Influence of Fly Ash and Its Partial Replacement by Slag on the Leaching Behavior of Blended Cement Pastes

Abstract

An accelerated leaching experiment, using 6M NH4Cl solution, on specimens of pure cement paste, binary cement paste with different fly ash contents (20, 30, and 50%), and ternary cement paste with the replacement of 10, 20, and 30% fly ash by slag was conducted to investigate the influence of fly ash and its partial replacement by slag on the leaching behavior of blended cement pastes. The leaching depth, porosity, calcium-silicon mass ratio (Ca/Si), phase composition, and microstructure morphology of the specimens were characterized by using the phenolphthalein indicator, saturation-drying weighing, scanning electron microscopy with energy dispersive spectrometer (SEM/EDS), and X-ray diffraction (XRD). Results show that higher fly ash content mixed in pure cement paste causes a greater leaching depth, with slower microstructure deterioration, lower porosity increment, and reduction of Ca/Si and calcium hydroxide content. Higher replacement of fly ash by slag in binary cement paste leads to a more compact microstructure and higher Ca/Si after complete leaching of calcium hydroxide. The fly ash can reduce the leaching degree of pure cement paste but accelerate its leaching process; however, the partial replacement of fly ash by slag in blended cement paste not only improves its compactness and the stability of its microstructure, but also slows down its leaching process. The ternary cement paste with a water-binder ratio of 0.45, 20% fly ash, and 30% slag has the optimum leaching resistance.