Tu, Wenlin; (2024) Multiscale Study of Behaviour of Alkali-Activated Fly Ash-Slag Paste at Elevated Temperatures. Doctoral thesis (Ph.D), UCL (University College London).

Preview

Text WENLIN TU-PHD THESIS.pdf - Accepted Version Download (12MB) | Preview

Abstract

Alkali-activated materials (AAM) are studied as a novel sustainable binder to substitute Portland cement in concrete. They can possibly reduce up to 80% CO2 emissions by consuming wastes and industrial by-products, which are rich in aluminosilicate, such as fly ash, slag and metakaolin. Compared with alkali-activated fly ash and alkali-activated slag, alkali-activated fly ash-slag (AAFS) exhibits an excellent balance between engineering properties and practical fabricability. However, the behaviour of AAFS paste at elevated temperatures remains unclear in terms of high temperature performance, which impedes its potential application as future high-temperature/fire-resistant construction materials. To date, the microstructural evolution and micromechanical properties of AAFS paste at elevated temperatures have not been extensively explored from a multiscale viewpoint. Also, the in-depth investigation on the microstructure-property relationships in AAFS is still lacking. Hence, it is vital to gain a comprehensive understanding of AAFS paste and explore the inherent damage mechanisms at elevated temperatures. To tackle this challenge, for the first time, this thesis presents a systematic study on the behaviour of AAFS paste at 20 to 800 °C in terms of multiscale microstructural characterisation, micromechanical, thermal and mechanical properties. A series of advanced characterisation techniques including nuclear magnetic resonance (NMR), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) are undertaken to characterise the nanostructures and phase assemblage of AAFS paste after exposure to 20, 105, 200, 400, 600 and 800 °C. Meanwhile, the microstructural characteristics of AAFS paste in terms of morphology changes, crack development and pore structure evolution are monitored using backscattered electron microscopy (BSEM), mercury intrusion porosimetry (MIP) and X-ray microcomputed tomography (XCT). Thermal and mechanical properties are determined from thermogravimetric analysis (TGA), thermal deformation, compressive and flexural strength tests. Based on the findings above, the multiscale microstructural characteristics of AAFS paste are discussed in three different levels: (1) Level 0: solid gel particles (N-A-S-H, C-A-S-H and N-C-A-S-H), (2) Level I gel matrix (solid gel particles and gel pores), and (3) Level II paste (unreacted particles, reaction products and pores). The nanostructure of solid gel particles in AAFS paste is significantly altered as temperatures rise beyond 200 °C, resulting in the decomposition of C-A-S-H gels and domination of N-A-S-H gels at 800 °C. At Level I, the gel matrix in AAFS paste experiences refinement at up to 200 °C, followed by a continuous drop in gel porosity to around 5% when reaching 600 °C. After 800 °C, new crystalline phases in terms of nepheline and gehlenite are observed, taking up around 26.3% and 21.5% of the crystalline phases by volume. At the paste level, results indicate that the compressive strength of AAFS paste rises by 77.5% at 200 °C, followed by a mitigation from 200 to 600 °C and a regain at 800 °C. Different phases in AAFS paste including unreacted particles, reaction products and pores take up 30%, 67.7% and 2.31% at ambient temperature, and 4.95%, 84.8% and 10.3% after exposure to 800 °C, respectively. The decomposition of binder gels occurs while gel pores are filled at elevated temperatures up to 800 °C, along with the crack development, whereas micro-cracks are healed by melting and viscous sintering. Therefore, the relationships between microstructural evolution and mechanical properties of AAFS paste at elevated temperatures are explored and discussed in depth to gain insights into the underlying degradation mechanisms. These damage mechanisms are summarised at three temperature ranges including Stage 1 (20-200 °C): further geopolymerisation and pore pressure build-up, Stage 2: (200-600 °C) thermal gradient and phase decomposition, and Stage 3 (600-800 °C): recrystallisation and viscous sintering. To sum up, this thesis significantly advances the understanding of AAFS paste at elevated temperatures and relevant damage mechanisms, offering crucial insights for its application as sustainable, high-temperature resistant materials in buildings and other infrastructures.

Download activity - last month

Export as JSONXMLCSV

Download activity - last 12 months

Export as JSONXMLCSV

Downloads by country - last 12 months

Export as XMLCSVJSON

Archive Staff Only

View Item