The Effect of Various Si/Al, Na/Al Molar Ratios and Free Water on Micromorphology and Macro-Strength of Metakaolin-Based Geopolymer

Abstract

The current work aimed to explore the effect of Na/Al ratios of 0.43, 0.53, 0.63, 0.73, 0.83, and 0.93, using NaOH to alter the molar ratio, on the mechanical properties of a geopolymer material, with fixing of the Si/Al molar ratio. While fixing the Na/Al molar ratio, alteration of the Si/Al ratios to 1.7, 1.75, 1.8, 1.85, 1.9, 1.95 was used, with silica fume and sodium silicate as a silica corrector. The influence on the micromorphology and macro-strength of samples was characterized through SEM, EDS, and compressive strength characterization methods. The results show that Si/Al and Na/Al molar ratios play a significant role in the microstructure and mechanical behavior of MK-based geopolymers, and revealed that the optimal molar Si/Al and Na/Al ratios for attaining maximum mechanical strength in geopolymers are 1.9 and 0.73, respectively. Under various Si/Al ratios, the macro-strength of the geopolymer mainly relies on the formation of NASH gel, rather than zeolites or silicate derivatives. The appropriate Na/Al molar ratio can contribute to the geopolymerization, but a ultra-high Na/Al molar ratio caused a high alkali state that destroyed the microstructure of the geopolymers. Regardless of the amount of water contained in the initial geopolymer raw material, the water content of Si/Al = 1.65 and Si/Al = 1.75 after curing for 10 days was almost the same, and the bound water content of the final geopolymer was maintained at about 15%. Structural water exists in geological polymer gels in the form of a chemical structure. It has effects on the structural performance strength, while free water affects the volume stability of the geological polymer. Overall, the current work provides a perspective on the elemental composition analysis, combined with the molecular structure and micromorphology, to explore the mechanical performance of geopolymers.

Geopolymer-Based Artificial Aggregates: A Review on Methods of Producing, Properties, and Improving Techniques

“…Therefore, an increased Si/Al ratio led to a denser microstructure of geopolymers produced, thus contributing to enhanced strength performance, and this was concluded by He et al [ 64 ], which proved that a denser microstructure with increasing Si/Al ratio contributed to high mechanical property performance. The findings were also supported by Wang et al [ 65 ], who determined the effects of Si/Al, Na/Al, and free water on micromorphology and the macro strength of metakaolin-based geopolymers. The characteristics of SiO 2 and Al 2 O 3 , which are used to control the glass phase viscosity, are closely related to the properties of artificial aggregates, and according to Kwek et al [ 66 ], aluminate plus silicate reactions are faster than silicate reactions alone.…”

Effect of Elevated Temperature on the Microstructure of Metakaolin-Based Geopolymer

“…The silicate modulus (Ms) of the sodium silicate solution controls the extent of gepolymerization and hence affects parameters like setting, viscosity, and strength. − Commonly used ratios fall within the range of 0.6 to 2 with the authors reporting various optimum values. − It was necessary to determine the minimum Ms that would help to achieve a workable solution. Figure compares the behavior of alkaline solutions with Ms of 0.7, 0.9, and 1.1.…”

Sludge‐based geopolymer materials: A review

“…In 2005, Palomo et al 20 conducted a comprehensive investigation of the microstructural morphology of FA-based geopolymers at different material phases using scanning electron microscopy (SEM) and transmission electron microscopy and proposed a descriptive model for geopolymer at the microscopic level based on the "Glukhovsky" model. In 2010, Criado proposed a nanostructure model for geopolymer, 21 followed by the discovery in 2013 by Myers and coauthors that the [C-(N)-A-S-H] gels formed in slag-based geopolymers can be substantially cross-linked to each other. They proposed the cross-linked substituted tobermorite model (CSTM), which can more accurately depict the spectral and density information of the material and estimate the average chain length of C-(N)-A-S-H gel.…”