Behavior and Mechanism of Electromigration in Controlling Low-Temperature Sulfate Attack on Cement-Based Materials

Low-temperature sulfate attack is a major degradation mechanism in concrete. Unlike conventional sulfate attack dominated by ettringite- and gypsum-induced expansion and cracking, an exposure below 20 ℃ promotes thaumasite as a primary product. Thaumasite directly decomposes calcium silicate hydrate as a principal binding phase in cementitious materials, converting concrete into a non-cohesive mass with a loss of structural integrity. This poses critical challenges for infrastructure in western cold regions and coastal zones where high sulfate levels coincide with low ambient temperatures.

Conventional prevention focuses on mix design. Strategies include densifying the matrix, reducing tricalcium aluminate, and consuming calcium hydroxide with pozzolans. However, the effectiveness is limited and can be even counterproductive at a low temperature. Sulfate-resistant cements that perform under ambient conditions can accelerate thaumasite formation in the cold. Some mineral admixtures also alter a hydration-product stability, producing variable resistance. These limitations motivate alternative protection technologies. Electrochemical approaches show a promising application potential (for instance, electrochemical chloride extraction achieves 30%–60% removal within 4–8 weeks), and the mechanistic differences between chloride and sulfate attack affect applicability.

This work was to investigate a preventive electromigration control method that applied direct current electric fields in the direction opposite to sulfate diffusion, inhibiting a sulfate ion penetration during early service life and weakening subsequent degradation reactions. Unlike conventional material-based methods whose effectiveness depends on cement hydration, electromigration could operate through electric field-driven ion migration control, maintaining effectiveness in low-temperature environments where hydration reactions could be retarded.

Cement-based materials containing 30% limestone powder (by mass) at a water-to-binder ratio of 0.40 were employed. Portland cement P·I 42.5 grade and limestone powder were used. To simulate actual reinforced concrete structures, specimens were embedded with 5 mm diameter stainless steel rods at their centers serving as cathodes for reverse electromigration. After standard curing, specimens underwent electromigration treatment. The electromigration system comprised a direct current power source, electrode system, and electrolyte. External titanium mesh anodes and embedded stainless-steel cathodes formed an electrochemical circuit with specimens serving as electrolytes. The degradation environment utilized 5% mass concentration sodium sulfate solution. The entire experimental process was conducted in a low-temperature chamber at 5 ℃ for 175 d. To investigate electric field intensity effects on sulfate attack control, two voltage parameters were established, i.e., 1.0 V (EF1 group) and 2.0 V (EF2 group). The comprehensive characterization techniques were employed to evaluate the protection efficacy and mechanisms.

The macroscopic performance evaluation demonstrates a significant degradation prevention due to electromigration treatment. Sulfate-immersed specimens exhibit typical low-temperature sulfate attack characteristics including progressive microcracking at 84 d, crack propagation with surface spalling and softened products at 140 d, and severe cracking with extensive spalling at 175 d. In contrast, electromigration-treated specimens maintain a superior appearance integrity throughout exposure, with EF2 specimens displaying only minor microcracks at 175 d, while EF1 specimens show a relatively more pronounced cracking.

The electrical resistivity evolution provides insights into degradation progression and protection effectiveness. Sulfate-immersed specimens exhibit a rapid resistivity increase during the first 60 d primarily due to pore filling by degradation products including gypsum and ettringite, temporarily increasing specimen densification. However, a continued exposure leads to a surface cracking, causing a resistivity growth to plateau after 140 d and declining to 18.63 Ω·m at 175 d as cracks expand and softening products accumulate. The electromigration-treated specimens, particularly EF2, demonstrate a more gradual resistivity increase and maintain relatively stable high values throughout testing, with EF2 retaining the maximum resistivity at 175 d despite minor edge cracking.

The depth-dependent sulfate content distribution reveals an electromigration effectiveness in suppressing sulfate penetration. All the specimens exhibit decreasing sulfate trends from surface to interior, reflecting external-to-internal penetration processes. The 2.0 V treatment achieves a 38.2% reduction in surface layer sulfate content, compared to sulfate immersion, with deeper regions approaching water-cured baseline levels, demonstrating an effective blockage of sulfate penetration for 175 d exposure period.

The thermogravimetric analysis and X-ray diffraction patterns with Rietveld quantification indicate a degradation product suppression. Sulfate-immersed surface layers (i.e., 0–5 mm) contain the maximum ettringite content of 11.95%, compared to water curing, while EF1 and EF2 specimens contain 6.25% and 4.98%, respectively. The calcium hydroxide content analysis reveals a significant consumption in sulfate-immersed surface layers (i.e., 9.27%) versus EF2 specimens (i.e., 12.9%), indicating that electromigration substantially inhibits calcium hydroxide reaction with sulfate ions. The attenuated total reflectance Fourier transform infrared spectra detects a characteristic absorption peak at 625 cm^-1 in sulfate-immersed surface layers, which is similar to silicon hydroxide group vibrations reported for thaumasite, indicating a minor thaumasite formation in severely degraded specimens.

The result by mercury intrusion porosimetry reveals the effect of electromigration on the pore structure evolution. Sulfate-immersed surface layers exhibit 7.6% reduction in cumulative pore volume relative to water curing (i.e., from 0.144 cm³/g to 0.133 cm³/g) due to degradation product pore filling, while EF2 specimens achieve a further reduction to 0.118 cm³/g, demonstrating an enhanced surface densification. The analysis by scanning electron microscopy with energy-dispersive spectroscopy shows a spatial visualization of elemental distribution and microstructural changes. The quantitative line scan analysis demonstrates that electromigration treatment fundamentally alters a sulfate spatial distribution within materials, reducing maximum sulfate content from 9.89% (in mass fraction) in sulfate-immersed specimens to 3.95% in EF1 specimens (i.e., 60.06% reduction) and 2.80% in EF2 specimens (i.e., 71.69% reduction).

This work could validate an electromigration-based control method for low-temperature sulfate attack on cement-based materials. The application of 2.0 V electric field significantly suppressed a sulfate penetration, reducing a surface layer sulfate trioxide content by 38.2% relative to untreated specimens. The electromigration treatment densified a surface layer structure, decreasing a cumulative pore volume to 0.118 cm³/g, while reducing a threshold pore diameter from 101.41 nm to 85 nm. The energy-dispersive spectroscopy analysis demonstrated that electromigration fundamentally altered sulfate distribution within specimens, reducing maximum sulfate content from 9.89% to 2.80%, effectively inhibiting degradation product formation. This proactive electrochemical control technology could offer an effective solution for protecting concrete structures in sulfate-rich environments in western cold regions and coastal areas.