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....</1.0>