Degradation Mechanism of Low-Heat Portland Cement Paste Under Semi-Immersion Magnesium Sulfate Attack with Wetting–Drying Cycles

Concrete structures in the saline–alkaline regions of western China frequently suffer from combined sulfate and magnesium attack. Semi-immersion and wetting–drying cycles in the water-level fluctuation zone make up a typical service scenario. This study investigates the performance evolution and microstructural degradation of low-heat Portland cement (LHPC) paste exposed to 5% MgSO4 solution over 90 days under semi-immersion with wetting–drying cycles, using full immersion as a macroscopic reference. The results show that MgSO4 attack under this regime exhibits a two-stage behavior, featuring early strengthening followed by later softening. In the early stage, Mg2+ is inferred to react with Ca(OH)2 to form a brucite layer that fills pores and impedes ion ingress. Once this layer is compromised, Mg2+ is inferred to penetrate the matrix and cause decalcification of the C-S-H gel, converting it into non-cementitious M-S-H. Under semi-immersion with wetting–drying cycles, capillary suction and evaporative concentration are considered to enrich Mg2+ in the evaporation zone, accelerating this transformation and leading to a 29.03% loss in compressive strength at 90 days, exceeding the 22.25% loss under full immersion. Microscopic characterizations focusing on the semi-immersed specimens indicate that the loss of cementitiousness associated with M-S-H formation is likely an important contributor to the strength loss, while pore coarsening appears to be a secondary effect. The XRD patterns are consistent with M-S-H formation, and the SEM-EDS results show Mg and Si enrichment and Ca depletion in the evaporation zone, providing complementary evidence. The low C3A content of LHPC is expected to limit ettringite-related expansion, whereas the abundant C-S-H gel derived from C2S hydration may provide a substantial substrate for Mg2+ attack, making structural softening the dominant deterioration risk under the tested conditions. For the specific LHPC paste and exposure conditions investigated, strength degradation is recommended as a more informative indicator than expansion for magnesium sulfate attack. These findings help to clarify the degradation mechanism of LHPC under accelerated semi-immersion conditions. The limitations of the accelerated paste test should be considered when extrapolating these results to field service conditions.