Enhancement of Attached Mortar in Recycled Aggregates of Returned Concrete by Wet Carbonation

A significant volume of ready-mix concrete is returned to the plant for various reasons such as cancelation of orders, concrete not meeting the specification at the site, and orders of more quantity than required. The returned concrete is typically disposed of in dumping sites, leading to substantial financial and environmental repercussions. After crushing the hardened discarded concrete, the resulting recycled aggregates can be introduced into new concrete production. However, owing to insufficient compaction and curing processes, the characteristics of this discarded concrete aggregate diverge significantly from those of conventional recycled concrete aggregates. This study employed the wet carbonation technique as a treatment method for returned concrete aggregates and conducted a comprehensive investigation into their properties following carbonation. Substantial advancements were observed following a 3-h wet carbonation treatment, with water absorption decreasing by 25.8%. The compressive strength of concrete made with carbonated recycled aggregates exhibited a substantial improvement. The findings from mineralogical analysis such as thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) indicated the formation of a substantial amount of calcium carbonate and silica gel. Furthermore, microstructural analysis revealed the development of well-defined calcite on the surface and within the inner regions of the aggregate. A close inspection of the interfacial transition zone (ITZ) demonstrated the improved bonding capability of the carbonated aggregate with new mortar enhancing the strength of recycled aggregate concrete. These findings indicate that the wet carbonation method effectively enhances returned concrete recycled aggregates, suggesting a more sustainable and efficient use of returned concrete.