Mechanistic Insights into Shrinkage Mitigation in Low-Heat Portland Cement by the Combined Use of MEA and SRA

Drying-shrinkage cracking is an important durability concern in railway concrete, especially for ballastless track structures that require strict dimensional stability. Combining low-heat Portland cement (LHC) with an expansive agent and a shrinkage-reducing admixture (SRA) may provide a promising approach to mitigate such cracking. This study investigates the combined effect of an MgO-based expansive agent (MEA) and SRA on the drying shrinkage of LHC pastes. Individually, 10% MEA and 2% SRA reduced the 28 d drying shrinkage of LHC paste by 12.8% and 40.6%, respectively, while their combined use achieved a 52.2% reduction. SRA retarded the hydration of both LHC and MEA, whereas MEA promoted the early hydration of LHC. Meanwhile, the combined use of MEA and SRA decreased the fraction of 5-50 nm pores, which reduces capillary force and further suppresses drying shrinkage. Compared with the reference paste, the combined system showed lower early-age compressive strength, elastic modulus, and microhardness, together with a higher internal relative humidity during drying. These results suggest that the combined system developed a weaker early-age matrix and a less severe internal drying state, which were favorable for facilitating shrinkage compensation and reducing capillary-shrinkage tendency, respectively.