Foamed cement paste (FCP) is a type of cementitious geotechnical material with a high porosity structure, prepared by mixing cement paste with preformed foam and sometimes incorporating soil materials such as clay, silica fume, fly ash, and sand. Due to its lightweight nature, excellent self-supporting ability, easy construction, and environmental friendliness, FCP has been widely used in embankment fills, bridge abutment backfills, slope reinforcement, and retaining wall applications. However, in saline soil regions, FCP is subjected to chemical attacks, particularly sulphate attack, in addition to environmental factors such as temperature fluctuations and water infiltration. The early stage of sulphate attack plays a critical role in determining the long-term durability of FCP. Unlike conventional concrete, which may experience initial strength gain due to ettringite and gypsum formation, FCP's unique pore structure may result in different degradation patterns. At this stage, the interaction between pore water, sulphate ions, and cement hydration products can lead to either early-stage degradation or partial densification. However, the mechanisms linking early-stage mechanical behavior and pore evolution remain unclear. This study focuses on the mechanical behavior and pore structure evolution of FCP during the early stage of sulphate attack. Specimens were exposed to Na₂SO₄ solutions (0%, 0.5%, 2.5%, 5.0%) for 28 d and 56 d, representing the early-stage exposure period. A combination of uniaxial compression testing, X-ray computed tomography (X-CT), and nuclear magnetic resonance (NMR) analysis was used to evaluate the changes in strength, aquiferous pore size distribution, and aquiferous porosity. The findings provide key insights into the initial degradation mechanisms of FCP, highlighting the competing roles of skeleton softening due to water absorption and pore densification due to sulphate reaction products, which are of great significance for further understanding of the influence of service environment on the engineering performance of FCP.

FCP specimens were prepared using P.C42.5 composite Portland cement, anionic foaming agent LG-2258, and tap water. The water-to-cement ratio (m_w/m_c) was set at 0.45, and the foam volume ratio (V_f/V_w) was controlled at 3.5 to achieve a final wet density of approximately 0.9 g/cm³. The specimens were cast in cubic molds (70.7 mm × 70.7 mm × 70.7 mm), demolded after 24 h, and cured under standard conditions [(20 ± 2) ℃, RH > 95%)] for 28 d. To eliminate the influence of initial moisture content on sulphate attack, all specimens were oven-dried at 60 ℃ to a constant weight before sulphate attacking. A multi-field coupling test device was used to control sulphate attack conditions, maintaining a constant temperature of 20 ℃ and an osmotic pressure of 10 kPa. The specimens were exposed in Na₂SO₄ solutions (0%, 0.5%, 2.5%, 5.0%) for 28 d and 56 d, simulating the early stage of sulphate attack. After attacking, NMR and X-CT analyses were conducted to assess aquiferous pore structure and aquiferous porosity, followed by uniaxial compression testing at a loading rate of 2 mm/min. Each experiment was repeated at least three times to ensure accuracy and repeatability.

Uniaxial compression test results showed that, during the early stage of sulphate attack, FCP specimens initially experienced strength reduction compared to the standard condition. However, partial strength recovery was observed after 56 d, particularly at 2.5% Na₂SO₄ concentration, where the peak strength increased by 19.28% and the residual strength increased by 55.56%. This indicates a transition from initial degradation to partial densification, likely due to the formation of sulphate reaction products. Unlike traditional concrete, which often exhibits a continuous strength increase in the early sulphate attack phase, FCP's unique pore structure resulted in a more complex response. At low sulphate concentrations (0.5%), sulphate attack increased porosity, accelerating early-stage degradation. However, at higher sulphate concentrations (2.5% and 5.0%), reaction products partially filled existing pores, leading to strength recovery. X-CT and NMR analyses revealed that sulphate attack significantly altered the aquiferous pore structure of FCP in the early stage: 1) Macropore frequency (>100 µm) decreased as sulphate concentration increased, indicating pore filling by reaction products. 2) Small pore frequency (1-10 µm) increased, particularly at 2.5% and 5.0% Na₂SO₄, due to the formation of ettringite and gypsum. 3) Total aquiferous porosity of sulphate-attacked specimens was higher than that of water-immersed specimens, suggesting that sulphate ions modified water retention behavior. A strong exponential correlation was observed between aquiferous porosity and mechanical strength. As aquiferous porosity decreased, both peak and residual strength increased, confirming that sulphate-induced microstructural changes significantly influence mechanical performance.

The main conclusion of this work are summarized as following: 1) Sulphate attack in the early stage first weakens FCP, then triggers partial strength recovery. Peak and residual strength initially declined, but after 56 d, peak strength increased by 19.28% and residual strength by 55.56%, particularly at 2.5% Na₂SO₄ concentration. 2) FCP's early-stage sulphate attack behavior differs from traditional concrete. At low sulphate concentrations (0.5%), sulphate attack increased aquiferous porosity, accelerating early-stage degradation. However, at higher concentrations (2.5% and 5.0%), sulphate reaction products partially filled pores, leading to strength recovery. 3) Sulphate attack altered FCP's aquiferous pore structure in the early stage. Macropores (>100 µm) decreased, while small pores (1-10 µm) increased, indicating that sulphate products influenced pore connectivity and densification. 4) A strong exponential relationship was identified between aquiferous porosity and strength. As aquiferous porosity decreased, both peak and residual strength increased, demonstrating that sulphate-induced pore transformations impact mechanical stability. These findings highlight the critical role of the early attack stage in determining the long-term durability of FCP, offering new insights for optimizing its application in sulphate-rich environments.