Surface modification of fly ashes and its effect on strength and autogenous shrinkage of cement pastes

The surface of GradeIIIfly ash particles was modified via addition of dolomite and calcination at 950°C for 1h. The microstructure, mineral phase and chemical compositions of the modified fly ash were analyzed by X-ray diffraction (XRD), scanning electron microscope and energy dispersive spectrometer. In addition, the effect of modified fly ash on the autogenous shrinkage and compressive strength of cement pastes was investigated, and the microstructure of cement pastes was determined by mercury intrusion porosimetry and back scattered electron microscope. The results show that the β-C₂S phases were formed on the surface of fly ash particles after the surface modification. Due to the hydration of β-C₂S, the porosity of cement pastes with the modified fly ashes was decreased, and the early strength of fly ash cement pastes was increased. The hydration of MgO due to the decomposition of dolomite can generate the expansion, leading to the compensation of the autogenous shrinkage of cement paste.