Carbonation treatment of recycled concrete aggregate: Effect on transport properties and steel corrosion of recycled aggregate concrete

Old cement mortars attached on recycled concrete aggregates (RCA) seriously affects the durability of the recycled aggregate concrete (RAC). Revealing the variation of the transport properties of cement mortar after subjecting to accelerated carbonation treatment is crucial with respect to understanding the effects of carbonated RCA on the durability of RAC. In this paper, cement mortars were used to study the transport properties after they were treated by accelerated carbonation. The corrosion behavior of steel bars in the concrete incorporating the carbonated RCA was also evaluated. The experimental results indicated that the water absorption and sorptivity, resistance to chloride ion penetration, as well as the bulk electrical conductivity of the cement mortar was decreased after subjecting the accelerated carbonation treatment. Extending the treatment from 1 day to 7 days only resulted in a marginal improvement of the transport properties. A significant decrease in the local porosity at the edge of cement mortar determined by Scanning electron microscopy-backscattered image was observed, agreeing well with the concentrated calcium carbonate (CC) contents detected in the surface layer by TGA. The determination of the spatial gradients of the porosity and carbonates/portlandite contents illustrated that even only with the external layer (0-10 mm) carbonated, a considerable improvement of transport properties of the cement mortar could still be achieved. The corrosion test in the new concrete prepared with the carbonated RCA also confirmed that the corrosion resistance of steel bars in the RAC was improved.