High-volume fly ash replacing cement helps to improve the fluidity, volume stability, durability, and economy of self-compacting concrete (SCC). Sulfate attack is the most common form of the durability damage of hydraulic concrete; in particular, the performance degradation at the water level change position is more significant. Therefore, research on the influence effect and mechanism of fly ash on the durability is of great significance. In this paper, the change regularity of the SCC physical and mechanical properties with the fly ash replacement percentage and dry–wet cycles were studied by 60 dry–wet cycles of sulfate attack test. The 6 h electric flux, MIP, and SEM were used to study the performance degradation mechanism of SCC cured for 56 days, which had also been attacked by sulfate. The results show that the physical and mechanical properties of SCC increased first and then decreased with the dry–wet cycles of sulfate attack. After 10–15 cycles, the corresponding properties increased slightly, and then decreased gradually. When the fly ash content was 40%, the corrosion resistance coefficient, relative dynamic elastic modulus, and flexural strength retention were higher than those of the control specimen. However, when the fly ash content was 50%, they were close to the control and deteriorated obviously with the further addition of fly ash. For pore sizes in the range of 120–1000 nm, the porosity of SCC cured for 56 days was inversely proportional to the 6 h electric flux and the retention of mechanical properties, indicating that the porosity of the large pores is the decisive factor affecting the chloride ion permeability and corrosion resistance. The incorporation of fly ash in SCC can change the sulfate attack products and destruction mechanism. The sulfate attack damage of SCC with 40% of fly ash and the control specimen was dominated by ettringite crystallization and expansion, while those with a fly ash content of 50% and 60% had no obvious corrosion products, and the microstructures became looser. The appropriate fly ash replacement percentage could significantly improve the corrosion resistance of SCC.

Sulfate attack is the most common form of the durability damage of hydraulic concrete, and the performance degradation of cracked structural components is more significant at the position of water level change. Fly ash, a widely utilized supplementary cementitious material, can effectively improve the durability of concrete. In this paper, fly ash was used to partially replace Portland cement at 0 w%, 40 w%, 50 w%, 60 w%, and 70 w%, respectively. Through the three-point bending beam test with notch and the dry–wet cycle of sulfate attack, the change law of the fly ash content on the fracture mechanical properties of steel fiber reinforced self-compacting concrete (SFSCC) and its degradation mechanism under sulfate attack was studied. The results show that the load–crack mouth opening displacement curve of SFSCC changed from a steamed bread peak to a sharp peak under 30 dry–wet cycles of sulfate attack. The fracture toughness, peak load, and fracture energy of SFSCC with a high-volume fly ash increased with the increase in the fly ash content, while they reversed after sulfate attack. When the percentage of fly ash was 70 w%, the retention ratio of the fracture parameters was lower than that of SFSCC without incorporating fly ash, and when the percentage of fly ash was 50 w%, SFSCC had good bearing capacity, fracture mechanical properties, and corrosion resistance. The corrosion product of the reference SFSCC with 30 dry–wet cycles of sulfate attack was ettringite, whereas the SFSCC with a high-volume fly ash had no obvious corrosion products and the microstructure became looser.

This study examines the effects of ultrafine recycled powder (URP) obtained from construction and demolition waste on the hydration kinetics, setting behaviour, and chemical shrinkage of Portland cement pastes. The presence of ultrafine particles in the recycled powder provides more sites for nucleation, thereby promoting the hydration process and accelerating the rate of nucleation. As a result, the setting time is reduced while chemical shrinkage is increased. Incorporating URP improves the early-age mechanical properties. When 7.5% URP is added, the highest compressive strength and flexural strength of cement mortar at a curing age of 3 d are 23.0 MPa and 3.7 MPa, respectively. The secondary hydration between the hydration product and reactive silica from URP contributes to gel formation and enhances mechanical property development. This research provides theoretical insights into utilizing recycled powder in cement-based materials and enhances our understanding of its impact on hydration kinetics.

In this study, push-out tests were conducted on 20 specimens to explore the bond–slip performance of geopolymer concrete-filled steel tubes. The investigation focused on the effects of various design parameters such as length–diameter ratio, diameter–thickness ratio, concrete strength, and internal structural measures of the steel tube on the bond–slip performance. Analysis of the test phenomena, load–slip curves, and strain distribution curves of each specimen revealed insights into the shear strength calculation methods for welded stud structure and ring rib structure specimens. The results indicated a slight buckling deformation at the loading end of the steel tube in the structural specimen, while no significant deformation was observed in the non-structural specimen. The strain distribution along the height direction of the steel tube exhibited a triangular pattern, with the strain increasing gradually. Improvements in the interfacial bonding performance were noted with reductions in length–diameter ratio and diameter–thickness ratio of the steel tube, as well as increases in concrete strength. When the steel tube wall thickness t increases from 3.5 mm to 4.5 mm, the peak load of GC30-1 increases from 382.13 kN to 419.59 kN, an increase of 9.81%. After improving the concrete strength of GC30-1 and GC30-3 specimens, the peak load increases from 382.13 kN and 274.54 kN to 436.46 kN and 306.12 kN, respectively, an increase of 14.2% and 11.5%. Furthermore, the welding structure of the steel tube significantly enhanced the shear bearing capacity of the interface. The ratio of load calculation value to test value fell within the range of 0.917 to 1.098, indicating good agreement between the calculated and experimental values. These research results can provide reference for engineering applications of geopolymer concrete.