Graphene oxide modified, clinker-free cementitious paste with principally alkali-activated fly ash

This work reports the development of a clinker-free cementitious paste with principally alkali-activated class C coal fly ash (PAFA) as the binder and with graphene oxide (GO) as an admixture, cured under ambient conditions. With desirable strength, short setting time and slight volume expansion, this PAFA shows great potential to be used in cementitious materials for rapid repair or renewal. This laboratory study started from a statistical design of experiments to investigate the influence of six constituent materials (waterglass, lime, recycled glass powder, triisopropanolamine, cement kiln dust, and an encapsulating agent F500) on the compressive strength of hardened pastes at 7 days and 28 days, respectively. The mix design showing the best mechanical strengths features the mass ratios of fly ash: waterglass: lime: triisopropanolamine: sodium sulfate: GO equal to 100:6:3.6:1.2:1:0.02, water to dry binder of 0.24, and F500 to fly ash of 0.0375. This target sample and its control (with the same mix design but without GO) were selected for further investigation. The compression strength, setting time, workability, water absorption, and volume change of these pastes were tested to clarify the effect of GO on the macro properties of PAFA. The microscale investigations including scanning electron microscopy, thermogravimetric analysis, and X-ray diffraction shed light on the mechanistic role of GO. The Ca, Si, Al contents and the Ca/Si, Al/Si, Ca/(Al + Si) mole ratios, obtained from energy-dispersive X-ray (EDX) spectroscopy, were statistically compared to elucidate the hydration mechanisms of this PAFA binder.