| Author: | Zhou, Yifan |
| Title: | Recycling of incinerated sewage sludge ash in lime-pozzolan cements for stabilisation/solidification of marine sediments |
| Advisors: | Poon, Chi Sun (CEE) Cheeseman, Chris (CEE) |
| Degree: | Ph.D. |
| Year: | 2022 |
| Subject: | Sewage sludge -- Recycling Sewage sludge ash -- Recycling Binders (Materials) Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxx, 254 pages : color illustrations |
| Language: | English |
| Abstract: | Due to the booming population growth and many upgrading works of sewage treatment systems, approximately 1,200 tonnes of sludge, a by-product of the sewage treatment process, are generated per day in Hong Kong and this number is estimated to increase to 2,000 tonnes in 2030. The moisture cotent of the dewatered sludge was approximately 70%. This substantial amount of sludge undoubtedly causes a significant burden on landfills which has been a common way for disposal of sludge. However, due to the scarcity of landfill capacity and advocation of sustainable waste management, incineration has been found to be an effective and reliable method to handle sewage sludge. After high-temperature combustion, the sludge is converted to ash which is known as incinerated sewage sludge ash (ISSA) associated with up to 90% reduction of volume to be dumped in the landfills. To further mitigate the burden on the environment and facilitate waste recycling, a wide range of ways to reusing ISSA such as being used as supplementary cementitious material (SCM) and being applied in soil stabilisation are attractive and burgeoning. This mostly benefits the concrete-related industry with respect to not only the conservation of natural resources but also energy and carbon footprint reduction. This thesis is comprised of three segments and focuses on reusing ISSA into lime-pozzolan cements as an alternative binder for stabilisation/solidification (S/S) of marine sediments. The lime-pozzolan binders had been widely used since Roman times but their usage declined after the invention of Portland cement, but it regained prevalence with the aim of exploring sustainable eco-friendly alternative materials to Portland cement concrete. However, the lime-pozzolan reactions are complex and each pozzolan with its unique compositions may also react differently with lime. For this reason, this research conducted an in-depth and systematic study on investigating the reaction mechanisms of the lime-ISSA binder and evaluating its feasibility on using it in the S/S of dredged marine sediments. In the thesis, firstly, the reaction mechanisms of ISSA with different types of lime were studied. Then, the influence of seawater on the lime-ISSA binder was investigated. Finally, the lime-ISSA binder was applied for the treatment of marine sediments which can be used as a fill material. After a detailed literature review, the experimental work was carried out on comparing the hydration of lime and ISSA with Ordinary Portland cement (OPC) and ISSA. Methods for characterising the mixtures involved isothermal conduction calorimetry (ICC) for heat of hydration, compressive strength, X-ray diffraction (XRD) analysis, scanning electron microscopy-energy dispersive X-ray (SEM-EDX) analysis, and thermogravimetric analysis (TGA). The results revealed that the cement hydration was accelerated, and the strength of the OPC-ISSA mixtures was improved by incorporation of ISSA. This was mainly attributed to the lower effective water-to-binder (w/b) ratio. In the lime-ISSA mixtures, ISSA reacted rapidly with lime and produced a large amount of heat which facilitated the pozzolanic reactions of lime and ISSA. Most importantly, the formation of brushite, calcium phosphate hydrates, and CSH gels contributed to strength gain in the hydrated matrix. The thesis next presented comparative studies on applying pulverised coal fly ash (FA), a common industrial by-product, and ISSA as the lime-pozzolan binders. ISSA was ground in a ball mill for two hours to obtain fine ISSA (FSSA) for enhancing its pozzolanic activity. Also, the effects of using anhydrous lime (quicklime) and hydrated lime on hydration and strength development of the mixtures were studied. Through investigations on hydration evolution, compressive strength, microstructures, and reaction degree of calcium hydroxide, it was confirmed that the heat generated from the quicklime hydration was conducive to pozzolanic reactions, while there was little heat evolved from the mixtures prepared with ISSA- hydrated lime. Regardless of the type of lime, FSSA achieved a higher strength than FA. Subsequently, given the possibility of using the lime-ISSA binder for S/S of dredged marine sediments, the effects of seawater on the properties of the lime-ISSA binder were investigated. A series of experiments were performed including heat of hydration, compressive strength, XRD, SEM-EDX, SEM-back scattered electron (SEM-BSE) imaging analysis for assessing the porosity at the microscale, mercury intrusion porosimetry (MIP), and TGA. The findings indicated that the lime-ISSA binder attained a higher hydration rate and compressive strength by mixing with seawater compared with those prepared with deionised water. The formation of Friedel's salt was identified in the seawater mix, which might contribute to its more compact and denser microstructure. Besides, using seawater led to a reduction of the total porosity of the hydrated matrix. The hydrated lime-ISSA binder was found to have a higher strength when mixed with seawater mixing at early ages (less than 28 days) only, which was different from the quicklime-ISSA binder. Owing to the complex constituents of seawater, the effects of major salts in seawater were studied individually on the hydration of the lime-ISSA binder. The experimental results found that the use of MgCl2 shortened the setting significantly, and the mix prepared with MgCl2 also exhibited the highest compressive strength from 7 to 60 days. With the inclusion of NaCl, MgCl2, and Na2SO4, the average pore diameter was found to be largely reduced. The reactions between ISSA and Ca(OH)2 were controlled by the dissolution of reactants, precipitation of products, or the reaction taking place on the surface of ISSA particles. Finally, the lime-ISSA binder was employed on the S/S of marine sediments. In studying this application, two real sediments (i.e. clean and contaminated) were collected and used to evaluate the performance of the lime-ISSA binder through comparing with pure cement and the cement-ISSA binders for different civil engineering uses including fill materials and (non-)load-bearing masonry units. The main findings were after the S/S by using the lime-ISSA binder, the compressive strength was improved, and ISSA played a significant role in addressing the high moisture content of the sediments due to its high water absorption ability. Also, the results of two leaching tests validated that the contaminated sediments after S/S complied with the regulatory limits. Therefore, it is concluded that the lime-ISSA binder can be used as an alternative to replace cement to stabilise/solidify marine sediments for the direct reuse as fills. |
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| Access: | open access |
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