Pozzolanicity of fly ash modified by fluidized bed reactor-vapor deposition

Fly ash (FA) can improve the workability and durability of concrete. However, the utilization of FA is usually limited due to its low pozzolanic activity. In this study, a fluidized bed reactor was designed to modify FA by the fluidized bed reactor-vapor deposition (FBR-VD), using NaOH solution as the activation material. We investigated the pozzolanicity of FBR-VD modified FA (MFA), and the workability of cement paste. The results were compared against traditional chemical activation method, using Ca(OH)2, NaOH, Na2SO4, or NaCl. The microstructures and hydration products were analyzed by scanning electron microscopy (SEM), and thermogravimetric-differential scanning calorimetry (TG-DSC). The experimental results show that FBR-VD accelerates the early-age pozzolanic reaction of FA, without affecting the workability of the paste. The 7-day and 28-day activity indices of the paste and mortar with MFA were higher than those with raw FA, while the 60-day activity index of the paste became even close to that without FA. The SEM results indicate that FBR-VD can make the activation material coat onto the surfaces of the FA particles, breaking down part of the Si-O and Al-O bonds in the FA. The remaining NaOH crystals on the FA surface also promote the growth of Ca(OH)2 crystals during the hydration process, so the FA particles are fixed tightly to the hardened cement paste, resulting in a higher compressive strength. TG-DSC shows that the MFA has a stronger effect on consuming the Ca(OH)2 in the paste than traditional activation materials and methods, indicating an enhanced pozzolanic activity of the FA.