<p>Clayey soils pose significant challenges in civil engineering due to their low strength, high shrink-swell potential, poor drainage, and susceptibility to frost heave. Cement stabilization is widely used to improve these soils, but its high energy demand and environmental impact have prompted research into alternative additives. Coal char, a lightweight byproduct of coal pyrolysis, offers a promising solution because of its low energy production cost and ability to enhance cementitious matrices. While previous studies have examined coal char in natural soils, limited research exists on its application in problematic soils with high bentonite content. This study focuses on the influence of coal char on the performance of cement-stabilized soils intended for sub-base applications. The experimental program employed a fixed cement dosage of 10% (by dry soil weight) in a composite mixture comprising 75% sodium bentonite and 25% sand. Coal char was incorporated at four replacement levels, 0%, 10%, 20%, and 40% of the cement weight. To evaluate the impact of char addition, a comprehensive geotechnical tests were conducted, including Atterberg limits, optimum total water content determination using the Harvard Miniature Compactor, unconfined compressive strength (UCS), triaxial compression, soaked California Bearing Ratio (CBR), consolidation, and freeze–thaw durability assessments. Results indicate that UCS improves with curing time, cement content, and char proportion. Modified Proctor compaction energy was applied for CBR testing to assess sub-base strength. While cement stabilization increased soaked CBR values by approximately 2.45 times, further improvement was not observed with char addition, likely due to its brittle and porous structure, which may increase susceptibility to penetration. However, swelling during soaking was reduced as char content increased. Overall, the findings suggest that incorporating coal char can be advantageous for sub-base layers exposed to frequent wetting conditions.</p>