Research on the mechanical properties and microstructure of low-carbon ultra-high performance concrete using carbonated recycled fine aggregate

The application of carbonated recycled fine aggregate (CRFA) as fine aggregate for prepared ultra-high performance concrete (UHPC) has proven to be an efficient method of reusing construction waste and shows significant potential for CO2 capture. This paper first investigated the effects of CRFA replacing 25-100 % quartz sand on the mechanical properties and microstructure of UHPC under a water-to-binder ratio of 0.16, compared with UHPC prepared with recycled fine aggregate (RFA). The results exhibit that the UHPC incorporating 25-100 % RFA is 3.7-14.5 % lower than the 152.5 MPa of the original UHPC, which is attributed to the high porosity and weak interface transition zone (ITZ) between the old mortar on the surface of RFA and the new cement matrix. Furthermore, the compressive strength of 25-50 % CRFA incorporated UHPC shows an increase of 1.82-3.22 % relative to the original UHPC, resulting from the enhanced physical properties of CRFA and optimized ITZ through carbonation modification. However, UHPC with 75-100 % CRFA is 7.10-11.54 % lower than UHPC containing 50 % CRFA, indicating excessive CRFA induces the number of old ITZ enhancements and the tight packing effect decrease. The X-ray diffraction, fourier transform infrared spectroscopy and thermal gravimetric analyses were adapted to further identify the phase composition, while the scanning electron microscopy and energy-dispersive X-ray spectroscopy test analyses were conducted to explore the microstructure morphology of UHPC prepared with quartz sand, RFA and CRFA, respectively. Overall, CRFA can be utilized as an excellent material for replaced quartz sand as fine aggregate even under a high replacement level of 50 % with acceptable workability and mechanical properties of UHPC.