Reaction, Phases, and Microstructure of Fly Ash-Based Alkali-Activated Materials

Abstract

The alkaline activator has a significant effect on the microstructure and mechanical properties of fly ash-based alkali activated products. This paper aims to identify the reaction products of fly ash (FA) activated with alkaline activators of different concentrations of Na₂O and different SiO₂/Na₂O ratios. NaOH solution (NA), a mixture of NaOH and sodium silicate solution (MIX) and sodium silicate solution (LG) were used to activate fly ash.

Different techniques were used to characterize the reaction products of activated fly ash including X-ray diffraction (XRD) with Rietveld refinements, attenuated total reflectance Fourier transformer infrared (ATR-FTIR), ²⁹Si dipolar-decoupling (DD) magnetic angel spinning/nuclear magnetic resonance (²⁹Si MAS/NMR) and ²⁷Al MAS NMR, field emission scanning electron microscopy (FE-SEM) attached with energy-dispersive X-ray (EDX) analyzer. Also, the fly ash-based alkali activated products were indentation tested using a Vickers indenter.

The results revealed that the main reaction product of alkali activation of fly ash is a sodium aluminosilicate gel (geopolymer) with high Si/Al ratio when sodium silicate was used as alkaline activator, while zeolite appeared as minority phase when fly ash was activated with NaOH solution or a mixture of NaOH and sodium silicate solution. Moreover, Na₂O content and SiO₂/Na₂O ratio in the activator solution influence on the amount of amorphous phase and Q⁴(mAl) units formed in the alkali activated materials and that reflect on the mechanical properties.