Microstructure Evolution and Mechanism of Strength Development of Fly Ash Paste

Three types of activators such as sodium hydroxide, calcium oxide and triethanolamine (TEA) are used to establish different activation environments to address the problems associated with the process of activating fly ash paste. We conducted mechanical tests and numerical simulations to understand the evolution of microstructure, and used environmental scanning electron microscopy (ESEM) and energy dispersive spectroscopy (EDS) techniques to analyze the microenvironments of the samples. The mechanical properties of fly ash paste under different activation conditions and the changes in the microstructure and composition were investigated. The results revealed that under conditions of low NaOH content (1%-3%), the strength of the sample increased significantly. When the content exceeded 4%, the rate of increase in strength decreased. Based on the results, the optimal NaOH content was identified, which was about 4%. A good activation effect, especially for short-term activation (3-7 d), was achieved using TEA under high doping conditions. The activation effect was poor for long-term strength after 28 days. The CaO content did not significantly affect the degree of activation achieved. The maximum effect was exerted when the content of CaO was 2%. The virtual cement and concrete testing laboratory (VCCTL) was used to simulate the hydration process, and the results revealed that the use of the three types of activators accelerated the formation of Ca(OH)(2) in the system. The activators also corroded the surface of the fly ash particles, resulting in a pozzolanic reaction. The active substances in fly ash were released efficiently, and hydration was realized. The pores were filled with hydration products, and the microstructure changed to form a new frame of paste filling that helped improve the strength of fly ash paste.