EFFECTS OF CARBONATED RECYCLED CONCRETE AGGREGATES ON THE MECHANICAL PROPERTIES OF CONCRETE AND THE MICRO-PROPERTIES OF THE INTERFACIAL TRANSITION ZONE

Abstract

The purpose of this paper was to study the effects of carbonated recycled concrete aggregates (CRCA) on the mechanical properties of recycled aggregate concrete (RAC) and the micro-properties of the interfacial transition zone (ITZ), and to determine their strengthening mechanisms. Recycled concrete aggregate (RCA) soaked with saturated lime water was treated by accelerated carbonation under a pressure of 0.30 MPa for 24 h, under the replacement ratios of 0 %, 30 %, 70 %, 100 %. The mechanical property tests included 7-day and 28-day compressive, splitting tensile, and flexural strength tests. The micro-property tests included scanning electron microscope-energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD), and Vickers micro-hardness (VMH). The results showed that the pores of the old mortar (OM) and ITZ-2 in the CRCA were filled with calcite, which improved the properties of the CRCA. Therefore, with the replacement ratio of the CRCA, the improvement degrees of the mechanical properties of the RAC gradually increased. Furthermore, compared to the other phases in the RAC, ITZ-3 had the lowest properties, but had the highest improvement degree. Besides, the strengthening mechanism of the micro-properties of ITZ-3 by the CRCA can be explained by the nucleation and chemical effect of the calcite and the micro-bleeding effect of the ITZ-3. Namely, the calcite on the surface of the CRCA provided nucleation sites for the calcium silicate hydrate (C–S–H) and reacted with the aluminate in the ITZ-3 to form hemicarbonate (Hc) and monocarbonate (Mc). In addition, a decrease in the water absorption of the CRCA alleviated the micro-bleeding effect of the ITZ-3.