Investigation of the effect of synthesis parameters and formation kinetics on the structure, mechanical performance and methylene blue adsorption of alkali activated magnesium silicates

Abstract

Alkali activated materials (AAM), commonly referred as geopolymers, are environmentally friendly alternatives to Portland cement with their remarkable mechanical strength. In this thesis, the main motivation is to investigate the effects of synthesis parameters and formation kinetics on the structure, mechanical performance, and methylene blue (MB) adsorption properties of alkali activated magnesium silicates. The raw materials and the SAAMs are characterized by combining X-ray fluorescence (XRF) spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). Attenuated total reflectance Fourier- transform infrared (ATR-FTIR) spectroscopic data is collected at different reaction times to shed light on the relation between the geopolymerization kinetics and the compressive strength. The results indicated that increasing pretreatment temperature has a negative impact, while increasing Si/Mg ratio has a positive impact on the compressive strength. The kinetics of geopolymerization is quantified by fitting exponential functions to the position of the main FTIR peak over time. The analysis demonstrated that early geopolymerization (time scales < 4 h) is a big contributor to the compressive strength. The highest MB removal capacity was obtained as 110.42 mg/g with a molar Na/Mg ratio of 1.3, a sepiolite treatment temperature of 750 °C and a molar Si/Mg ratio of 2.5. The ANOVA analysis showed a correlation coefficient (R2) of 0.865 and lack of fit was found to be non-significant. From the model, the optimum values of molar Na/Mg ratio, temperature and molar Si/Mg ratio are found to be 1.3, 697 °C and 2.4, respectively. The findings of this thesis demonstrate that SAAMs can be economical and sustainable alternatives for construction and wastewater treatment applications with their remarkable mechanical strength and enhanced adsorption performance.