Mechanical Behavior of Biocarbonizing Reactive Magnesia–Stabilized Soft Soil: Strength Control and Prediction Framework Based on Effective Carbonate-to-Magnesia Ratio

Soft soil is widely recognized as problematic due to its poor physical-mechanical properties. The biocarbonation of reactive magnesia (BCRM) technology has shown promise for the efficient stabilization of soft soil, but excessive carbonate ions (CO32−) addition was found to lead to significant strength degradation, limiting its practical applicability. To date, no effective solution has been identified for this issue. To address it, this study introduces the effective carbonate-to-magnesia ratio (ECMR) as a crucial indicator for strength control and prediction of biocarbonated soil, based on a systematic investigation of the effects of urea concentration (UC) and MgO content (MC) on strength enhancement using pickling-drainage and unconfined compressive strength (UCS) tests. Results revealed that the urea hydrolysis ratio decreased with increasing UC, which could be accurately modeled using a dose-response function. The hydrolyzed urea concentration (HUC), representing CO32− content, peaked at a UC of 3.0 mol/L. The effects of UC and MC were fundamentally attributed to HUC and reactive magnesia content (RMC). UCS first increased and then decreased with increasing HUC, whereas it consistently increased with higher RMC. The interactions between HUC and RMC on UCS improvement were captured by ECMR. The optimal ECMR values for maximum UCS enhancement varied with RMC levels but could be accurately determined by an established linear function model, enabling effective avoidance of strength degradation via HUC modulation. Furthermore, a comprehensive functional model associated with ECMR was constructed to predict UCS development across different RMC levels. This study provides valuable insights into the strength evaluation of biocarbonated soils, advancing the practical implementation of BCRM technology for soft soil stabilization.