In the saline soil environments of the western China, bridge piers often experience "root rotting" in the adsorption zone about 50 cm above the ground/water surface due to dual sulfate erosion. This phenomenon manifests as severe spalling and cracking, significantly shortening the service life of the bridges. It is thus urgent to take effective measures in the section to retard sulfate transport under capillary action and enhance the durability of concrete. Physical coating layers such as epoxy resin can effectively block sulfate ingress into the piers, but a long-term exposure to ultraviolet radiation and high temperatures can lead to aging and failure. Although fiber-reinforced composites can isolate erosion and restrain concrete, they exhibit a poor high-temperature resistance and may release toxic gases in case of fire. In contrast, steel tube-confined concrete improves load-bearing and seismic capacity and effectively inhibits expansion cracking of concrete caused by sulfate erosion. This study was to investigate the influence patterns and mechanisms of sulfate ion transport in steel tube-confined concrete. The effect of steel tube confinement on the sulfate ion transport behavior in concrete was investigated at different steel tube wall thicknesses, C₃A contents, and sulfate concentrations. In addition, a corresponding sulfate ion transport model was also established.

Specimens were cast using cylindrical molds made of PVC and steel tubes with the dimensions of φ100 mm×50 mm. After 24 h demolding, the specimens were standard-cured for 28 d. After curing, the specimens in PVC molds were removed, and their sides were sealed with epoxy resin to serve as unconfined specimens, ensuring one-dimensional sulfate ion transport.

All the specimens were immersed in a Na₂SO₄ solution (replaced every two weeks), and radial expansion and ion content were measured at specified ages. The radial expansion of confined and unconfined concrete specimens was determined by a ±0.001 mm electronic micrometer (Sanliang Co., Japan). The total sulfate ion concentration in the concrete was characterized by an inductively coupled plasma atomic emission spectrometer (ICP-AES). The pore structure of sulfate-eroded concrete was determined by a low-field nuclear magnetic resonance (LF-NMR) technology (Suzhou Niumag Analytical Instrument Co., Ltd., China)

Compared with unconfined concrete, steel tube confinement reduces the sulfate ion content and diffusion coefficient in concrete. Increasing the C₃A content, sulfate concentration, and steel tube wall thickness further decreases the sulfate ion content and diffusion coefficient in steel tube-confined concrete. This is because sulfate erosion causes a concrete expansion, and the confinement effect of the steel tube restrains this expansion, putting the steel tube in tension and the concrete in compression, effectively inhibiting expansion cracking of the concrete. This reduces ion transport channels and delays ion migration into the interior. A larger wall thickness (i.e., a higher steel ratio) provides a stronger circumferential confinement, thereby increasing concrete compactness and inhibiting ion ingress. However, the improvement in confinement stress due to wall thickness is limited, thus slightly enhanceing its hindering effect on ion diffusion. As sulfate concentration and C₃A content increase, the formation of expansive sulfate products such as ettringite increases, leading to a higher confinement stress. This effectively inhibits expansion cracking caused by erosion, thereby slowing down ion diffusion.

Under sulfate erosion, steel tube confinement effectively inhibits expansion cracking of concrete caused by expansive products such as ettringite. As the erosion age increases, the increase in sulfate ion concentration significantly accelerates the chemical erosion process, promoting the formation of gypsum crystals. This reaction consumes calcium hydroxide in the system and triggers the decalcification and decomposition of calcium silicate hydrate (C-S-H) gel, leading to a noticeable discoloration of the paste structure. As the stability of hydration products decreases, the microstructure of the material deteriorates, manifested by an increase in the proportion of harmful pores and total porosity. The evolution of the pore structure further alters transport pathways, forming fast channels for ion migration, ultimately enhancing the transport efficiency and erosion diffusion range of sulfate ions within the concrete. Based on this mechanism, a time-varying porosity sulfate ion transport model for concrete considering the effect of steel tube confinement can be established.

Under sulfate erosion, the steel tube could restrain a concrete expansion through the confinement effect, putting itself in tension and the concrete in compression, thereby inhibiting cracking. Steel tube confinement could reduce sulfate ion content and diffusion coefficient in concrete. Increasing C₃A content, sulfate concentration, or steel tube wall thickness further decreased sulfate ion content and diffusion coefficient.

A time-varying porosity-based sulfate ion diffusion model was established, accurately predicting sulfate ion concentration in steel tube-confined concrete under sulfate erosion.