Multi-method study on the effect of biochar on the properties of alkali-activated slag

초록

Alkali-activated slag (AAS) is regarded as a promising low-carbon alternative to Portland cement; however, the increasing scarcity of slag and the need for further sustainability improvements motivate the exploration of renewable and carbon-based additives. Biochar, as a porous and carbon-rich material derived from biomass waste, has shown potential in cementitious systems, yet its role and mechanism in AAS remain insufficiently understood. To address this gap, a systematic experimental study was conducted to investigate the effects of biochar on the reaction kinetics, microstructure, and engineering properties of AAS. Biochar was used to partially replace slag at 1, 2, and 5 wt%. Isothermal calorimetry, XRD, FTIR, SEM, and TG analyses were employed to characterize reaction behavior and products, while workability, compressive strength, ultrasonic pulse velocity (UPV), electrical resistivity, and chloride diffusion tests were performed to evaluate macroscopic performance. The results showed that biochar increased the 7-day cumulative reaction heat and promoted late-age CASH formation, while not altering the main reaction products, indicating a predominantly physical role. The porous structure of biochar provided internal space for alkali activation but also introduced air bubbles during mixing, leading to reduced absolute compressive strength and UPV. Despite this strength reduction, moderate biochar additions (1-2 wt%) had limited adverse effects on durability-related properties and showed improved relative strength development at later ages. Overall, this study clarifies the mechanism by which biochar influences AAS systems and demonstrates the feasibility of partially replacing slag with biochar, providing guidance for the development of more sustainable alkali-activated materials.

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