Fly Ash Based Geopolymer for Concrete Infrastructure: Development, Characterization, Application, and Life Cycle Assessment

Zhipeng Li

doi:10.7273/000006384

To address the significant energy consumption and carbon emissions issues of the traditional concrete industry, there is an urgent need for a low-carbon, eco-friendly, and sustainable alternative to conventional ordinary portland cement (OPC). Geopolymer binders, also known as alkali-activated aluminosilicate binders, have emerged as a promising solution for tackling these challenges. This study primarily centers on the development, characterization, and application of a chloride-free fly ash-based geopolymer (FAGPR) binder, also known as alkali-activated fly ash binder, which is enhanced through the incorporation of graphene oxide (GO). Additionally, carbon fiber-reinforced plastic (CFRP) tubes are employed to enable the possible structural application of this geopolymer composite, and the associated life cycle assessment is conducted as well.Laboratory testing results indicate that the incorporation of 0.02% GO (by weight of fly ash) in the fresh mixture improves the mechanical and durability properties of the hardened FAGPR composite. This innovation allows the FAGPR composite to exhibit comparable performance to traditional OPC composite. The role of GO is revealed through analyses of hydration precursors, hydrates morphology, elemental distribution, and chemical structure of the FAGPR, by employing scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), thermogravimetric (TGA), Fourier-transform infrared spectroscopy (FTIR), and 29Si/27Al nuclear magnetic resonance (NMR) techniques, respectively. Experimental findings suggest that GO accelerates the hydration process of FAGPR binder and adjusts the Ca/Si, Si/Al, and Ca/(Si+Al) molar ratios of the hydration products. The NMR analysis also demonstrates that GO enhances the polymerization degree of FAGPR binder by inducing the formation of high-polymerized silicate-oxygen tetrahedra, thereby increasing its strength and durability. More specifically, NMR analysis confirms that GO also plays a crucial role in reducing the deterioration of the FAGPR binder in cold regions. In the unmodified FAGPR composite, the cyclic freeze/thaw process leads to the leaching of calcium, resulting in the depolymerization of hydrates and the degradation of hydration products. However, the incorporation of GO in the binder significantly inhibits calcium leaching by strongly attracting and retaining it. As a result, the chemical and physical structure of hydrates is preserved even in harsh environments, effectively mitigating the degradation of the FAGPR composite. Under the assumed service scenarios, the addition of GO is beneficial not only for the environmental sustainability of FAGPR concrete-filled CFRP tube cylinders but also for their economic sustainability. The primary reason is that GO enhances the strength of the core concrete, reducing the need for CFRP tubes, which are costly and feature high Global Warming Potential (GWP). Both FAGPR and GO-modified FAGPR (GFAGPR) concrete-filled CFRP tube cylinders exhibit lower GWP, and thus superior environmental sustainability compared to their counterparts filled with OPC concrete. However, the latter shows better economic sustainability. In the case of concrete-only cylinders, the relatively weak F/T resistance of (G)FAGPR leads to frequent reconstructions, subsequently causing significant CO2 emissions and imposing notable economic expenses. Even though they are relatively more environmentally and economically sustainable in the initial stage. As advice to engineers, when dealing with temporary or less important structures like some columns, piers for pedestrian bridges, or pile foundations for roads, concrete-only members could be given priority due to their lower carbon emissions. However, for these structural members that are more critical and will serve for a long time, concrete-filled CFRP tube composites are more desirable. In specific scenarios, such as earthquake-prone areas, concrete-filled CFRP tube composites should be prioritized due to their excellent ductility. Overall, the application of nanotechnology relaxes the restrictions of using fly ash in concrete. This study reveals the potential role of GO in the hydration of FAGPR, contributing to a better understanding of the microstructure of GFAGPR composites. GO also plays critical roles in harsh environments by mitigating the deterioration and preserving the integrity of FAGPR composites. Such universal insights can be extrapolated and serve as references for other cementitious materials as well. Part of the innovations reported in this work are protected by a US patent application.

Fly Ash Based Geopolymer for Concrete Infrastructure: Development, Characterization, Application, and Life Cycle Assessment

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