Haigh, Robert (2023) Engineered mortar and concrete composites using fibres derived from recycled cardboard. PhD thesis, Victoria University.

Abstract

The extraction of natural resources for building and construction materials is growing exponentially. Concrete and mortar materials require cement as the primary binder, but cement production negatively impacts the environment. Cement substitution has been studied by many researchers over the years, primarily focusing on industrial wastes as a partial replacement. However, the accumulation of common waste materials is a challenging problem due to the abundance of waste generated across all economic sectors. Due to recent exportation bans, countries are now searching for alternative methods to recycle their common household waste products. Cardboard materials have a limited recycling rate due to the contamination and weakening of the constituent fibres (kraft fibres). This leads to the disposal of cardboard contributing approximately 2.2 million tonnes of waste annually to landfills across Australia. To reduce cardboard waste accumulation, there is potential for kraft fibres (KFs) to be further incorporated into cement-based composite materials. Nonetheless, limited research outputs have been conducted on the successful integration of KFs. KFs are natural fibres which are susceptible to degradation in high alkaline environments, such as cement-based materials. When fibres degrade, the mechanical integrity of the material is compromised, and the service life of the application is often reduced. For the application to be widely accepted in the construction industry, further research is required. As the drive toward the circular economy framework becomes prominent, this research addresses the issue of cardboard waste accumulation and reducing cement consumption in concrete and mortar materials. This thesis presents a study that integrates matrix and fibre modification techniques on KFs derived from cardboard waste. Silica fume (SF) is used as a fibre pre-treatment, creating silica fume kraft fibres (SFKFs) and metakaolin (MK) as a matrix modifier. Integrating both SF and MK with KFs is a novel concept for minimising the degradation caused by calcium hydroxide in concrete on natural fibres. The target compressive strength was 25MPa, with KFs replacing 5-20% of cement in concrete. Compressive, tensile, and flexural testing was conducted to determine the mechanical capabilities of the novel mix designs. As expected, density and compressive strength declined with increasing fibre integration. However, matrix modification with 5% MK alongside 5% SFKFs increased the tensile strength by 10%. To ascertain the long-term effects of the novel composite, durability experiments were required. These included accelerated ageing simulations of wet and dry cycles, as well as water absorption, water immersion, carbonation, and chloride ion permeability experiments. The ageing simulation results produced a lower mechanical strength for all composite materials, including the control. The integration of 5% MK increased the surface absorption of the fibre composites, whereas 10% SFKF composites exhibited the lowest water absorption at 365 days. Raw KFs had a lower carbonation rate than matrix and modified fibres due to the morphology of the supplementary cementitious materials. The morphology and microstructure of the fibres were analysed using a scanning electron microscope (SEM) and energy dispersion x-ray spectroscopy (EDS). This application of SF revealed less degradation on the outer fibre walls, thus increasing service life of the fibre and the corresponding composite. A Fourier transform infrared spectroscopy (FT-IR) was employed to gain insights into the fibres chemical nature. The thermal, calorimetric and combustion attributes of the fibres were measured using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and pyrolysis combustion flow calorimetry (PCFC). SFKFs showed a lower heat release capacity, demonstrating a lower combustion propensity compared to raw KFs. Furthermore, a 45% decreased peak heat release rate of SFKFs highlighted the overall reduction in the fire hazards associated with these materials. Moreover, SEM images illustrated advanced petrification on raw KFs compared to SFKFs within the composite. The increase of cement products on the fibre wall contributes to the associated mechanical strength reductions. In conjunction with the experimental data, life cycle assessments (LCA) and mix design optimisation were used to determine the environmental and economic effects when using waste cardboard KFs. The LCA determined the environmental effect of waste cardboard integration. A sensitivity analysis using a Monte-Carlo (MC) simulation investigated the transportation and energy manufacturing greenhouse gas (GHG) emission variables. Two KF composites were subsequently evaluated. SFKF5 mix design contained 5% KFs and SFKF105 contained 10% KFs with 5% MK. Both composites integrated SF as a fibre modification technique for durability purposes. LCA results of SFKF105 showed savings of 11%, 8%, 4% and 1% for terrestrial acidification potential, global warming potential (GWP), terrestrial ecotoxicity potential (TEP) and human toxicity potential, respectively. SFKF5 revealed savings of 3%, 2% and 4% for GWP, TEP and marine eutrophication potential, respectively. The additional travel requirements of KFs and MK to the cement batching plant for composite production did not surpass the embodied energy and travel emissions of the control. However, this was negated due to the additional energy requirements to manufacture KFs. Optimising the economic and environmental trade-offs via the use of a nondominated sorting genetic algorithm revealed three key regions. Region 1 was the most economical but also created the most carbon emissions. Region 2 was a compromise between both economical and environmental factors and region 3 demonstrated the lowest carbon emissions but also produced the highest costs. Utilising the acquired data from the associated regions produced additional mix designs for mechanical analysis. The findings of this study provide a detailed understanding of the effect of KFs on cementitious composites when integrated with SF and MK. As a result, material engineers may use the results described herein to incorporate cardboard waste materials into composite designs. The results provide opportunities to utilise the novel composites within low stress grade concrete such as non-structural civil applications, residential slabs, and driveways. In addition, researchers may use similar concepts to integrate other novel waste materials, contributing to the circular economy and enhancing the sustainability of the building and construction sector.

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