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

This study aimed to improve the mechanical properties and microstructure of recycled aggregate concrete (RAC) by incorporating carbon fibers (CFs) and nano-SiO₂ (NS) to promote the optimal utilization of RAC. The mechanical properties of the RAC were enhanced by both single and hybrid additions of CFs and NS, and the hybrid addition had a better strengthening effect. From the experimental results, it was found that the addition of CFs could increase the 28 d compressive strength and splitting strength of the RAC by 9.05% and 22.36%, respectively. The hybrid CFs and NS were more conducive to improving the mechanical properties of the RAC, and the enhancement effect increased first and then decreased with an increase in the NS content. The optimal content of NS was 0.8 wt%, which increased the 28 d compressive strength and splitting strength of the RAC by 20.51% and 14.53%, respectively. The microstructure results indicated that the addition of CFs had little effect on the optimized pore structure of the RAC, but the crack inhibition action of the CFs could improve the mechanical properties of the 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 capillary pores and harmful pores into harmless capillary pores and gel pores, thus improving the mechanical properties of the RAC. Therefore, the use of hybrid CFs and NS was more conducive to enhancing the performance of RAC for building materials.