Ft-ir and si-29-nmr for evaluating aluminium-silicate precursors for geopolymers

Geopolymers are systems of inorganic binders that can be used for sustainable, cementless concrete and are formed by alkali activation of an aluminium–silicate precursor (often secondary resources like fly ash or slag). The type of aluminium–silicate precursor and its potential variations within one batch may influence geopolymer performance. Therefore, if geopolymers are to be applied for sustainable concrete with reproducible quality, a characterization tool for quality control of precursors and optimizing mix design is needed, which links the (variable) secondary resource characteristics to the performance of geopolymers. This paper shows the potential of using FT-IR and solid state 29Si-NMR for a (semi-)quantitative evaluation of secondary resources suitable for geopolymers. More specifically, the aim is to investigate whether the presence of particular aluminium–silicate phases may have a dominant influence on the strength of a simple geopolymer system. Based on deconvolution of FT-IR and NMR spectra of 9 source materials (fly ash and slag) and their geopolymers, relative amounts of different (aluminium–silicate) phases were estimated and trends with strength were evaluated. A clear trend is observed for both NMR and FT-IR results: the precursors containing a higher amount of ‘active bonds’, predominantly from silicates with a medium amount of aluminium and alkalis incorporated in their framework, result in higher paste strengths. As such, due to its relatively quick and easy measurements, FT-IR for estimating relative amounts of these aluminium–silicate phases, can be a useful tool for evaluating geopolymer precursors.