Effect of nano metakaolin on compressive strength of recycled concrete

In order to solve the problem of waste concrete caused by demolition of old buildings, nano metakaolin was added into recycled aggregate concrete to form nano metakaolin recycled concrete. The cube compressive strength test was carried out on it, and the influence of nano metakaolin replacing ordinary Portland cement on compressive strength of recycled concrete was further studied. In this study, the internal microstructure of recycled concrete was observed by scanning electron microscope, aiming at revealing the mechanism of nano metakaolin to improve the performance of recycled concrete. BET multi-point method and BJH method were used to test the specific surface area and total pore volume of recycled concrete. The characteristics of pore size distribution were analyzed also. The results showed that when the replacement rate of recycled coarse aggregate was constant, the 28d compressive strength gradually increased with the increase of nano metakaolin content, but the increase trend was gradually slowed down. When the replacement rate of recycled aggregate were 30% and 50%, and the content of nano metakaolin was 5%, the compressive strength value each were the largest, and the increase of the compressive strength values were 17.0% and 8.0%, respectively. The compressive strength of fully recycled aggregate concrete increased most obviously, and the 28d compressive strength of fully recycled concrete with 7% nano metakaolin increased by 27.1%. There were pores in the recycled concrete with pore sizes of 3.5–4 nm mainly. The total pore volume of recycled concrete decreased after adding nano metakaolin, and the refinement of internal pore enhanced the structure compactness. The strength of recycled concrete with different substitution rates could be increased due to the incorporation of nano metakaolin. Furthermore, when the replacement rate of recycled aggregate was 30%, added 5% nano metakaolin could not only fill the micropore and microcrack of recycled aggregate itself, improve the contact surface of new and old mortar, but also fill the micropores inside the new mortar, so that the compressive strength of recycled concrete exceeded that of ordinary concrete.