Author: Yang, Shuqing
Title: Maximizing the utilization of recycled glass aggregates in concrete blocks
Advisors: Poon, Chi Sun (CEE)
Degree: Ph.D.
Year: 2021
Subject: Glass waste -- Recycling
Alkali-aggregate reactions
Concrete -- Defects
Hong Kong Polytechnic University -- Dissertations
Department: Department of Civil and Environmental Engineering
Pages: xxv, 200 pages : color illustrations
Language: English
Abstract: Utilization of crushed waste glass as aggregates in concrete production is environmentally friendly to relieve the burden on landfills and conserve depleting natural aggregates resources. The non-hydrophilic nature of the glass aggregates contributes to the increase of workability, the enhancement of resistance to chloride ion penetration, and the reduction of drying shrinkage on the concrete. However, Alkali-silica reaction (ASR) of the glass aggregate is the most challenging issue that restricts the extensive application of waste aggregates in cement-based materials. Recent investigations showed compared with the excessive ASR expansion in wet-mixed mortars, ASR expansion of the glass aggregates in dry-mix mortars was minimized. In addition, previous studies have shown the use of glass aggregates could improve the high temperature performance of the wet-mix concrete, which is thought it can be extended to the dry-mix concrete. Therefore, the principal objectives of this research are to demonstrate and understand the mitigation mechanism of ASR of glass aggregates and the extent and enhancement mechanism of the high temperature performance by the dry-mix casting method. This aim is to maximize the application of glass aggregates in partition wall block products. For the purpose of understanding the different ASR formation mechanisms in the wet-mix and the dry-mix concrete, the non-destructive X-ray computed micro-tomography (X-ray μCT) technique was first applied to compare the different 3D structure of the wet-mix and dry-mix glass concrete, to determine their porosities using 3D volumes and to investigate the in-situ progress of cracks during ASR development. The result of the 3D structure showed the irregular macro pore geometry in the dry-mix concrete associated with the dry casting method was the essential difference in pore structure between the wet-mix and the dry-mix concrete. After the ASR test, the porosity (the macro pores measured by 3D volume) of the dry-mix glass concrete decreased. However, no obvious change was observed in the porosity of the wet-mix glass concrete. Through the in-situ observation using 3D X-ray μCT images, no ASR induced cracks were found in the dry-mix glass concrete during the progressive development of ASR, whereas numerous ASR cracks were densely distributed in the wet-mix glass concrete, which led to the failure of the concrete matrix. Further investigation was carried out by higher resolution microstructural analysis (scanning electron microscopy-backscattered image analysis) of the distribution of the ASR gel in the mortars revealed different types of ASR gel were produced in the dry-mix and the wet-mix mortars. The microstructural analysis results showed the better ability of the dry-mix mortars to mitigate the ASR expansion was mainly due to the distinctive different distribution of ASR gel. Compared to the ASR gel formed in the pre-existing cracks of the glass aggregate in the wet-mix mortars, the ASR gel formed in the dry-mix mortars was mainly located along the surface of glass aggregates in the larger pores (between 10-100 μm). The swelling pressure of the ASR gel was relieved by the macro pore in the dry-mix blocks. The long-term field monitoring results also validated the findings of the laboratory study.
The high temperature performance of the dry-mix and wet-mix concretes prepared with the different amounts of glass aggregates before and after exposure to elevated temperature (cooling to elevated temperatures (cold test)) was investigated and compared. Regardless of the casting methods, the increasing amount of glass aggregates in the blocks reduced the thermal conductivity at room temperature and improved the residual compressive strength after exposure to 800°C. This was because the surface of the glass aggregates was partially melted at 800°C and they re-solidified after cooling, which served to enhance the bond between the glass aggregates and the cement paste. With a constant content of glass aggregates, the dry-mix concrete had a lower thermal conductivity at room temperature and a higher residual compressive strength after high temperature exposure than the wet-mix concrete. The smaller particle size of the glass aggregates further improved the post-high temperature exposure compressive strength of the dry-mix concrete. This was due to the larger softened surface area of the smaller glass aggregates as revealed by X-ray μCT observations. However, the glass would become too soft after reaching the softening point to resist loading during a fire. Therefore, the performance of the glass concrete subjecting to a constant dead load during heating (hot test) was further investigated. Unlike the improvement conditions and mechanism found in the cold test, the optimal performance of the mortars prepared with 15% glass aggregates to replace river sand in the hot test was attributed to the penetration of the softened glass into the empty pores and surrounding cracks during loading, filling surrounding pores and cracks. However, further increasing the glass aggregate content to above 15% led to a sharp reduction of compressive strength and the softening of the test sample. This was because the semi-melted glass behaved like "soft" aggregates which were unable to sustain stress (loading) under the hot test. Overall, the glass aggregates were effective in improving the high temperature performance of the concrete products, but the percentage of the glass aggregates should be controlled when the concrete is subjected to a dead load during the elevated temperature exposure.
Rights: All rights reserved
Access: open access

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