Early Hydration Behaviours and Kinetics of Portland Cement Composites Incorporating Low-Calcium Circulating Fluidized Bed Fly Ash

Low-calcium circulating fluidized bed fly ash (LCFA) exhibits obvious potential as a supplementary cementitious material (SCM) due to its minimal impact on concrete volume stability. However, its early hydration behavior remains unclear. This study investigates the hydration characteristics of cementitious composites incorporating varying LCFA dosages. Setting time, hydration heat, pore solution ion concentrations (Ca2+ and SO42−), and XRD analysis were employed. Hydration kinetics were described using the Krstulovic–Dabic model, with corresponding kinetic parameters calculated. The results demonstrate that LCFA inhibits the formation of calcium hydroxide (CH) and C-S-H precipitation while delaying sulfate depletion. Consequently, LCFA incorporation significantly extends both initial and final setting times. Hydration kinetics were effectively described by the Krstulovic–Dabic model, identifying three distinct stages of nucleation and crystal growth (NG), interactions at phase boundaries (I), and diffusion (D). Increasing the LCFA dosage reduced the rate constant for the NG process (KNG′) but increased the rate constants processes of I (KI′) and D (KD′). Furthermore, LCFA increased transition points of NG → I (α1) and I → D (α2).