Urban waste accumulation has changed the landscape of cities through environmental pollution. This detrimental change is pushing the development of advanced and green waste management strategies for reshaping green cities, given that the traditional disposal approaches of incineration and landfilling cause air, soil, and water pollution. The utilization of urban waste in concrete shows immense potential as an alternative, as it brings about environmental and economic benefits. However, we are encountering some long-term existing technical barriers that impede the utilization of urban waste in concrete. This thesis primarily focuses on the use of large quantities of waste generated in urban areas, specifically used engine oil (UEO) and thermoset plastic waste. These two materials serve as reference models for integrating other urban waste types, i.e., organic liquid and solid waste with cement-based construction materials, such as concrete and mortar. The objective is to address the challenges of poor dispersion and weak interface between waste materials and cementitious binders in concrete with UEO and thermoset waste, respectively, ultimately promoting sustainable and innovative solutions for urban waste management.

To address the problem of poor dispersion in concrete with UEO, a typical superplasticizer was employed to achieve good dispersion of UEO in ternary blended concrete with fly ash and silica fume. The workability and mechanical performance of concrete admixed with UEO were investigated through slump and compression tests. The experimental results reveal that ternary blended concrete containing a high dosage of UEO (5% by weight of cementitious materials) compares favorably to ordinary concrete without sacrificing workability and compressive properties. High strength concrete (C60) with a 2% dosage level of UEO exhibits the maximum improvement in compressive strength by 4.4%. In addition, a scanning electron microscope (SEM) was used to reveal the microstructural origins behind the alterations in the mechanical properties. Microscale images demonstrate that the addition of UEO aids in the microstructure densification of concrete.

To further prove the technical feasibility and promote safe application of UEO concrete, its fire performance has been thoroughly evaluated. The fire performance of UEO concrete following exposure to elevated temperatures was assessed by measuring its compressive strength and elastic modulus. Experimental outcomes suggest that concrete with 2% UEO by mass of cementitious materials shows enhanced fire performance and that concrete admixed with 5% UEO exhibits comparable mechanical properties to reference concrete. In ascertaining the microstructural origins behind the mechanical performance changes, scanning electron microscopy analysis was conducted to analyze the surface morphology of UEO concrete. The analysis demonstrates that concrete with 2% UEO features densely packed fibrous calcium silicate hydrates (C-S-H) at 200°C that contribute to strength increase. In addition, the surface characteristics of UEO concrete in terms of color changes, cracks, and pore density were analyzed to supplement the fire performance information, with the intention of facilitating structural health assessments following fire attacks. Moreover, the environmental and economic assessments of the proposed UEO disposal approach concerning carbon dioxide emissions, energy consumption, and costs, respectively, are discussed and contrasted with the mainstream disposal options. The discussion suggests that the present recycling strategy can reduce carbon dioxide emissions by 8,050–10,750 kg, energy consumption by 2.87-4.13 billion MJ, and disposal costs by HK$3,250–9,450 per tonne of UEO.

To solve the weak interface problem in cement mortar with thermoset waste, a green recycling solution for recycling thermoset waste as fillers with the help of a green surface adhesion promoter – a silane coupling agent – was proposed. The feasibility of the proposed approach was experimentally validated in terms of workability performance and compressive strength in order to meet requirements in practice. The results show that thermoset waste can substitute for at least 15% of sand in cement mortar, following workability requirements. In addition, the optimal replacement level of thermoset waste for sand in cement mortars with the highest compressive strength is 5%, beyond which the compressive strength exhibits a continuing drop. The strength alteration mechanisms were elucidated by SEM analyses. The SEM results demonstrate that the developed thermoset cement mortars with 5% thermoset waste have great potential to achieve a highly dense microstructure, robust interfacial transition zone, and better hydrate crystal growth, which are the microscopic origins determining the inherent strength enhancement.

This thesis has innovatively devised green recycling processes and presents technically viable solutions for recycling used engine oil UEO and thermoset waste. These ecofriendly concrete products and recycling technologies will facilitate the development of environmentally conscious formulations for waste-enhanced, sustainable cement-based construction materials that exhibit desired practical performance for large-scale civil infrastructure applications. Additionally, the research advances a comprehensive urban waste management strategy that supports the long-term goal of decarbonizing the built environment of human society.