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The influences of nanoparticles on the mechanical properties and microstructures of recycled aggregate concrete (RAC) were investigated in this study. The compressive and flexural strengths of RAC with different dosages of nanoSiO2 (NS) and nanoCaCO3 (NC) were tested. Both the scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) techniques were applied to analyze the microstructures and porosity of interfacial transition zones (ITZs) in RAC. Based on the comparison on compressive strength and flexural strength, the NS is more effective than the NC for improving the mechanical properties of RAC. Dispersion of NC particles by a superplasticizer can somewhat improve the early-age strength of RAC but is unlikely to enhance the 28-day mechanical strengths. This may be attributed to the better dispersion and pozzolanic activity of NS compared to NC. The results show that a denser microstructure and a reduction of porosity within ITZs were observed by incorporating NS, which occurred along with the improvement of the mechanical strengths of RAC. Moreover, the NS-modified RAC exhibits less total porosity than the NC-modified RAC. The NS-modified RAC mainly contains small size pores, but the NC-modified RAC has more large pores than the NS-modified RAC, which has negative effects on the mechanical properties of RAC.

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