Carbonaceous admixtures in cementitious building materials: Effect of particle size blending on rheology, packing, early age properties and processing energy demand.

Application of biochar, produced from locally generated wastes, as admixture in cement is a strategy to upcycle biomass waste and produce durable building materials. This research explores the influence of particle size and porosity of biochar, prepared from coconut shell and wood waste, added at 2 wt% of cement, on rheology, setting time, hydration and early age strength of cement mortar. For each biochar type, three particle size gradations are explored - coarser biochar (d = 45-50 μm) (obtained by sieving), finer biochar (d = 10-18 μm) (obtained by ball milling) and combination of coarser and finer biochar (d = 15-25 μm). Experimental findings suggest that combination of coarser and finer biochar improves workability and rheological properties of binder pastes compared to that with (only) coarser biochar. Depending on biochar type, hydration and rate of setting are accelerated compared to control. Inclusion of finer biochar and combination of finer and coarser biochar improve packing density and degree of hydration of pastes compared to coarser biochar and control, leading to 12-19% enhancement in compressive strength at 7-day age. Micro-structural investigations show that the macro-pores of coarser biochar can be filled with dense hydration products, although some macro-pores may remain unfilled. This offsets improvement in strength that can be achieved through enhancement in packing density. The approach of blending coarser and finer biochar reduces the energy demand and cost associated with ball-milling by 23-37% and SGD 2.30-4.80 per ton respectively compared to only finer (ball-milled) biochar per cubic meter of concrete. Overall, the findings from this research demonstrate that blending of biochar of different particle size distributions can enhance physical properties of cement-based materials, while reducing associated energy consumption.