Compressive strength and electrochemical impedance response of red mud-coal metakaolin geopolymer exposed to sulfuric acid

The performance of the geopolymer synthesized using red mud and coal metakaolin when exposed to sulfuric acid solutions was evaluated. The effects of different soaking times on mechanical properties, electrochemical properties, and microstructural changes to the materials were investigated. The degradation of the geopolymer was studied using corrosion depth and weight testing, uniaxial compression test, electrochemical impedance spectroscopy (EIS), nuclear magnetic resonance (NMR), X-ray diffractometry (XRD), and scanning electron microscope-energy dispersive spectrometry (SEM-EDS). The EIS results indicate that the diameter of the impedance arc in the intermediate frequency region decreases with prolonged immersion. In addition, there was a significant correlation between the compressive strength and the impedance parameters (Rs and R1). Microstructural examination showed that the deterioration of the geopolymer observed was connected to depolymerization of the aluminosilicate polymers and destruction of microstructure in acidic media, which lead to loss of strength.