Using sodium alginate aided bio-treatment for improving the uniformity of precipitates on recycled aggregates

The recycled concrete aggregates have high porosity and water absorption, which hinders their utilization in concrete production. Microbial-induced calcium carbonate precipitation was regarded as a very promising method for strengthening recycled aggregates. However, the uneven distribution of CaCO₃ on surface of aggregates encountered in the current bio-deposition treatment weakened the efficiency, especially in the aspect of the decrease of water absorption. Therefore, this study innovatively applied a sodium alginate aided bio-deposition treatment to improve the uniform distribution of biogenic CaCO₃. The principle was that sodium alginate was used to uniformly "fix" the bio-agents (urea or bacterial cells) on the surface of recycled aggregates, which was supposed to promote the uniform in-situ precipitation of CaCO₃ on the surface of aggregates, and hence effectively blocking surface pores, and reducing the water absorption of the aggregates. Two concentrations of sodium alginate (0.2w% and 0.5w%) and four sodium alginate aided bio-deposition treatments were studied. It was found that CaCO₃ (a mass increase of 4.05%) was formed on the aggregates after the suitable sodium alginate aided bio-deposition treatment. The participation of sodium alginate made CaCO₃ uniformly deposited on full surface of the aggregates, resulting in a significant decrease (42.10%) of water absorption. The biogenic CaCO₃ showed limited mass loss under ultrasonic attack, indicated a strong cohesion and bonding strength with aggregates. The results demonstrated that sodium alginate-aided bio-deposition treatment can enhance the efficiency, which was beneficial to improve the quality of recycled aggregates and their utilization of recycled aggregates in concrete production. KEY POINTS: • The SA-aided bio-treatment promoted the distribution uniformity of CaCO₃ on aggregates. • The water absorption of aggregates decreased by 42.10%. • The formed CaCO₃ showed excellent cohesion and adhesion.