Abstract This study investigates the rheological behaviour of clay–cement sealing suspensions modified with dry size-fractionated coal fly ashes of contrasting chemical composition. High-calcium S1 ashes and siliceous S2 ashes were separated into ultrafine ( 150 Pa, and exceeds 400 Pa at 30wt% S1 loading, accompanied by steep increases in storage modulus (G′ up to 10⁷–10⁸Pa for ultrafine fractions). In contrast, S2 ashes disperse the clay–cement network, lowering yield stress to ~ 15–60 Pa even at 40 wt% replacement and maintaining moderate viscoelastic stiffness (G′~10⁴–10⁵Pa). Particle fineness amplifies these trends: S1.UF promotes early hydration products precipitation and slip-layer formation, whereas S2.UF preserves fluidity. Sodium-silicate activation introduces a distinct threshold—at 1.0–1.5wt% Na₂SiO₃ the S1 systems undergo a sol–gel transition, with τ₀ surging above 200 Pa, while S2 systems respond gradually. These results quantitatively shows how ash chemistry and granulometry govern early yield stress, viscosity, and G′/G″ ratios, and identify composition windows for either rapid sealing (Ca-rich, fine fractions) or long-distance pumpability (Si-rich, fine fractions). The work demonstrates that fractionated fly ash provides an effective design tool for tailoring rheology of low-cement, sustainable sealing slurries for hydraulic and geotechnical applications.
Delihowski et al. (2026) studied this question.