The influence of ambient pH on fly ash-based geopolymer

This study reports the comprehensive observation of the influence of ambient pH on the changes of fly ash-based geopolymer in an aqueous solution at various time periods up to 720 days. The aim of the study was to find a relationship between ambient pH, the change of composition, the structural changes and mechanical properties. The results of XRF, NMR, XRD showed that, Na can still leach into solutions of pH ≤ 13. The percentage of Na2O decreased over time in solutions of pH ≤ 13, and the decreasing rate of the Na2O percentage increased at low pH. The structural changes still proceeded for specimens in water, the number of Al-O-Si bonds increased over time. The cleavage Si-O-Si stopped, when specimens were immersed in the solution of pH = 1(HCl) due to the fast leaching of Na to solution and neutralization. In a high pH environment (NaOH), the Al-O-Si bond was more consistent than the Si-O-Si bond. The phase change was recorded only in the solution of pH = 14 with the small amount of Na-P1 zeolite. Even though the chemical composition and structure of specimens changed over time, the mechanical properties of the geopolymer were quite stable even when specimens were immersed in solutions of extreme pH (pH = 1, 2 or up to pH = 14).