Utilization of waste sludge-derived biochar in sustainable planting ecological concrete: Integrating structural,...

Porous ecological concrete integrates structural stability with plant growth support. However, its sustainability is often limited by high cement content and insufficient nutrient availability. Meanwhile, the continuous generation of waste sludge necessitates its effective utilization as a resource in construction materials due to the high cost of conventional disposal and its unsustainability. Sludge-derived biochar (SB), characterized by porous structure, reactive mineral phases, and intrinsic nutrient content, is a promising multifunctional modifier for ecological concrete. In this study, SBs derived from river sediment, industrial, domestic, and livestock sludge were characterized using SEM-EDS, XRD, and BET. The characterized SBs were incorporated into sustainable planting ecological concrete (SBEC) at a 10% cement replacement ratio. Hydration kinetics and microstructural evolution were analyzed using isothermal calorimetry, TGA, and ²⁹Si MAS NMR. The physical properties, vegetation compatibility (Cynodon dactylon and Festuca arundinacea), and environmental sustainability (life cycle assessment, LCA) were systematically evaluated. SB incorporation enhanced the water retention, alkalinity regulation, freeze-thaw resistance, and sustained N/P release of the resulting composite. SBECs containing industrial and domestic SBs achieved 28-day compressive strengths of 16.94 and 17.12 MPa, respectively, and exhibited the lowest strength loss during freeze-thaw cycles. In contrast, livestock SB contributed to the highest hydration degree (67.7%) in SBECs and rapid alkalinity reduction, which significantly improved root biomass and nutrient accumulation. LCA confirmed that partial cement replacement with SBs significantly reduced environmental burdens. Overall, this work provides a high-value valorization pathway for waste sludge in sustainable multifunctional concrete, with different sludge sources enabling SBECs to achieve structural stability, vegetation support, or balanced multifunctionality.