Effect of magnesium phosphate cement on the mechanical properties and microstructure of recycled aggregate and recycled aggregate concrete

This paper aims to investigate the effects of magnesium phosphate cement (MPC) on the mechanical properties and microstructure of recycled aggregates (RA) and recycled aggregate concrete (RAC). Two surface modification methods, namely, the MPC wet and dry mix methods, were designed and applied to improve physical properties of RA, respectively. Further, the mechanical properties of the RAC mixes with modified RA (MRA) were assessed in terms of compressive, splitting tensile strength, and elastic modulus. In addition, X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS) were carried out to illustrate the effects of surface modification methods on the microstructure of RAC. The results indicated that the MPC dry mix method shows more appropriate on the quality enhancement of RA compared to MPC wet mix method, improving the apparent density, specific gravity, water absorption, and the crushing value of RA by 3.6%, 7.4%, 26.7%, and 3.0%, respectively. Meanwhile, the corresponding mechanical properties of the concrete are enhanced, particularly the compressive strength getting increased by up to 41.5%, due to the new hydration reactions. Based on the microstructural analyses, the coarse surface of RA is changed more homogeneous and denser when modified by MPC, and the MPC is observed at both interfacial sides of recycled aggregate mortar and fresh concrete mortar for the reinforcement of the interfacial strength between the new and old mortar.