Effect of fly ash with or without mechanical activation on early-age cement hydration: A comparative case study by boundary nucleation and growth model

In this study, the modified boundary nucleation and growth (BNG) model is adopted to elucidate the effect of fly ash, with or without mechanical activation, on early-age cement hydration kinetics. Results show that compared with ordinary Portland cement paste, as-received fly ash has impaired the nucleation density. This might be attributed to the aluminate ions in the pore solution, released from the dissolution of cement and fly ash, which have passivated the surface of particles and suppressed nucleation and growth of the main hydration products, i. e., calcium-silicate-hydrate (C-S-H) at the early age. However, mechanical activation towards fly ash has improved its specific surface area, further boosting its reactive area fraction in the nucleation and growth pro-cess. The increased boundary area per unit volume for mechanically activated fly ash is less affected by the hindering effect of aluminate ions, and favors the C-S-H nucleation in the paste, which causes the nucleation density for the paste containing mechanically activated fly ash to be higher than the one containing as-received fly ash.