Quality Improvement of Recycled Concrete Aggregate by Accelerated Carbonation under Different Pressure

Due to the presence of old mortar (OM) and interfacial transition zone (ITZ), recycled concrete aggregate (RCA) is inferior to natural aggregate (NA). The purpose of this paper was to study the effect of accelerated carbonation on the macro-properties and micro-properties of RCA under different pressure (0.05, 0.15, 0.30 MPa). The macro-property tests included colour change, apparent density, water absorption, and crushing value of RCA. The micro-property tests included scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetry-differential scanning calorimetry (TG-DSC), and Vickers micro-hardness (VMH). The results showed that the change trends of apparent density, water absorption, and crushing value of RCA displayed exponential relationships as pressure increasing, with the optimum pressure of 0.30 MPa. SEM images indicated that the calcite caused by the hydration products in RCA and the Ca(OH)(2) derived from saturated lime water improved the properties of RCA; as the apparent density increased, the water absorption and crushing value decreased. The results of XRD and TG-DSC indicated that, as the pressure increased, the masses of Ca(OH)(2) in carbonated RCA gradually decreased, while those of CaCO3 gradually increased, which demonstrated that the carbonation degree gradually increased. Besides, ITZ-2 was the weakest phase in RCA, but its improvement degree of VMH by accelerated carbonation was higher than that of OM. However, RCA was not completely carbonated, but only carbonated in a certain depth after 24 h accelerated carbonation.