| A comparison study on the recycled aggregate with different strength grades is studied in this paper. It is found that the properties of the natural aggregate are similar to those of the recycled aggregate and that the recycled aggregate is in correspondence with the relevant requirements of the building industry on aggregate and in agreement with the building codes of our country. In terms of the requirements on aggregate, it is feasible for the recycled aggregate is used for normal concrete making. In addition, the physical and mechanical properties of the recycled and natural aggregate are studied and the relationship between the crushing index, water absorptivity and volume fraction of coarse aggregate is established.Based on a large amount of experiments, the strength formula of natural and recycled aggregate is particularly studied. The experimental results show that the strength of concrete is related not only to the water/cement ratio, but also to the strength of the used coarse aggregate. A unified Bolomey's formula between the compressive strength of concrete at 28 days, and the crushing index of coarse aggregate, the nominal water/cement ratio and the real water/cement ratio is established and can be used for the mix proportions of recycled concrete. By comparing the measured values with the calculated values, it is found that the relative error is small, indicating the validity of theunified Bolomey's formula for estimating the strength of concrete at 28 days for a given cement and water per unit volume of concrete.The surface treatment of the recycled aggregate by direct coating of pozzolanic admixture slurry is used for recycled concrete. Compared with normal methods, the strength of concrete at 3 and 28 days is increased to a certain extent. The strength of concrete at 28 days is increased by 20 to 50%. Therefore, the surface treatment of the recycled aggregate by direct coating of pozzolanic admixture slurry is effective. In addition, with the surface treatment of the recycled aggregate by direct coating of pozzolanic admixture slurry, the accumulated and oriented calcium hydroxide in the interface zone between the recycled aggregate and cement paste can be absorbed, and the interfacial weak zone can be improved or even eliminated. Furthermore, the effect of interface strengthening on the interfacial adhesive strength of recycled concrete is analyzed. With the help of the widely used method for analyzing the microstructure of normal concrete, i.e., scanning electron micrographs, the interface strengthening mechanism of recycled concrete is vestigated.
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