The effect of alkali metals on the formation of geopolymeric gels from alkali-feldspars

Abstract The effect of alkali metal cations in alkaline solutions and alkali metals structured in alkali-feldspars on the formation of gel phase and the formed geopolymers has been investigated by using ICP, PAS-FT-IR, Raman, 29Si MAS NMR and SEM/EDX techniques. In a concentrated alkaline solution, the dissolution of alkali-feldspars is found to be inhibited, which is partly due to the reversible dissolution and precipitation reactions. DSC results indicate that K contained in the gel phase, either derived from KOH solution or from the dissolution of K-feldspar, is observed to increase the extent of disorder of the gel phase as well as the mechanical strength of the formed geopolymers. The reason for this effect is determined by 29Si MAS NMR analysis as being the higher polymerising activity between silicate species and between silicate and aluminosilicate species caused by the presence of K. The elemental analysis of gel phase using SEM/EDX shows that when the molar ratio of Na/K ranges from 3.5 to 85.6, the geopolymers synthesised from alkali-feldspar/kaolinite matrices demonstrate a high compressive strength. This study also reveals that a higher dissolution tendency as well as the involvement of K can increase the compressive strength of the geopolymers synthesised from alkali-feldspar/kaolinite matrices in alkaline solutions.

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