Dry–Wet Cyclic Sulfate Attack Mechanism of High-Volume Fly Ash Self-Compacting Concrete

Abstract

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.