Balancing strength, durability, and environmental impacts in sewage-sludge-modified cemented clay

This study assesses the mechanical performance, freeze-thaw durability, and cradle-to-gate environmental impacts of cement-stabilised clay where cement is partially replaced with fresh sewage sludge (SS). Mixes included 5 %, 7 %, and 10 % cement (by dry soil) and SS replacement levels of 0 %, 3 %, 5 %, 7 %, and 10 % (by cement mass). Specimens were compacted to dry densities of 1.50 and 1.70 g/cm 3 , cured for 28 days, and tested for 28-day unconfined compressive strength (UCS). Freeze-thaw resistance was measured using accumulated mass loss (ALM) and observed failure behaviour. The environmental assessment employed openLCA with ReCiPe 2016 Midpoint (H) for a functional unit of 1 m 3 of stabilised clay at the target density. UCS increased with cement content and dry density. SS replacement resulted in a non-monotonic strength response, typically peaking at 3–5 %, indicating improved cement-efficiency at low replacement levels. Under the tested conditions, UCS increased from 2,421 kPa (C10-SS0, 1.50 g/cm 3 ) to 3,814.5 kPa at SS3 in the same series. The freeze-and-thaw performance also improved significantly in the C5 series at 1.70 g/cm 3 . The control sample failed at Cycle 5 (ALM = 77.74 %), but SS3 remained intact through Cycle 12. ReCiPe midpoint impacts increased with cement content and density but decreased consistently as SS increased, resulting in 7–8 % reductions compared to controls. Strength-normalised impacts showed the greatest eco-efficiency gains at SS3-SS5, maximising environmental benefits per unit of performance. Durability-coupled indices further favoured mixtures achieving both lower impacts and reduced ALM, highlighting the importance of jointly optimising strength, durability, and environmental performance.