| Author: | Qian, Lanping |
| Title: | Development of artificial aggregates from wastes and by-products for sustainable construction |
| Advisors: | Dai, Jian-guo (CEE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Recycling (Waste, etc.) -- Environmental aspects Waste products as building materials Sustainable construction Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxv, 258 pages : color illustrations |
| Language: | English |
| Abstract: | Transforming by-products and wastes into artificial aggregates is an effective solution which eases the environmental burden of both exploitation of natural resources and landfills. Artificial aggregates are generally manufactured by sintering and cold-bonded techniques. However, sintering process is energy-intensive and costly. Although cold-bonding process occurs at temperature below 105°C, cold-bonded aggregates are typically manufactured by using cement as binding materials. The cement industry accounts for around 5% of the total CO2 emissions worldwide. Therefore, the development of alternative technologies to manufacture the cost-effective artificial aggregates with lower environmental impact is required. To work around this issue, the use of geopolymerization technology to prepare geopolymer aggregates (GPA) has emerged in recent years. Geopolymer, synthesized by combining aluminosilicate precursors with alkaline activator, have significantly lower environmental impact and comparable mechanical properties to traditional cementitious materials. The current researches on GPA are mainly about the feasibility of recycling various wastes in GPA and the properties of produced aggregates, few studies focus on the fundamental manufacturing technology. Furthermore, the enhancement of environmental sustainability and economic development, and the failure mechanisms of GPA concrete need to be intensively studied. First step of this study was to explore the bulk utilization of waste materials by combining common commercial sintered aggregates with geopolymer technology. Next, detailed investigations on GPA manufactured through pelletization method using disc pelletizer and crushing technique were carried out. Hazardous wastes were carefully introduced as alternative precursors and activator to produce more sustainable GPA. Furthermore, the failure mechanisms of geopolymer aggregate concrete (GAC) were studied experimentally. The dissertation comprises an introductory chapter (Chapter 1), a chapter of literature review (Chapter 2), manufacturing methods and properties of GPA and GAC (Chapter 3-7) and a summary section (Chapter 8). The contents of Chapter 3-7 are explicated in succinct detail below. Chapter 3 introduced the concept of "full-volume fly ash geopolymer mortar", which utilizes fly ash geopolymer as a binder and sintered fly ash aggregates to completely replace conventional river sand. The primary objective of this approach is to conserve natural sand resources by utilizing fly ash, a byproduct of coal combustion, which is abundantly available. The proposed geopolymer mortars were experimentally evaluated to investigate the effects of sintered fly ash aggregates, alkali concentration, and curing regime on their physical, mechanical, microstructural, and mineralogical properties. The compressive strength of proposed mortars achieved slightly lower or almost similar values compared to the control mortars, which was due to the porous properties of sintered aggregates and chemical reaction between geopolymer binder and the external layer of sintered fly ash aggregates. The porous aggregates could also serve as micro reservoirs for internal curing and contributed to lower drying shrinkage. Chapter 4 demonstrates the manufacturing of GPA using both one-part and two-part pelletization methods. It covers the effects of various factors, such as the type of coal fly ash (i.e., low reactive fly ash and high reactive fly ash), activator alkalinity, and pelletization methods (one-part and two-part) on the properties of pelletized GPA. The research targets were the pelletization process and the properties of GPA, including pelletization efficiency, particle size distribution, oven-dried particle density, water absorption, individual pellet strength, and porosity. In comparison with one-part method, two-part method showed a more reliable and stable pelletization process regardless of the precursor types and the produced aggregates had higher individual pellet strength. Higher internal porosity was observed in the one-part GPA. However, this could lead to a lower oven-dried particle density than the two-part ones. In Chapter 5, the potential for large-scale industrial production of GPA is explored through a crushing technique. Initially, one-part geopolymer technology is used to cast geopolymer cubes, which are subsequently crushed into coarse aggregates with angular shapes. The influences of geopolymer mix proportions (i.e., the alkalinity and fly ash, ground granulated blast-furnace slag and cement contents) on the fundamental properties of GPA were comprehensively investigated. The produced GPA were further used as coarse aggregates to produce geopolymer aggregate concrete (GAC). Results indicated that a minimum early strength of around 1.0 MPa is still needed to facilitate enough cube stiffness for crushing. The increase in alkalinity and the addition of GGBS resulted in a higher one-day compressive strength, which would make better aggregate shapes, but need higher crushing energy and might cause initial damage to the aggregates. Generally, a higher aggregate strength of GPA could result in a better GAC performance. Chapter 6 proposes simultaneous utilization of red mud and flue gas residues as a partial replacement for traditional precursors and energy-intensive activator, respectively. The reaction mechanisms of geopolymer pastes were investigated through isothermal calorimetry, X-ray diffraction, thermogravimetry and infrared spectroscopy, by varying the dosage of red mud and flue gas residues. GPA were produced from the optimal mix proportion through the crushing technique. The comparison between the modified GPA in this chapter and the previous research results proves their superiority in engineering properties, cost analysis and environmental impact. When GPA were used to prepare concrete, GAC could achieve a similar strength to natural aggregates concrete (NAC) and had an improved interfacial zone compared to NAC as observed by the failure patterns of samples and microstructural analysis. Chapter 7 investigates the mechanical properties and failure mechanisms of GAC. Coarse aggregates with high strength GPAs were produced and utilized in concrete with varying strength grades by adjusting water-to-binder ratios. The mechanical properties of the GAC, including compressive stress-strain behavior, splitting tensile strength, and failure pattern, were comprehensively investigated using Digital Image Correlation (DIC) technology. The underlying mechanisms were further elucidated through microhardness tests and Back Scattered Electron (BSE) imaging coupled with Energy Dispersive Spectroscopy (EDS). Although GPA showed inferior strength than natural aggregates, both the compressive and splitting tensile strengths of GAC were slightly higher than those of NAC. For the failure modes of GAC, cracks penetrated through both GPA and matrix while more aggregate/matrix interfacial cracks were observed as the water-to-binder ratio increased. The micro-level observations demonstrated the existence of dense microstructures in GPA/matrix interfacial regions. In conclusion, GPA have the potential to achieve industrial production whether by pelletization technique or crushing technique. Simultaneous replacement of both precursor and activator by appropriate mix design can solve the current bottleneck of relatively high environmental impacts and costs. Fundamental understandings of the mechanical properties and failure mechanisms of GAC are explored. The developed GPA technology fulfills the pressing socio-environmental need for sustainable infrastructure development in Hong Kong, China and beyond. |
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| Access: | open access |
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