Alkali activation behaviour of un-calcined montmorillonite and illite clay minerals

Using alkali activation, un-calcined soils have potential as precursors for low carbon, low cost, geopolymer-stabilised construction materials. This technology has been recently promoted as a lower impact alternative to cement stabilisation for walling materials in construction around the world. There is a lack of fundamental understanding around the alkali activation of un-calcined montmorillonite and illite, which, along with kaolinite, are clay minerals commonly found in soils. Kaolinite, as a 1:1 clay mineral, has been shown to form crystalline hydrosodalite when alkali-activated, but 2:1 montmorillonite and illite could form stronger geopolymer structures due to the higher Si:Al ratio in the precursor mineral. The lack of understanding of the underlying mechanisms at work with 2:1 clay minerals is a barrier to knowing how viable un-calcined geopolymer stabilised soil materials are for the range of soil types found in nature. In this study, montmorillonite and illite precursors were activated with a range of sodium hydroxide concentrations, compacted, and then cured at 80 °C for 24 h. The cured samples were characterised using a variety of advanced analytical techniques, including powder XRD, SEM, TGA, 27Al and 29Si-MAS-NMR, and FTIR. For the first time it was confirmed that alkali activation of uncalcined montmorillonite forms a NASH or (N,C)ASH geopolymer as the major product phase, which increases in quantity with increasing Na:Al molar ratio of the system. Although it has a similar Si:Al ratio, alkali activation of illite seems to result in structural alteration and increased porosity for Na:Al ≥ 0.5. The behaviour of these individual clay minerals suggests that the alkali activation of un-calcined 2:1 clay minerals is complex. Although alkali activation of montmorillonite can form a geopolymer, alkali activation of soils containing illite may lead to poor quality materials. This research has shown that the focus of future development work should be around montmorillonite-based clays.