement at 2, 5 and 8% by weight in pre-soaked condition. The rate and total heat of hydration measured by isothermal calorimetry were consistently higher for biochar samples, with coarse biochar samples generating up to 30% more heat at 96-h mark compared to the control mix. The degree of hydration, estimated from thermo-gravimetric analysis using Bhatty's method, was found to increase from 42% (in control mix) to 59% (in samples with 5% by wt. fine biochar). Fine biochar at 5% replacement also showed comparable 28-days compressive strength (144 MPa) as the control mix (150 MPa). Microstructure of hydrated pastes studied through scanning electron microscopy showed deposition of cement hydrates on the surface and inside the surface pores of biochar, with dense interfacial transition zone, further suggesting the efficacy of biochar in improving hydration. (C) 2019 Elsevier Ltd. All rights reserved.