Organic chemistry

Alkali metal

Kaolinite

Geopolymer

Engineering

Catalysis

Composite material

Mineralogy

Inorganic chemistry

Chemistry

Materials science

Montmorillonite

Illite

Clay minerals

Chemical engineering

Calcination

Fly ash

Applied Clay Science

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.

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