Self-healing cementitious composites by encapsulation techniques

Abstract

Cracking of cementitious composites is almost inevitable, which requires manual repair and maintenance to extend service life of cracked cementitious composites. However, manual remediation may not be practical in case of inaccessible cracks and would cause high carbon emissions and cost. To address this issue, cementitious composites with novel components for self-healing have been developed, but current self-healing agents for cementitious composites face challenges due to high cost, low compatibility with cementitious composites and limited autonomous healing capacity. To overcome these challenges, some new approaches of autonomous healing in cementitious composites have been proposed, attempted, and analyzed in this thesis, which are summarized as below.

To lower down cost of self-healing agents, expanded graphite as one component for lightweight and high thermal conductivity cementitious composites was leveraged to immobilize bacterial spores. After immobilization, properties of sound cementitious composites were not significantly undermined. In the meanwhile, immobilized bacterial spores could revive then present high urease activities. Hence, after cracking of cementitious composites, immobilized endospores induced more calcium carbonate, which led to higher crack width reductions, more recovery in compressive strength and restoration in thermal conductivity, which could be associated with the precipitation at expanded graphite/cement matrix interfaces.

To improve compatibility between self-healing agents and cementitious composites, modified hydrogels and polyethylene glycol (PEG)-based granules were designed to encapsulate endospores. Hydrogels were modified at nano and macro scales. At nano scale, hydrogels were reinforced by polydopamine (PDA) @ carbon nanotubes (CNTs) and doped by graphene oxide (GO) respectively. Due to the PDA@CNTs reinforcement, hydrogels could be spherical after drying. Owing to the modification by GO, hydrogels could reduce pH of cracks, bond well with surrounding matrix and alter morphology of calcium carbonate from cubic to needle shapes. At macro scale, hydrogels were coated by calcium sulphoaluminate (CSA) cement. Afterwards, the modified hydrogels with CSA coating could protect encapsulated endospores under highly alkaline and calcium concentrated solution for 28 days. By incorporating the modified hydrogels into cementitious composites, autonomous healing of cementitious composites was enhanced in terms of reduction of crack width and water permeability due to biologically mediated calcium carbonate precipitation, and flexural strength could be completely regained in the presence of graphene oxide modified hydrogels with encapsulated endospores.

By using PEG-based capsules, compatibility with cement matrix was further improved as reflected by mitigation of commonly observed compressive strength reduction due to addition of endospores and nutrients. Moreover, encapsulated endospores and nutrients were released rapidly within 4 hours owing to high solubility of PEG. After release of these substances, crack sealing of cementitious composites was accelerated by calcium carbonate precipitated in cracks, thereby reducing water permeability. According to the self-healing process, a mathematical model based on stereology and reaction kinetics was established to simulate bacteria-based self-healing, and modelling results were generally consistent with that of the experimental results.

To enhance autonomous healing capacity, a new strategy by in-situ alginate crosslinking was attained in cracks of cementitious composites using the PEG-based granules. After the in-situ gelation, 1-4 mm wide cracks were sealed, and water permeability was reduced accordingly. In terms of the in-situ gelation and subsequent volume growth of hydrogels, a model for simulations of crack sealing evolutions by generated hydrogels was developed and simulation outcomes were in agreement with experimental findings.

Based on the research in this thesis, it can be concluded that autonomous healing of cementitious composites can be enhanced with these novel encapsulation or immobilization techniques and the new self-healing strategy, whilst future research in robust autonomous healing, cost reductions and life cycle analysis can be conducted to further upgrade autonomous healing cementitious composites.