An innovative sludge-derived capsule for self-healing cementitious materials

An innovative eco-efficient capsule utilizing drinking water treatment sludge (DWTS) as a healing agent for concrete crack sealing was developed. In this study, the core materials comprised a mix of DWTS and calcium hydroxide, granulated by poly (ethylene glycol). A non-toxic polymer, Ethyl cellulose (EC), is applied as the protective shell material. The morphology, internal structure, and leakage mechanism of capsules, self-healing performance of cracked mortars and the composition of resulting healing products were assessed. Obtained results indicated that EC uniformly covered the spherical core material following the designed coating procedures. The main elements released from the capsules were Ca, Al and Si after dissolution of polyethylene glycol (PEG), which would contribute to pozzolanic reactions inside the matrix. Cracks with an initial width of 400 μm were completely healed after 7 days of curing. Such a healing process also led to an 73% enhancement in compressive strength at a healing age of 28 days. The water tightness of capsule-based samples improved by more than 90% in the first 7 days, compared to only 10% in control samples. The predominant healing products were calcium carbonate in the form of calcite, and some content of aluminium-bearing phases derived from the pozzolanic reaction of DWTS. • Mix of by-product sludge and calcium hydroxide used as a green healing agent in capsules. • Up to 400 μm crack healing was achieved within 7 days of water curing. • Capsule incorporation improved the compression and water tightness recovery rates of mortar samples.