Enhancement of recycled aggregates and concrete by combined treatment of spraying Ca²⁺ rich wastewater and flow-through carbonation

Accelerated carbonation, regarded as one of the sustainable and economical methods for enhancing recycled concrete aggregates (RCAs), has a potential to be further enhanced through pretreating the RCA with Ca²⁺ rich solutions before carbonation as the externally provided Ca²⁺ was expected to yield more precipitation of calcium carbonate in the pores. This study utilized wastewater sourced from a ready-mix concrete plant combined with a flow-through carbonation method in order to enhance the quality of RCA as well as recycled aggregate concrete (RAC). The density and water absorption of (i) natural aggregate (NA), (ii) RCA, (iii) carbonated RCA (C-RCA), and (iv) wastewater pretreated and carbonated RCA (WW-C-RCA) were compared. The density, water absorption, sorptivity, compressive strength, chloride penetration of the concrete prepared with these four types of aggregate were analyzed and compared. The results showed that (1) pretreating RCA by the Ca²⁺ rich wastewater could further enhance the effects of the flow-through carbonation for RCA; (2) the improvement in RCA quality resulted in a better RAC performance; and (3) the microscopic analysis revealed the enhancement of RCA was attributed to a reduced porosity and an increased microhardness value of the adhered mortar of WW-C-RCA. This study demonstrated a feasible approach to combine three waste resources (RCA, wastewater, and waste CO₂) to produce green concrete.