Eco-efficient Cement Composites Reinforced with Recycled Carbon Fibres


Student thesis: Doctoral Thesis

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Awarding Institution
Award date3 Aug 2021


Carbon fibre-reinforced plastic (CFRP) is typically employed in the automotive, aerospace, and wind energy industries, where weight reduction is reckoned as an essential factor. These advanced materials are conducive to the expansion of the aforementioned industries by dint of their high specific strength and specific stiffness, lightweight nature, and remarkable corrosion resistance properties. Such distinctive merits have led to a conspicuous demand for CFRP, which, in turn, poses serious environmental challenges. Approximately 97,000 tonnes of CFRP waste from commercial aeronautical and wind power sectors is anticipated to be accumulated in the landfills of China by 2044, if not recycled properly. The traditional disposal approach to leftover composites of carbon fibres involves either waste disposal or combustion. These disposal solutions, however, lead to consequential environmental impacts, such as excessive land use, groundwater pollution, or the emittance of hazardous fluids and gases. Therefore, CFRP recycling and reuse of recycled fibres are the sustainable approaches to mitigate the CFRP waste accumulation in the landfills. The problem arises when the major stakeholders, i.e., aerospace and wind energy industries, reject the use of recycled carbon fibres (rCFs) recovered through CFRP waste due to a decrease in mechanical strength during the recycling process. For this reason, the intent of this study is to methodically assess the resourceful recycling of CFRP waste in construction industry and investigate the effects of rCF as a reinforcement on the mechanical performance and environmental impacts of cement composites.

This research aims to provide detailed insights into the influence of rCFs on the properties of cement composites. Therefore, a comprehensive research study was conducted to investigate the effect of rCFs in strength improvement, microstructure, crystalline phase change, and hydration products of cement composites over time. A detailed characterisation of rCFs revealed that when they are procured through the pyrolysis technique, they have surface defects, grooves, and traces of residual epoxy, which provide nucleation sites and better bond with the surrounding cement matrix. rCFs promote the growth of hydration products by providing inert nucleation sites without any additional chemical reaction. The experimental results resolved that the addition of rCFs to cement composites can provide a marked improvement in mechanical performance. Among other notable results, cement composite reinforced with 1% by volume of milled rCFs exhibited an optimum mechanical performance with an increase in flexural strength, compressive strength, splitting tensile strength, elastic modulus, and fracture toughness.

In addition, the scope of this thesis encompasses the investigation of the effects of elevated temperatures (up to 900°C) on the reinforcement mechanism of rCF-based cement composites. A rigorous investigation was conducted to examine the interaction of rCF with hydration products at different temperatures (25°C, 200°C, 400°C, 600°C, 800°C, and 900°C) by using X-ray diffraction (XRD), differential thermogravimetric (DTG) analysis, and scanning electron microscopy (SEM) techniques. Furthermore, the mechanical properties, including compressive strength, splitting tensile strength, and flexural strength, were experimentally studied as a function of rCF dosage and elevated temperature. From the obtained results, it is evident, that the reinforcing effect of rCFs was promising up to 600°C because of the release of high-pressure steam through channels on the surface of rCFs. At 900°C, rCFs were fully decomposed and resulted in empty tunnels, which subsequently affected the performance of cement composites. 

Further research topics evaluated the environmental performance of rCF-reinforced cement composites. For this purpose, life cycle assessment (LCA) was performed and the overall performance of six different fibres (rCF, basalt fibre (BF), glass fibre (GF), polypropylene fibre (PPF), polyvinyl alcohol fibre (PVAF), and virgin carbon fibre (vCF)) reinforced cement composites was evaluated on the basis of the mechanical strength, environmental performance, and cost. Based on the multicriteria performance score, the fibre-reinforced cement composites can be ranked as: M-rCF > M-PPF > M-BF > M-GF > M-PVAF > M-vCF.