Investigating Gel Molecular Structure and Its Relation with Mechanical Strength in Geopolymer Cement Based on Natural Pozzolan Using In Situ ATR-FTIR Spectroscopy

In situ attenuated total-reflectance Fourier transform infrared (ATR-FTIR) spectroscopy was used to investigate the gel formation of a natural pozzolan-based geopolymer and its relation to mechanical strength. For this purpose, geopolymer samples were prepared with different concentrations of SiO2/Na2O (Si/Na) and Na2O/Al2O3 (Na/Al) molar ratios in an alkali activator. The in situ ATR-FTIR showed that Si─O─T (T=Si, Al) stretching bond shifted to lower wave numbers with any increase in Si/Na molar ratio because of an increase in the geopolymerization reaction rate. The key finding of the present work is the relation between changes in gel molecular structure and the strength results demonstrated in different Si/Na and Na/Al molar ratios. Results showed that in high Na/Al and Si/Na molar ratios, Si─O─T bonds are formed at a lower wave number due to the presence of more Si─O species and increasing Al substitution in the silicate network of the geopolymer, which led to higher compressive strength.