The synergistic effects of straw biochar and MgO carbonation on improving the physico-mechanical properties of silty clay

• Straw biochar serves as a multi-functional medium to enhance CO 2 transport and product nucleation in MgO carbonation. • The strength of biochar-MgO carbonated soil achieved 4.51 MPa, 3.1 times that of biochar-free carbonated soil. • Biochar promotes dense composite structures by fostering key carbonate crystals. • A recycling-sequestration-enhancement pathway is established for low-carbon geotechnics. To overcome the CO 2 diffusion and uneven carbonation when using reactive MgO carbonation in high-water-content soil, straw biochar was introduced as a multi-functional synergistic medium, creating a novel approach integrating "waste recycling-CO 2 sequestration-soil enhancement". An orthogonal experimental design was employed to investigate the effects of biochar content (0∼6 %), MgO content (10∼15 %), and initial water content (30∼35 %). The physico-mechanical properties were evaluated through unconfined compressive strength (UCS) tests, mass/volume change measurements, and pH analysis. The strengthening mechanism of biochar-synergized MgO carbonation was revealed by SEM and XRD analyses and compared with traditional cement-stabilized soil. The results indicate that biochar significantly enhances the CO 2 carbonation degree and mechanical performance. The optimal mix (6 % biochar, 15 % MgO, 30 % water content) achieved a UCS of 4.51 MPa after 12 h of carbonation, which is 3.1 times the strength of the biochar-free control and 131 % higher than 28-day cured cement-stabilized soil with the same binder dosage. Microstructural analysis revealed that biochar created preferential pathways for CO 2 , provided nucleation sites for carbonate crystals, and facilitated the formation of a dense composite structure cemented by nesquehonite and dypingite. The synergistic channeling-adsorption-nucleation mechanisms of straw biochar were identified, establishing a novel green stabilization approach that integrates waste recycling, CO 2 sequestration, and soil enhancement. The findings provide a green improvement method characterized by carbon sequestration and toughness enhancement for rapid stabilization of soft soil, offering technical support for the utilization of agricultural waste and low-carbon geotechnical stabilization.