A multi-method investigation into rheological properties, hydration, and early-age strength of cement composites with admixtures recovered from inorganic and bio-based waste streams

The selection of mineral admixtures to develop low-carbon cement composites must be diversified to balance the increasing demand and low supply of certain widely used admixtures. The research aims to advance scientific knowledge into the comparative influence of several admixtures recovered from different waste streams including silica fume (SF), fly ash (FA), pulverized glass powder (PGP), biochar (BC), and high-performance ash (HPA) on static yield stress, dynamic yield stress, plastic viscosity, setting, micro-structural build-up, degree of hydration and compressive strength of cementitious paste. Experimental findings show that adding 10 wt% silica fume results in a 300% increase in yield stress (under static and dynamic shear rate) and plastic viscosity than control, attributed to its fine particle size and higher gelling rate during early hydration. HPA leads to an 83% increase in static yield stress compared to control at 70 mins, but the application of a high shear rate leads to a marginal increase (18%) in yield stress than control. PGP leads to improvement in workability by reducing the dynamic and static yield stress by 50 – 55% than the control without significant change in plastic viscosity. Comparing the influence of BC, FA, and PGP, which have comparable particle size distribution in this research, the addition of biochar accelerates micro-structural build-up and final setting, evident from higher ultrasonic pulse velocity and faster hydration kinetics by 1.20 h. Cement pastes with FA, PGP and HPA demonstrate significantly lower compressive strength at 3-day and 7-day age than control but offer similar 28-day strength. The addition of SF and BC do not compromise 3-day and 7-day strength while offering 16% enhancement in 28-day strength than control attributed to a higher degree of hydration due to pozzolanic and filler effects respectively. The findings suggest that PGP, BC, and HPA could be potential alternatives to more widely used FA and SF as supplementary mineral admixtures in cement-based construction materials.