Optimization of sodium alginate aided bio-deposition treatment of recycled aggregates and its application in concrete

Bio-deposition treatment based on microbial induced carbonate precipitation is a promising method to enhance the properties of recycled aggregates, because the generated biological precipitates can repair the defects of recycled aggregates. However, the main problem of current bio-deposition treatment is the uneven distribution of the precipitates on aggregates, which greatly diminishes the enhancing efficiency. Therefore, sodium alginate (SA) was innovatively utilized to improve the uniformity of the precipitates on aggregates surface during the bio-deposition treatment in this study. The principle was that SA and the network structure of Ca-alginate were applied to uniformly “fix” the bacteria on the surface of recycled aggregates, which was supposed to promote the uniform in-situ precipitation on the surface of aggregates. The influences of the concentrations of bacteria and deposition solution (containing equimolar urea and Ca-nitrate), on the treatment efficiency were studied. The results indicated that the optimized SA-aided bio-deposition treatment group was under 0.2 wt% SA, 10⁹ CFU/mL of bacterial solution, and 0.5 M of deposition solution, the water absorption of aggregates was reduced by 39.63% on average, the effective mass increase of calcium carbonate was increased by 3.14%, and the crush value decreased by about 13.8%. Using aggregates subjected to the optimized SA-aided bio-deposition treatment to produce concrete, it was found that, comparing with the untreated recycled aggregate concrete, the saturated water absorption of the treated recycled aggregate concrete decreased by about 15.3%. It can be concluded that the optimized SA-aided bio-deposition treatment can improve the strength of aggregates and durability of recycled concrete.