Geopolymer Coating of Bacteria-containing Granules for Use in Self-healing Concrete

Concrete structures are often reinforced with steel. In order for the reinforcement to take over tensile forces, concrete has to crack. Through such cracks, water and compounds that are harmful to concrete can enter. This can cause durability issues like leakage, concrete degradation and reinforcement corrosion. In situ repair of cracks is often labour-intensive and inefficient.Preferentially, cracks are autonomously healed from the inside out in an early stage, preventing the ingress of water. This can be achieved by incorporating healing agent particles composed of nutrients and bacterial spores into the concrete matrix. The bacteria will germinate when water is available, plugging cracks with calcium carbonate. However, a coating is needed to protect the water-soluble healing agent from water during mixing. In order to allow the bacteria access to water for activation after the concrete has hardened, such a coating should break whenever a crack occurs in the concrete. Therefore, it should adhere well to the concrete matrix. It is possible to achieve this by protecting the particles with a brittle geopolymer coating.For this study, healing agent particles are coated with geopolymers following different mixture recipes. Metakaolin is used as an aluminosilicate source and sodium silicate as well as sodium aluminate are used as activator liquids. The particles are coated by granulation in a low-shear granulator. In order to improve the coating process, the operating window and the granulation mechanism are determined for all activator liquids used. Leaching and strength tests are performed and coated particles are incorporated in cement paste in order to determine the feasibility of application of the particles in concrete.Results show that the prepared particles are better protected from leaching than untreated particles. Using a high pressure single-fluid nozzle to improve nebulisation when coating produces more particles of the desired size than coating with a low pressure single-fluid nozzle with poor nebulisation. Furthermore, particles prepared with a high pressure nozzle sprayer perform better when incorporated into cement paste than particles prepared with a low pressure nozzle sprayer.