Fang, Guohao; (2021) Multiscale Characterisation of Microstructure and Mechanical Properties of Alkali-Activated Fly Ash-Slag Concrete. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Alkali-activated fly ash-slag (AAFS) concrete manufactured through the reaction of alkaline activator with industrial aluminosilicate by-products (fly ash and slag) is considered as a promising alternative to Portland cement (PC) concrete because of its environmental benefits (e.g. low CO2 emission and low consumption of natural resources) and superior engineering properties under ambient curing condition. There is about 55% less CO2 emissions in the production of AAFS comparing with the production of PC concrete. In addition, AAFS concrete can achieve a good synergy between fresh properties, mechanical properties and durability under ambient curing which cannot be achieved by the sole alkali-activated concrete, e.g., alkali-activated fly ash (AAF) and alkali-activated slag (AAS). AAF needs to be cured under an elevated temperature (60 ~ 85 °C) to gain early-age strength, whereas AAS concrete has some drawbacks including poor workability and quick setting. The mechanical properties of AAFS concrete are highly dependent on its heterogeneous microstructure with multiscale (nano- to macro-scale) and multiphase (pore, reaction products, unreacted fly ash and slag particles, and aggregate). Although the microstructure and mechanical properties of AAFS concrete have been studied for decades, a systematic understanding of the microstructure and micromechanical properties of individual phases within AAFS concrete and their corresponding relationships with the macroscopic mechanical properties is still lacking to date. More specifically, the following aspects for AAFS concrete have not been fully understood: (i) reaction mechanism of fly ash and slag particles in AAFS system; (ii) microstructure evolution of interfacial transition zone (ITZ) in AAFS concrete; (iii) multiscale micromechanical properties of AAFS concrete; (iv) multiscale microstructure-mechanical properties relationship in AAFS concrete. To fill these research gaps, this thesis aims to systematically characterise the microstructure and mechanical properties of AAFS concrete cured at ambient temperature at multiscale from nano- to macro-scale and to investigate the microstructure-mechanical properties relationship in AAFS concrete in depth. The multiscale features of AAFS concrete are identified based on four length levels: Level 0 (solid gel particle: 1 nm ~ 10 nm), Level I (gel matrix: 10 nm ~ 1 µm), Level II (paste: 1 µm ~ 100 µm), and Level III (concrete: 1 mm ~ 10 cm). Regarding the multiscale characterisation of microstructure, the nanostructure of solid gel particle at Level 0 is characterised using nuclear magnetic resonance (NMR), while the chemical composition of gel matrix at Level I is evaluated by means of X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The in-situ monitoring of microstructure evolution of fly ash and slag particles in AAFS paste at Level II is achieved using X-ray microcomputed tomography (XCT), providing new insights into their reaction mechanism. The microstructure evolution of ITZ in AAFS concrete at Level III is characterised using backscattered scanning electron microscopy (BSEM) and energy dispersive spectrometry (EDS), which delivers needed insight into the mechanism of ITZ evolution. The results of microstructural characterisation provide a systematic understanding of the microstructure of individual phases in AAFS concrete and their inherent relationships at different length scales. With respect to the multiscale characterisation of micromechanical properties, nanoindentation is used to evaluate the micromechanical properties (elastic modulus and hardness) of individual phases at Level I. It is the smallest material length scale that can be measured through experimental tests. The effective mechanical properties of AAFS paste at Level II are estimated using the self-consistent continuum micromechanics model by assuming that each nanoindentation test serves for a single phase in the material. Afterwards, the micromechanical properties of ITZ in AAFS concrete at Level III are evaluated through a series of statistical analysis. The multiscale micromechanical analysis offers the first-hand information of micromechanical properties of different phases in AAFS concrete and their contributions to the macroscopic mechanical properties of AAFS concrete. Lastly, the relationships between chemistry, microstructure, and mechanical properties of AAFS concrete from Level 0 to Level III are established based on the experimental results obtained above, which enable us to better understand the development of overall mechanical properties of this new type of concrete. The experimental and simulated results indicate that the dissolutions of fly ash and slag particles in AAFS system are not uniform due to their inherently heterogeneous characteristics, which would consequently lead to the formation of non-uniform reaction products, mostly accumulating within the boundary of the original particles. The polymerisation degree and cross-linking of reaction products are improved over curing age, potentially through the initial formation of C-A-S-H gels followed by the gradual development of N-A-S-H and N-C-A-S-H gels with a higher cross-linking degree. Within these three types of reaction products, the N-A-S-H gels have a relatively low elastic modulus due to their high level of structural disorder and gel porosity. In addition, it is found that the elasticity of reaction products and their relative volumetric proportions mainly determine the macroscopic elasticity of AAFS paste, while the porosity and pore size distribution primarily condition its macroscopic strength. Furthermore, it is also observed that ITZ formed in AAFS concrete has a comparable microstructure and micromechanical properties to the paste matrix, which indicates that ITZ might be not the weakest region within this new type of concrete. The ITZ with compact microstructure and high micromechanical properties would help to improve its macroscopic mechanical strength, especially for the fracture properties.

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