Metakaolin-, fly ash- and calcium hydroxide-based geopolymers: effects of calcium on performance

Effects of calcium on the performance of geopolymers prepared from metakaolin, class C fly ash (CFA) and calcium hydroxide are studied, including setting times and compressive strength testing. Leaching concentrations of calcium from prepared geopolymers are investigated by inductively coupled plasma atomic emission spectrometry. Relative contents of geopolymeric products and hydration products are studied by acid dissolving testing. The mineral composition, morphology and chemical groups of geopolymeric products and hydration products are analysed by X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy. Geopolymer setting time is much faster when calcium content increases. Calcium has a positive effect on compressive strength of geopolymers. By acid dissolving testing, the ratio of geopolymer value/ordinary hydration product value in metakaolin-based geopolymer was 4·4 : 1, whereas in CFA-based and CFA + 6% calcium hydroxide-based geopolymers the values were 1 : 1·7 and 1 : 1·5, respectively. Thus, the main product in metakaolin-based geopolymer is geopolymeric products, whereas in CFA-based geopolymer, hydration products also occur. In the alkali-activated condition, CFA-based geopolymer is a composite system with geopolymeric gel and calcium–silicate–hydrate gel co-existing.