Mechanical properties and strengthening mechanism of fully recycled aggregate concrete prepared by nano-SiO₂ composite carbonated recycled aggregates

To explore the influence of carbonated aggregate on the mechanical properties of fully recycled aggregate concrete (FRAC), FRAC using NS composite carbonated recycled aggregate was prepared in this paper. Macroscopic mechanical properties of FRAC including compressive strength, splitting tensile strength and flexural strength were tested, the microscopic mechanical properties of FRAC were tested by nano-indentation. Combined with SEM microscopic test, the multiple interface strengthening mechanism of carbonated fully recycled aggregate concrete (CFRAC) was revealed. The results show that NS composite carbonation can achieve efficient carbonation modification of recycled aggregate, and the mechanical properties of CFRAC are significantly improved. The maximum increase of compressive strength, splitting tensile strength and flexural strength are 33.12%, 48.73% and 24.53%, respectively. The average elastic modulus and micro-hardness of CFRAC are significantly improved, and the maximum increase are 171.05% and 62.96%, respectively. The micro-mechanical properties of old mortar and aggregate-old mortar interface transition zone (ITZ₁) are increased most significantly. The strengthening mechanism of CFRAC is revealed by ' aggregate wall effect ' and ' ITZ carbonation strengthening effect '. The results can provide theoretical support for the research on the quality improvement of recycled aggregate and application of FRAC.