Biopolymer recovery from waste activated sludge toward self-healing mortar crack

Activated sludge (AS) offers great potential for resource recovery considering its high organic and nutrient content. However, low recovery efficiency and high costs are directing the focus toward the high-valuable resource recovery. This study extracted 71.5 ± 5.9 mg/g VSS of alginate-like exopolysaccharides from AS (ALE/AS) and applied it to mortar as a novel biopolymer agent for crack self-healing. With a mortar crack of 120 μm, addition of 0.5 wt% ALE/AS yielded a high crack closure ratio of 86.5 % within 28 days. In comparison to commercial healing agents, marginal flexural strength reduction with ALE/AS addition (17.9 % vs 30.2-50.5 %) was demonstrated. The abundance of COO^- group in GG blocks of ALE/AS resulted in a higher cross-link capacity with Ca²⁺, while the reduction of hydrophilic residues (e.g., COO^- and OH) after complexation engendered a lower swelling capacity, which facilitated self-healing and flexural strength maintenance. Molecular dynamics (MD) revealed that lower Ca²⁺ diffusivity, arising from the stronger electrostatic interactions between the COO^- groups and Ca²⁺, resulted in a high Ca²⁺ concentration around the cracks, leading to CaCO₃ deposition and healed cracks. The outcomes of this study provided light on ALE-based mortar crack healing and presented a possibility for multi-level AS resource recovery.