Rheological characterisation of clay-cement sealing suspensions with fractionated coal fly ashes

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 (< 10 μm), fine (5-20 μm), and middle (20-100 μm) fractions to isolate size-dependent reactivity. Rotational and oscillatory rheometry demonstrate that Ca-rich fractions induce rapid structuration: at 20wt% ash addition, the Bingham yield stress rises from ~ 20-30 Pa in reference suspensions to > 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.