Effect of thermal treatment, foaming and stabilizing agents on the synthesis of fly ash-based geopolymer foams using Raman spectroscopy and 29Si and 27Al MAS NMR

Increasing environmental concerns have challenged researchers to find greener materials to meet the sustainability goals of applied engineering industries. Geopolymers, which have desirable chemical, mechanical, and porous properties, are promising for a variety of applications, from wastewater treatment to the production of green or lightweight construction materials. This study aims to investigate the effect of thermal treatment (70°, 70°+300° and 70°+600°C), foaming (0.1, 0.5 and 0.7 % Al powder) and stabilizing agent (0.1, 0.3 and 0.5 % sodium dodecyl sulfate [SDS]) on the mechanical and microstructural properties of fly ash-based geopolymer foams (GF). Uniaxial compressive strength values of these foams were determined on prepared 35(Ø)× 70 mm cylindrical specimens. Fly ash and final products were characterized by X-ray fluorescence spectrometers (XRF), X-ray diffraction (XRD), Magic-Angle Spinning Nuclear Magnetic Resonance Spectroscopy (²⁹Si and ²⁷Al MAS NMR), scanning electron microscopy (SEM), specific surface area/porosity analysis and Raman spectroscopy. The results shows that increasing the content of foaming and stabilizing agents increases the porosity of the geopolymeric foams. However, this also leads to a decrease in the strength of the specimens. Additionally, exposing the geopolymer foam specimens to high temperatures caused the porosity to decrease from 71 % to 62 %. Raman spectra analysis revealed a shift towards lower wavenumber of the bands attributed to Si-O-Si, Si-O-Al or O-Si-O; this is indicative of a change in the structure of the geopolymeric network from Q4 to Q3. In addition, the ²⁷Al NMR spectra analysis confirmed the crystallization of a part of the gel at high temperatures.