Properties and microstructure of packing-optimised recycled aggregate concrete with different cement paste or sand contents

This paper investigates the influences of cement paste volume (CPV) and sand-to-aggregate volume ratio (Bs) on the properties and microstructure of recycled aggregate concrete (RAC). The recycled concrete aggregates (RCAs) in an optimal packing status were utilised to fully replace the natural coarse aggregates. The workability and mechanical properties of RAC were first examined, followed by characterisation of the interfacial transition zones and the pore structure inside RAC by using microhardness test and X-ray computed tomography, respectively. The results indicate that the slump of RAC increases with the CPV in a growing rate, which is mainly attributed to the increased lubrication effect of fresh paste on the aggregates. Under the constant CPV and water-to-cement ratio, increasing the Bs decreases the slump of RAC due to the reduced cement paste film thickness. Increasing the CPV decreases the macroporosity of RAC as it reduces the amount of RCAs with pores in old mortars, thereby enhancing its compressive strength and Young's modulus. However, excessive CPV increases the porosity and thickness of interfacial transition zones, which subsequently weakens the mechanical properties of RAC. The increase of Bs first enhances the granular skeleton by optimising the particle packing status, which is beneficial to RAC properties. However, excessive sand (i.e., high Bs) increases the macroporosity and mean pore size inside RAC, degrading the mechanical properties of RAC. Nonetheless, the flexural strength of RAC is marginally influenced by the CPV or Bs. This study guides the selection of CPV and Bs in the mix design of packing-optimised RAC to refine its microstructure and mechanical properties.