It is environmentally friendly to crush waste concrete as recycled aggregate for cyclic utilization, and it also can solve the shortage of natural aggregate. However, the performance of the recycled aggregate concrete was poor caused by the surface adhesion of old cement mortar on the aggregate and the micro crack properly generated during the crushing process. The pressure pretreatment on recycled coarse aggregate by CO₂ was used to improve its basic performances. The variables carbonization time (0h, 6h, 12h, 24h, 48h, 72h) and CO₂ carbide gas pressure (0.1MPa, 0.4MPa, 0.8MPa) were studied, and the basic properties of recycled coarse aggregate before and after carbonization were compared and analysed, and the degree of improvement on recycled aggregate concrete were also investigated to determine the optimal scheme for carbonation, which was in the container with 0.4MPa pressure for 24h. Based on the optimum carbonation scheme, the influence of recycled coarse aggregate on mechanical properties of recycled aggregate concrete was studied with the substitution rate of recycled coarse aggregate (30%, 50%, 70% and 100%) before and after carbonization respectively. The results showed that the CO₂ reinforced recycled coarse aggregate water absorption rate and crushing index was decreased, apparent density was increased. At the same substitution rate, the compressive strength and elastic modulus of carbonized recycled aggregate concrete was higher than that of non-carbonized recycled aggregate concrete.

Using recycled aggregate for producing concrete is an effective way to realize the resource utilization of construction and demolition waste. Understanding the variation law of the residual compressive strength of recycled aggregate concrete (RAC) under fatigue loading is significant for its practical application under repeated loads. Thus, this paper conducted uniaxial compression fatigue and static-load tests on recycled aggregate concrete with the different replacement ratios of recycled coarse aggregate, analyzed the probability distributions of the fatigue life and residual compressive strength of RAC and natural aggregate concrete (NAC), and revealed the attenuation law of their residual compressive strength. In addition, it studied and compared the evolution of fatigue damage to NAC and RAC by defining a damage variable based on residual compressive strength. The results show that the fatigue life of the RAC and NAC matches the two-parameter Weibull distribution, and their residual compressive strength follows the normal distribution. The attenuation law of the residual compressive strength of the RAC is similar to that of the NAC. In the initial stages of the fatigue loading, the attenuation is small, and the attenuation rate of the residual compressive strength of the RAC and NAC is low. In contrast, it gradually accelerates in the later stages; a sudden and significant decrease in the concrete strength is noticed in the final stage near failure. Compared with the NAC, the damage to the RAC develops faster in the initial stages of fatigue loading and slower in the later stages.

Using recycled aggregate in concrete is effective in recycling construction and demolition waste. It is of critical significance to understand the fatigue properties of recycled aggregate concrete (RAC) to implement it safely in structures subjected to repeated or fatigue load. In this study, a series of fatigue tests was performed to investigate the compressive fatigue behavior of RAC. The performance of interfacial transition zones (ITZs) was analyzed by nanoindentation. Moreover, the influence of ITZs on the fatigue life of RAC was discussed. The results showed that the fatigue life of RAC obeyed the Weibull distribution, and the S-N-p equation could be obtained based on the fitting of Weibull parameters. In the high cycle fatigue zone ($N\ge {10}^4$), the fatigue life of RAC was lower than that of natural aggregate concrete (NAC) under the same stress level. The fatigue deformation of RAC presented a three-stage deformation regularity, and the maximum deformation at the point of fatigue failure closely matched the monotonic stress-strain envelope. The multiple ITZs matched the weak areas of RAC, and the negative effect of ITZs on the fatigue life of RAC in the high cycle fatigue zone was found to be greater than that of NAC.

The fluidized bed combustion (FBC) ash was evaluated for value-added use in the production of geopolymer concrete where alkali aluminosilicate hydrates are the primary binder constituents. The FBC was supplemented with other raw materials in order to achieve chemical balance, and they were milled with CO₂ and air to evaluate the benefit from CO₂ uptake. An experimental study on the properties of the optimum FBC ash-based hydraulic cement and the concrete fabricated from it was conducted. Also, the corresponding properties of Portland cement concrete were also investigated as the reference group. For the FBC ash-based cement, tests were performed to measure the CO₂ uptake and heat of hydration; for FBC ash-based hydraulic cement concrete, measurements were made on the compressive strength of hardened material, residual compressive strength after immersion in boiling water and sulfates resistance. The results of this study suggest that, the FBC ash-based geopolymer with the mechanicochemical aviation can provide desired mechanical attributes, sulfates resistance when compared with normal Portland cement concrete.

Recycled aggregate concrete made with recycled aggregate was one of the effective ways to reuse the construction and demolition waste. Due to the weak complex interface transition zone of recycled aggregate concrete, the defect had great effect on the bearing capacity and durability of recycled aggregate concrete. The fracture performance of recycled aggregate concrete was studied, which was of great practical significance and engineering value. The wedge splitting test of 200mm × 200mm × 230mm specimens was carried out using the fatigue testing machine (MTS). The elastic modulus of the specimens were measured and calculated by strain gauge and dynamic resistance strain gauge, and the elastic modulus was about 4GPa lower than that of standard test. Based on double-K fracture model of recycled aggregate concrete, the crack initiation fracture toughness and the unstable fracture toughness was obtained. The test results show that the replacement level of recycled aggregate is not obvious to the crack initiation fracture toughness, and the unstable fracture toughness is lower than that of natural concrete. Using the exponential function to fit the tail of the P_H-COMD curve of the wedge splitting test, it is found that the fracture energy of the recycled aggregate concrete is not reduced with the increase of the replacement level of the recycled aggregate.