This study conducts push-out tests on eight geopolymer concrete filled steel tube column specimens with varying design parameters to evaluate the effects of steel tube wall thickness, concrete strength, and the presence of welded longitudinal ribs on the bond-slip performance at the interface between geopolymer concrete and steel tube. The study analyzes the influence mechanism of chemical bonding force, mechanical bite force, and friction resistance on bond strength and a three-stage bond-slip constitutive relationship is established. The results indicate that steel tube strain increases with height, with significant strain observed at the welding structure and the fixed end of the steel tube. The length-to-diameter ratio, diameter-to-thickness ratio, concrete strength, and construction measures of the steel tube are identified as key factors in enhancing interfacial bonding performance. The presence of welded longitudinal ribs on the inner wall of the steel tube significantly improves the interface bonding. The calculated bond strength ratios, compared with the test results, fall within an 11% margin, demonstrating good agreement.

To reveal the meso-mechanism of the axial compression failure of recycled aggregates stabilized by multi-source solid waste geopolymers and to overcome the limitation that traditional laboratory tests cannot observe the meso-state of the material, a three-dimensional numerical model of this material was established using PFC3D discrete element software to simulate an unconfined compressive strength test. The meso-contact parameters for the specific mix proportion were calibrated using the back-analysis method. The stress–strain curves obtained from the simulation and laboratory tests were compared and analyzed. The meso-rules of axial compression deformation and failure of the material were analyzed from the perspectives of crack evolution, particle movement, and force chain transmission. The results show that the descending branch and failure mode of the stress–strain curves from the discrete element simulation are highly consistent with those from laboratory tests, whereas slight differences exist in the ascending branch owing to model limitations and material randomness. The through-cracks of the specimen failure are mainly distributed in the middle and lower parts. The cracks initiate from the middle of both sides and propagate longitudinally. The increase in cementation bond breakage reached 349.5% during the rapid crack development stage. After failure, the particles in the middle and lower parts of the specimen flowed around, and the particles in the upper part moved downward slowly. The force chains were uniformly distributed in the elastic stage. However, a large number of force chains broke, and the high-force chains were concentrated in the middle and lower parts in the crack development stage. This study verifies the feasibility of the discrete element method in simulating the meso-mechanical behavior of this material and provides a meso-theoretical basis for its mix ratio optimization and engineering applications.

In order to study the workability and mechanical performance of high fluidity recycled concrete, parallel tests were carried out to prepare high fluidity recycled concrete by changing the amount of recycled aggregate (20%, 30%, 40%, 50% and 60%) and the sand ratio (0.37, 0.38, 0.39 and 0.40). The fluidity, compression strength, tensile strength and flexural strength of each mix were tested. The results show that the slump of a recycled concrete mixture is 120 mm when the content of recycled aggregate is less than 30%, and the mechanical strength satisfies the requirement of the high fluidity ordinary concrete. As the ratio of sand increases, the fluidity of the recycled concrete also improved. The best sand ratio is not consistent with the strength index. When the sand ratio is between 0.37 and 0.40 and the content of recycled aggregate is not more than 40%, the concrete of C60 can be prepared. Both the fluidity and the strength index can meet the design requirements and can be used in the practical engineering.