Abstract: To make full use of recycled concrete resources, using carbon fiber to modify the flexural strength of nano metakaolin recycled concrete and preparing recycled concrete with different carbon fiber contents and nano metakaolin contents as variables, combing with the mercury injection method and scanning electron microscope to study recycled concrete’s pore structure and microscopic morphological characteristics. Then analyzing the modification mechanism of carbon fiber on nano metakaolin recycled concrete. Results show that the strengthening effect of carbon fiber on flexural strength is noticeable. The suitable content of carbon fiber varies under different nano metakaolin contents. When the content of nano metakaolin is 7%, 10%, and 12%, the optimum content of carbon fiber is 0.1%, 0.15%, and 0.2%, respectively. When the content of nano metakaolin is 10%, and the content of carbon fiber is 0.15%, the flexural strength reaches the maximum, which is 59.1% higher than that of ordinary recycled concrete. The incorporation of carbon fiber refines the internal pore structure of recycled concrete, decreasing the proportion of harmful and multi-harmful pores and increasing the proportion of less harmful pores. Carbon fiber plays a role in bearing and cracking resistance in recycled concrete. The filling effect of nano metakaolin and the volcanic ash activity enhance the bonding strength of the interfacial transition zone, which makes the carbon fibers more effective and thus promotes the improvement of the strength of the recycled concrete.

Key words: carbon fiber, nano-scale metakaolin, recycled concrete, flexural strength, microstructure, composites