Microbial carbonate precipitation for the improvement of quality of recycled aggregates

High water absorption is the main drawback of recycled aggregates which greatly hinders the re-use of them in concrete production. In this study, bio-deposition treatment, based on bacterially induced CaCO₃ precipitation, was applied to improve the quality of the recycled aggregates. Two representative recycled aggregates, recycled concrete aggregates (CA) and mixed aggregates (MA) were used. The bacterial CaCO₃ precipitated on the surface and in the pores of the recycled aggregate worked as a barrier for the penetration of water, and hence the water absorption of the aggregates can be decreased. Firstly, the optimal treatment method was determined by screening among spraying and several immersion strategies. It was found that the two times immersion treatment was the best method. Samples subjected to this method had a high weight increase (2% for CA and 2.5% for MA) and largest extent of water absorption decrease (one percentage point drop for CA and two percentage points drop for MA). Furthermore, The biogenic CaCO₃ had a good cohesion and strong bond with the aggregate surface. Very limited (<0.1%) mass loss occurred on the bio-treated samples while the mass loss of the untreated aggregates was much higher (0.2% for CA and 0.5% for MA). This indicated that the surface of the aggregates was strengthened by the biogenic CaCO₃ as well. After using the bio-treated aggregates, the compressive strength was increased by 40% for CA concrete and 16% for MA; the water absorption was decreased by 27% for CA concrete and 20% for MA concrete.