Reaction and microstructure of cement–fly-ash system

The reaction and microstructure of the cement–fly-ash system were investigated in the present study. The reaction of cement–fly-ash system was described by a diffusion-controlled reaction process, leaving aside the nucleation and growth and boundary phase reaction processes at an earlier stage. The extent of hydration and the microstructure of hydrates were investigated by employing thermogravimetry, nitrogen adsorption/desorption analysis and scanning electron microscopy. A descriptive model of the reaction processes of the cement–fly-ash system was proposed to determine the importance of each process. In summary, (a) the blending of fly ash augments the local water-to-cement ratio and affects the heterogeneous nucleation of hydrates, promoting the hydration rate and extent; (b) the rapid hydration of cement coats the fly ash, retarding the dissolution and reaction of fly ash; (c) the continual dissolution of active silica-aluminate etches the surface of fly ash; and (d) the diffusion coefficient is estimated as 10−19 m2/s.