Large amounts of coastal silt are converted into subgrade soil by cement solidification for resource utilization, while the compressive strength was too low (<1.0 MPa) with as little cement usage as possible (<15%, w/w). Our previous studies have shown that biochar was added into the raw materials to achieve strength enhancement, but the specific mechanisms need to be further explored. Three biochars derived from peanut shells (PSBC), cow dung (CDBC) and sewage sludge (SSBC) at 300°C, 500°C and 700°C pyrolysis, respectively were used as exogenous additives with 1%, 2% and 5%. Results showed that all biochars significantly improved the compressive strength of the subgrade soil products by 20-110%, which was attributed to that biochar catalyzed cement hydration reactions to produce more Ca(OH)2, CaCO3 and calcium silicate hydrates (C-S-H gel). The catalytic capacity of different biochars was SSBC > PSBC > CDBC. Due to the suitable specific surface area and porosity of 500°C biochar, it is the most effective for strength enhancement. The low temperature biochar (BC300) or high temperature biochar (BC700) is not as effective as BC500 in improving strength due to its insufficient pyrolysis or large porosity. Especially, addition of 2% SSBC500 induced the greatest increase in 28 d-strength from only 1.0 MPa to 2.1 Mpa, while 5% addition induced 28% decrease of strength. As the amount of biochar added increases, the porosity of the subgrade soil material also increases, which leads to a decrease in strength. In addition to the enhanced strength, the addition of biochar also facilitated the CO2 capture in the hydration process from 55 mg·g-1 to 70 mg·g-1. The porous nature of biochar facilitated the physical pore filling and adsorption sites of CO2; the high alkalinity and water holding capacity contributed to the capture and dissolution of acidic CO2. More importantly, compared with CDBC and PSBC, the high content of Ca, Fe and other mineral fractions in SSBC helps to chemically convert CO2 to Fe(OH)2CO3, CaMg(CO3)2, etc. The results of carbon footprint analysis show that 11 kg-94 kg of CO2 can be sequestered by converting 1 ton of silt with 2% biochar into subgrade soil. This method provides a novel strategy of using biochar to strengthen the intensity of subgrade soil simultaneously achieve long-term carbon sequestration
