Doctor of Philosophy
Civil and Environmental Engineering
Nehdi, Moncef L.
The increasing concern for decarbonizing concrete manufacturing is calling for sustainable construction materials since portland cement production contributes about 8% of global anthropogenic carbon dioxide emissions. Alkali-activated materials (AAMs) have emerged as promising alternative binders to curb carbon dioxide emissions from portland cement production and allow more sustainable construction. In addition, AAMs have desirable mechanical properties and high resistance to acidic and sulphate attacks.
Like conventional concrete, AAMs are quasi-brittle materials with low tensile strength, demonstrating susceptibility to cracking. Cracking-initiated deteriorations such as reinforcing steel corrosion, sulphate attack, and freeze-thaw damage, pose severe threats to the service life of concrete structures. Yet, sustainability is not readily achievable even if structures were made of environmentally friendly AAMs. Increasing awareness of sustainability has promoted the development of crack self-healing technologies. While crack self-healing in cement-based materials has been a topic of extensive research, very few studies have so far investigated the self-healing of AAMs. Therefore, there is need to explore self-healing approaches for improving the crack self-healing capability of AAMs.
This dissertation is centered on exploring the possibilities of enhancing the self-healing efficiency in fibre-reinforced alkali-activated slag-based composites via using calcium hydroxide powder, crystalline additives, expansive minerals, and biomineralization. The self-healing effect was evaluated using a portfolio of techniques, including optical microscopy, compressive and tensile tests, sorptivity tests, mercury intrusion porosimetry (MIP), inductively coupled plasma optical emission spectroscopy (ICP-OES), and X-ray microcomputed tomography (μCT). Crack self-healing products were characterized using scanning electron microscopy with energy dispersive X-ray (SEM-EDS) analysis and Raman spectroscopy. Ultimately, a chemistry-informed machine learning model was proposed to estimate the compressive strength of AAMs, therefore guiding the design of AAMs that meet the requirements for construction. The results demonstrate that all healing strategies explored herein improved the crack closure ratio significantly. In addition, the mechanical properties, watertightness, and pore structure were enhanced. SEM-EDS revealed that the main self- iii healing product was calcium carbonate. The computational intelligence model developed permits accurate prediction of the mechanical strength of AMMs and the effects of influential mixture design parameters. The findings of this study should bridge the knowledge gaps in crack self-healing of AAMs, thereby removing the hindrances that impede broader implementation of AAMs in construction applications.
Summary for Lay Audience
Except for water, concrete is the most widely used material in this world. It plays a vital role in developing urbanization and modernization. However, as the primary ingredient of concrete, portland cement requires fossil fuels and causes substantial greenhouse gas emissions during its production, thereby accelerating climate change. The increasing concern for a sustainable world is calling for green construction materials. Alkali-activated materials (AAMs), the products of reactions between industrial by-products or wastes and alkaline solutions, can be a green alternative to cement, with less environmental impacts. AAMs showed desirable performances comparable to that of their concrete counterparts.
Both AAMs and concrete are susceptible to cracking due to their brittle nature, which may threaten the lifespan of structures. The problem with cracks in a structure is that they provide preferential ways for the ingress of water, oxygen and chloride ions to steel rebars, thereby causing costly corrosion, and potentially compromising the safety of structures. To make AAMs more durable in response to cracking, the self-healing techniques inspired by nature, where organs can heal wounds automatically, were adopted in this study.
This dissertation aims to explore the self-healing improvement of AAMs using various additives and limestone-forming bacteria. The self-healing performance was investigated using multiple techniques. The results demonstrate that all the self-healing approaches improved the crack sealing significantly. It was also confirmed that the main self-healing product that healed surface cracks was limestone. The findings in this study help enhance the understanding of self-healing in AAMs, paving the way for developing sustainable green construction materials.
Zhang, Lei, "Crack Self-Healing in Alkali-Activated Materials: From Mechanisms to Strategies for Enhancing the Healing Efficiency" (2021). Electronic Thesis and Dissertation Repository. 8226.
Available for download on Sunday, October 01, 2023