1 Department of Building, Civil and Environmental Engineering, Concordia University, Montréal, Quebec, Canada. Electronic address: elnaz.rajaei@concordia.ca.
2 Department of Building, Civil and Environmental Engineering, Concordia University, Montréal, Quebec, Canada. Electronic address: maria.elektorowicz@concordia.ca.
3 Civil Engineering Program, Al Ain University, Al Ain, United Arab Emirates. Electronic address: mousa.banibaker@aau.ac.ae.

The immobilization and stabilization of crude oil-contaminated soils pose significant challenges in geotechnical engineering, particularly in clayey matrices prone to hydrocarbon adsorption and structural degradation. This study evaluates a two-step synergistic grouting technique using calcium chloride (CaCl2) and sodium silicate (Na2SiO3) to enhance strength recovery and immobilize contaminants simultaneously. Laboratory experiments simulated field conditions on sand and sand-clay mixtures containing 1 %, 3 %, and 5 % bentonite under both clean and crude-oil-polluted conditions. Direct shear results revealed strength improvements of 28-120 %, with polluted clayey soils showing the most substantial gains. Cohesion increased from 29 kPa to 64 kPa, and the friction angle rose by 4-7°. In some cases, treated polluted samples outperformed untreated clean soils. Morphometric analysis confirmed microstructural enhancement: average pore area decreased by 37-52 %, major-to-minor axis ratio fell by 13-35 %, and eccentricity dropped from 0.83 to 0.66. These changes reflected the formation of calcium silicate hydrate (C-S-H) gels, which bridged particles and reduced void spaces. They created denser, more isotropic fabrics, improving load-bearing capacity, lowering permeability and limiting further hydrocarbon migration. The results showed that oil-polluted soils, which typically suffer pronounced mechanical deterioration, can be effectively rehabilitated using this method, in some cases exceeding the performance of untreated clean soils. Cost and carbon analyses confirmed the sustainability of the CaCl2-Na2SiO3 system, indicating significant reductions of 56 % in cost and 97 % in carbon emissions compared with Portland cement stabilization. Furthermore, hydraulic conductivity and leakage tests on polluted sandy soils indicated reductions of more than 90 % in conductivity and nearly 100 % in leakage for soil containing 5 % clay, confirming the method's effectiveness as both a stabilizer and a barrier. The results demonstrate that a two-step synergistic chemical grouting technique is effective for stabilizing petroleum-affected foundations, slopes, and containment barriers. Field trials are recommended to evaluate the material's long-term durability and environmental performance.