Freeze–Thaw Performance of Marl Soil Stabilized with Iron and Steel Cooling Water Sludge and Polypropylene Fibers: Toward Sustainable By-Product Reuse

This study investigates the mechanical performance, freeze–thaw durability, and microstructural characteristics of marl soil treated with iron and steel cooling water sludge (CWS) and polypropylene fibers (PPFs). The CWS used in this study is an Fe-rich industrial by-product generated from the cooling water circulation system of an iron and steel production facility. In an initial screening stage, mixtures containing 5–30% CWS and 0.2–1.4% PPF were evaluated using 7-day unconfined compressive strength (UCS) tests. The highest UCS values within the investigated ranges were obtained at 15% CWS and 1% PPF, increasing the UCS of untreated marl from approximately 254 kPa to 430 kPa with 15% CWS and 450 kPa with 1% PPF. These dosages were subsequently selected for the freeze–thaw durability program. Untreated and treated specimens were conditioned or cured for 7, 28, and 90 days and subjected to up to 10 freeze–thaw cycles. Repeated freeze–thaw exposure reduced the UCS of all the specimens; however, the magnitude of deterioration depended strongly on the treatment type and the curing duration. Untreated marl exhibited strength losses of 27.7–31.5% after 10 cycles. The 15% CWS-treated specimens showed comparatively stable freeze–thaw resistance, with strength losses of 20.5–22.6%, whereas the performance of 1% PPF was strongly curing-time dependent, with the strength loss decreasing from 58.2% at 7 days to 27.5% at 90 days. The combined 15% CWS + 1% PPF treatment exhibited the most favorable response at 90 days, with a strength loss of 20.7% after 10 freeze–thaw cycles. The SEM observations qualitatively indicated a comparatively more compact and continuous matrix in the treated specimens, while the XRD patterns showed mineralogical differences associated with CWS incorporation; however, specific cementitious or hydration products could not be conclusively confirmed. Overall, the combined CWS–PPF treatment improved the mechanical integrity and freeze–thaw durability of marl soil and may provide a practical route for the beneficial reuse of an iron and steel industry by-product, provided that its environmental suitability is confirmed.