Optimizing Carbonation Conditions for Low-Quality Recycled Concrete Aggregates and Sustainable Valorization in Cement-Based Systems

Abstract

This study aims to examine the parameters affecting the accelerated carbonation performance of low-quality recycled concrete aggregates (RCA), obtained from end-of-life buildings, with low mechanical and chemical properties, and to determine the effects of the accelerated carbonation process on the engineering performance of cement-based systems through a series of performance tests performed on mortar samples produced with carbonated RCA (CRCA), uncarbonated RCA and natural aggregate (NA) under the optimum parameters determined in the first section. The accelerated carbonation process was implemented using a newly designed laboratory-scale carbonation reactor that provides a dynamic carbonation process. Relative humidity (50-70-90%), ambient pressure (1-2-3 bar) and ambient temperature (50-90oC) were determined as the main parameters for the accelerated carbonation process. Carbonation treatments were applied to low-quality RCAs with five different particle size ranges: <0.85, 0.85-2.00, 2.00-4.75, 4.75-9.50 and 9.50-14.00 mm. The effects of carbonation durations (2-4-6-12-24-48-72-96-120 hours) on the carbonation performance of RCAs were also investigated. Thermogravimetric analysis (TG/DTA) was applied to measure the CO2 uptake rate of low-quality RCAs, and the water absorption capacities of CRCAs and RCAs were measured to obtain information about precipitated calcium carbonate. As a result of examining all parameters, the optimum carbonation condition was determined as 70% relative humidity, 1 bar ambient pressure, 50oC ambient temperature, 48 hours carbonation duration. The highest CO2 uptake rate occurred in the particle size range of 2.00-4.75 mm. In the continuation of this study, flowability, mechanical strength, water absorption capacity, capillary water absorption (sorptivity), freezing-thawing and chloride permeability properties of mortar samples produced with fine (0.85-4.75 mm) RCA, CRCA and NA were determined. The results showed that the losses in engineering properties observed in mortars when using RCA can be significantly eliminated by the accelerated carbonation method. The main reason underlying the gains brought by the use of CRCA is the improved chemical and mechanical properties of the high-void, high-permeable, low-density mortar residue and the interfacial transition zone (ITZ) on the RCA surface. Results showed that CRCA, rather than remaining only as a low-quality construction and demolition waste (CDW), can be actively used in cement-based systems to reduce their negative environmental impacts. In this way, the sustainability of cement-based systems produced with CRCA can be contributed.