Aluminium‐27 NMR investigation of the influence of cation type on aluminosilicate solutions

Abstract

The effects of tetraalkylammonium (TAA) and alkali metal cations on the equilibrium distribution of aluminosilicate oligomers in aqueous alkaline aluminosilicate solutions were investigated using 27 Al NMR spectra and their evolution with time. The results indicate that there are no differences in the initial equilibria involving solutions containing both TAA and alkali metal cations on the one hand and those containing alkali metal cations only. However, re-equilibration of the aluminosilicate species for TAA/Na aluminosilicates is slow (usually not detectable on the time-scale of the experiments), whereas when purely alkali metal cations are used, the spectra alter over a period of ∼1 h, such that resolution is degraded substantially. In the latter case, it is suggested that the anions aggregate into larger systems, although the solutions are still clear. 29 Si NMR evidence for slow equilibration of silicate and aluminosilicate solutions at higher concentrations is also discussed.

Effect of Alkali Cations on Aluminum Incorporation in Geopolymeric Gels

“…17 In aluminosilicate solutions with Si/Al ) 1, the majority of the aluminum is present in the form of Al(OH) 4 -(aq), in equilibrium with silicate oligomers and small quantities of oligomeric aluminosilicate species. 18 At Si/Al ratios of >5, aluminum is mostly incorporated into a variety of aluminosilicate oligomers that have been identified in detail elsewhere. 19 Of all the aluminosilicate species, the most stable is reported to be the cyclic trimer 20,21 with a Si/Al ratio of 2.…”

“…It has been shown that aluminum is incorporated into aluminosilicate oligomers where the nominal Si/Al ratio of these oligomers is greater than unity. 18 Hence, it is believed that, during polymerization, the rate of silicon precipitation from solution will exceed that of aluminum, reducing the Si/Al ratio in the solution phase. It is known that dilute alkali aluminosilicate solutions require up to 100 min to reach equilibrium, which might be extended due to the high viscosity of the geopolymeric slurry.…”

Effect of Curing Temperature and Silicate Concentration on Fly-Ash-Based Geopolymerization

“…These two factors seem to explain the differences in pore structure between Na and K geopolymers. Nonetheless, the effect of cation size on the mechanism of formation of aluminosilicate gels, such as geopolymer gels, is still ambiguous. , The increase in soluble cations is definitely known to affect the distribution of anionic species and the structure of porous aluminosilicate by increasing the depolymerization of larger silicate anions, thus enhancing nucleation and gel growth. Yet, the effects of cation size are complex and do not correspond to simple extension of the reasoning for alkali metal cations; rather, they are interrelated with various variables, such as the silicate and alkali concentrations.…”

Setting and nanostructural evolution of metakaolin geopolymer

“…It should be noted that liquid-state 29 Si NMR may not detect aqueous aluminosilicates at the concentration and temperature used here. 26,43 These species have been detected by 27 Al NMR, [44][45][46][47] which is more sensitive than 29 Si NMR mainly due to the higher natural abundance of 27 Al nucleus.…”