Microstructure evolution and mechanical properties of calcined kaolin processing waste-based geopolymers in the presence of different alkali activator content by pressing and casting

Abstract

In this work, microstructure evolution and mechanical properties of calcined kaolin processing waste-based geopolymers in the presence of low and high contents alkali activators were studied. Lower and higher contents of alkali activators were employed to synthesize geopolymers by pressing and casting methods, respectively. Chemical bonding analysis, microstructure/elemental analysis, phase composition analysis, and compressive strength test were performed using FTIR, SEM/EDX, XRD, and universal mechanical testing machine, respectively. Findings showed that geopolymer with the low content of alkali activator formed by pressing (pressed geopolymer) might promote in the higher degree of geopolymerization because of higher compacted matrices but geopolymer with the high content of alkali activator formed by casting (cast geopolymer) would hinder the degree of geopolymerization reaction. Microstructure of pressed geopolymer showed a denser structure, no cracks, and lower porosity in comparison to the cast geopolymer. The geopolymers contained the lower and higher contents of alkali activators resulted in the formation of geopolymeric gels, and of sodium carbonate, sodium hydroxide, and zeolite phases, respectively. Compressive strength of pressed geopolymer was approximately 24.39% higher than that of cast geopolymer. Highest compressive strength values of pressed and cast geopolymers were 27.74 and 22.30 MPa, respectively. Thus, pressed geopolymer contained a lower content of alkali activator and had higher compressive strength in comparison to the cast geopolymers contained a higher content of alkali activator.