Stabilization/solidification (S/S) is a low-cost and high-efficiency remediation method for contaminated soils, however, conventional cement-based S/S method has environmental constraints and sustainability concerns. This study proposes a low-carbon, cement-free, clay-based approach for simultaneous S/S of As and Pb in the contaminated soil, and accordingly elucidates the chemical interactions between alkali-activated clay binders and potentially toxic elements. Quantitative X-ray diffraction and 27Al nuclear magnetic resonance analyses indicated that the addition of lime effectively activated the hydration of kaolinite clay, and the presence of limestone further enhanced the polymerization of hydrates. X-ray photoelectron spectroscopy showed that approximately 19% of As[III] was oxidized to As[V] in the alkali-activated clay system, which reduced toxicity and facilitated immobilization of As. During the cement-free S/S process, As and Pb consumed Ca(OH)2 and precipitated as Ca3(AsO4)2·4H2O and Pb3(NO3)(OH)5, respectively, accounting for the low leachability of As (7.0%) and Pb (5.4%). However, the reduced amount of Ca(OH)2 decreased the degree of hydration of clay minerals, and the pH buffering capacity of the contaminated soil hindered the pH increase. Sufficient dosage of lime was required for ensuring satisfactory solidification and contaminant immobilization of the clay-based S/S products. The leachability of As and Pb in high-Ca S/S treated soil samples was reduced by 96.2% and 98.8%, respectively. This is the first study developing a green and cement-free S/S of As- and Pb-contaminated soil using clay minerals as an environmentally compatible binding material.

Biochar is a waste-derived material that can sequester carbon at a large scale. The development of low-carbon and sustainable biochar-enhanced construction materials has attracted extensive interest. Biochar, having a porous nature and highly functionalised surface, can provide nucleation sites for chemical reactions and exhibit compatibility with cement, asphalt, and polymer materials. This study critically reviewed the state-of-the-art biochar-enhanced construction materials, including biochar-cement composites, biochar-asphalt composites, biochar-plastic composites, etc. The efficacies and mechanisms of biochar as construction materials were articulated to improve their functional properties. This critical review highlighted the roles of biochar in cement hydration, surface functional groups of engineered biochar for promoting chemical reactions, and value-added merits of biochar-enhanced construction materials (such as humidity regulation, thermal insulation, noise reduction, air/water purification, electromagnetic shielding, and self-sensing). The major properties of biochar are correlated to the features and functionalities of biochar-enhanced construction materials. Further advances in our understanding of biochar’s roles in various composites can foster the next-generation design of carbon–neutral construction materials. Graphical Abstract