Coupled effects of emulsified asphalt solid content and emulsion-to-cement ratio on pore structure and durability of sulphoaluminate cement-asphalt mortar for ballastless-track filling-layer repair

Ballastless-track filling-layer repair requires rapid strength development, deformation compatibility, and environmental resistance. This study investigated sulphoaluminate cement-emulsified asphalt (SEA) mortar prepared with two cationic emulsions having nominal solid contents of 60% and 50% at A/C = 0–0.6, where A/C denotes the as-received emulsion-to-cement mass ratio. At a fixed A/C, changing solid content simultaneously changes the calculated nonvolatile-residue contribution and emulsion-introduced water, so the observed behavior reflects their coupled influence. As A/C increased from 0 to 0.6, the 28-day compressive strength decreased from approximately 68 MPa to below 10 MPa, and the dynamic elastic modulus decreased from approximately 12 GPa to 4 GPa. For the 60% solid-content emulsion (EA-A), the integrated mercury-intruded volume increased from 0.113 to 0.245 mL/g and D 50 increased from 17.7 to 332.5 nm, indicating substantial coarsening of accessible pore entries. Higher A/C improved relative retention under low-high temperature and wet-dry cycling and reduced the chloride migration coefficient by up to 47.4%, but the high-A/C mixtures retained much lower absolute strength and stiffness. At the same A/C, EA-A generally exhibited a better balance of mechanical and environmental performance than the 50% solid-content emulsion (EA-B), consistent with more nonvolatile residue and less emulsion water. SEM and XRD observations linked the exposure-related strength evolution with phase precipitation, local cracking, and interfacial changes. Considering strength, stiffness, shrinkage, and environmental retention together, EA-A with A/C = 0.2–0.4 is proposed as a preliminary composition window, whereas A/C = 0.6 is not recommended for applications requiring high support capacity.