Geopolymer synthesis using low-grade clays

Abstract

Due to the large contribution of cement production towards CO2 emission and rising concerns regarding climate change, substitution with alternative binders has become an established alternative to improve sustainability. Aluminosiliceous polymeric materials, known as geopolymer or alkali activated materials, have been proven to achieve a performance as good as and even superior to ordinary Portland cement (OPC). Various source materials have the potential to be used for the geopolymer synthesis depending on the local availability, among which fly ash, slag, and metakaolin have received most attention. A less established alternative is low grade clay, whether as the natural clay or the waste of construction work. These have the potential for geopolymer production due to their composition and abundance worldwide. However, due to the impurities present in these low-grade clays and slower dissolution rates, the synthesised geopolymers generally display lower mechanical performance. In this research, four different clays collected form construction sites in Melbourne, Australia, were characterised, and the optimum clay selected based on the amorphous content, specific surface area, and fineness. Furthermore, a relatively low energy-demand calcination regime was applied to the chosen clay, 550° C for 1 h. For the untreated clay-based geopolymer, a 7-day compressive strength of nearly 32 MPa was achieved, which increased to approaching 50 MPa after calcination at 550° C. A range of microscopy techniques, EDS, XRD, XRF, zetametry, 27Al MAS NMR, and computerised tomography were applied to characterise the microstructure and reaction kinetics of the synthesised geopolymer.