Application and impact of carrier systems and immobilization methods in microbial self-healing cement-based composites: A comprehensive review

As one of the intelligent and environmentally friend repair method, microbial self-healing technology, which enables autonomous healing in the early stages of crack formation in cement-based composites, shows its benefits to overcome the drawback of time-consuming, labour-intensive natures as well as ineffectively repair for internal cracks of the traditional passive repair work. However, the harsh environment inside concrete leads to a low survival rate of microorganisms, and consequently severely affects the self-healing performance of microbial concrete, which therefore, requires the suitable approach to load the microorganism. As the most important process of embedding microorganism, the carriers can provide a suitable micro-environment for the survive of microorganism, influencing the healing performance of cement-based composites. Therefore, impact of the carriers on the performance of microbial self-healing concrete, which unfortunately, has not been systematically summarized and discussed. This paper summarises the state-of-the-art of the carriers for microbial cement-based composites, in the perspectives from the manufacture to the performance and applications. Based on the deep review of the bacterial species and immobilization methods to the carriers, the manufacture of different microbial carrier used in microbial self-healing cement-based composites is carefully reviewed and their influence on the mechanical properties and self-healing performance of cement-based composites is extensively discussed. Finally, new research directions for carrier development are proposed.