Transforming waterworks sludge into controlled low-strength material: Bench-scale optimization and field test validation

In order to reduce landfill disposal of waterworks sludge (alum sludge) and incinerated sewage sludge ash (ISSA), this study proposed an innovative approach for upcycling them into value-added controlled low-strength materials (CLSM). Waterworks sludge interfered with cement hydration reaction and delayed the stiffening time of CLSM (>10 h) due to its high organics content (5%). The addition of triethanolamine (TEA) with a dosage of 0.1 wt% of binder effectively shortened the stiffening time to 4.5 h. The lab-scale results suggested that the optimal CLSM design (6% cement, 14% ISSA, 8% sludge, and 72% recycled fine aggregate at a water/binder ratio of 1.2) complied with the standard requirement of flowability (>200 mm), stiffening time (<5 h), and compressive strength (0.3–2.1 MPa). The pilot-scale field tests further confirmed that the sludge-incorporated CLSM achieved a high flowability (220 mm), short stiffening time (4 h), low compressive strength (1.38 MPa), and easy re-excavatability after 3 months. This study demonstrated that waterworks sludge can be potentially transformed into sustainable construction materials for urban development.