A study in Structural Concrete found that incorporating 6% rice husk biochar into alkali-activated slag concrete increased the number of drops required for the first visible crack by an impressive 185% compared to the control mix. This biochar-supplemented concrete also showed a 44.6% enhancement in 28-day compressive strength. Alkali-activated materials are emerging as greener alternatives to Ordinary Portland Cement (OPC) due to their lower carbon dioxide emissions and reduced need for natural resources. The production of ground-granulated blast furnace slag (GGBS), the main binder in alkali-activated slag concrete (AASC), results in significantly less energy consumption and carbon dioxide emissions compared to OPC. AASC itself is well-regarded for its exceptional mechanical performance, even when cured at ambient temperatures. While biochar, a material produced from biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More, is already recognized for its ability to improve the mechanical properties of concrete and mortar , the potential for enhancing its impact resistance was previously unexplored. Authors Harshani Egodagamage, Hiran Yapa, Samith Buddika, Thomas Loh, Satheeskumar Navaratnam, Yulin Patrisia, and Thuy Nguyen investigated this potential.

The experimental program examined five AASC mixes with different rice husk biochar (RB) contents: 0%, 2%, 4%, 6%, and 8%. The results confirmed that adding RB significantly improved the 28-day compressive strength of AASC, with the most notable increase occurring at the 6% RB level. Specifically, the 6% RB mix showed a 44.6% increase in compressive strength compared to the biochar-free sample, while the 8% RB blend yielded no additional gains. This enhancement is largely attributed to biochar’s high absorption and water retention capabilities. The porous biochar absorbs a portion of the mixing solution, effectively reducing the solution-to-binder ratio and lowering the evaporable free water content, thereby creating a denser microstructure and increasing the compressive strength. Additionally, the retained water acts as an internal curing agent, favoring the formation of hydration products and further increasing strength.

More critically, the study demonstrated a substantial augmentation of the concrete’s impact resistance with increasing RB content, up to the 6% dosage. The impact resistance was measured using a drop-weight test, which recorded the number of drops required for the first visible crack (L1​) and for complete failure (L2​). The 6% RB mix exhibited a maximum increase of 185% for L1​ and 175% for L2​ over the control mix. This improvement in impact performance is also credited to the densification effect of RB on the AASC microstructure, an observation supported by the scanning electron microscopy analysis. The porosityPorosity of biochar is a key factor in its effectiveness as a soil amendment and its ability to retain water and nutrients. Biochar’s porosity is influenced by feedstock type and pyrolysis temperature, and it plays a crucial role in microbial activity and overall soil health. Biochar More of the mixes showed a declining trend up to 6% RB, which agreed with the strength and impact resistance results. However, the beneficial effects diminished with an over-dosage of biochar. Increasing the RB level from 6% to 8% drastically reduced the impact resistance. Researchers suggest this reduction is due to the inherent brittle characteristics of biochar. The failure patterns of the AASC samples under impact further supported this conclusion. With the increasing biochar dosage, the samples displayed an increasing propensity to fracture along a single diagonal crack, a characteristic associated with increased brittleness in the composite. The biochar-free and lower-dosage mixes showed a more spread-out, multi-directional crack pattern.

The findings suggest that a replacement level of rice husk biochar between 4% and 6% is optimal for achieving both improved strength and enhanced impact resistance in alkali-activated slag concrete. This makes biochar a promising, sustainable additive for developing concrete structures—such as road barriers and rock-fall protection walls—that are specifically designed to resist impact loads.

Source: Egodagamage, H., Yapa, H., Buddika, S., Loh, T., Navaratnam, S., Patrisia, Y., et al. (2024). Enhancement of impact resistance of alkali-activated slag concrete through biochar supplementation. Structural Concrete, 25(5), 3630–3647.