Insight into Mechanical Properties and Microstructure of Recycled Aggregate Concrete Containing Carbon Fibers and Nano-SiO2

The mechanical properties and microstructure of recycled aggregate concrete (RAC) with carbon fibers (CF) and nano-SiO2 (NS) were investigated in this paper. The mechanical properties of RAC were enhanced by both single and hybrid addition of CF and NS, and the hybrid addition had a better strengthening effect. From the experimental results, it was found that the addition of CF could increase the 28d compressive strength and splitting strength of RAC by 9.05% and 22.36% respectively. Hybrid CF and NS were more conducive to improve the mechanical properties of RAC, and the enhancement effect increased first and then decreased with the increase of NS content. The optimal content of NS was 0.8wt%, which increased the 28d compressive strength and splitting strength of RAC by 20.51% and 14.53% respectively. The microstructure results indicated that the addition of CF had little effect on optimization pore structure of RAC, but the crack-inhibition action of the CF could improve the mechanical properties of RAC. The addition of NS reduced the content of CH and facilitated the formation of more (C–S–H) gel. The hydrated calcium silicate (C–S–H) gel significantly decreased the porosity and transformed harmful capillaries pores and harmful pores into harmless capillaries pores and gel pores, thus improving the mechanical properties of RAC. Therefore, the hybrid CF and NS was more conducive to enhance the performance of RAC for building materials.