Strength and Durability Properties of Self-Compacting Concrete Made with Recycled Glass and Metakaolin

Abstract

The sustainable approach for infrastructure development is governed by long lasting durable structures prepared used minimal natural resources. Sustainable construction requires effective management and conservation of limited natural reserves. One such approach is to use discarded waste material to produce concrete structures, which can conserve scarce natural resources such as aggregates. These materials can be used to replace any of the fundamental constituent of the concrete. This study examines the application of metakaolin (MK) and crushed recycled glass (CRG) for development a durable self-compacting concrete (SCC). Utilization of these two materials for production of SCC is a green approach of construction practices as it tackles the problems like depletion of natural stone reserves and global carbon dioxide emission. The influence of metakaolin (MK) and crushed recycled glass (CRG) on the fresh, mechanical, microstructural, durability and microstructural properties of SCC was investigated. A total of 24 SCC mixtures were prepared with varying proportions of MK and CRG, where cement was partially replaced with 0%, 4%, 8%, and 12% MK by weight, and fine aggregate was substituted with 0%, 10%, 20%, 30%, 40%, and 50% CRG by volume. Test results indicated that incorporating CRG improved the flowability of the fresh SCC mixes at all replacement levels. As the CRG content was increased in SCC, an increase in slump flow diameter and a decrease in T50 flow time was observed. However, the inclusion of MK increased the superplasticizer demand to maintain desirable flowability properties. The L-box and U-box tests showed that all SCC mixes exhibited good passing ability. The V-funnel flow times decreased with increase in CRG content; but increased as MK content was increased. Sieve segregation resistance test confirmed that the segregation ratio of all the mixtures was within the prescribed limit of 15% as per EFNARC standards. In terms of mechanical properties, a general decline in compressive strength, split tensile strength and flexural strength was observed with increasing CRG content, which was attributed to the weak interfacial bond between the smooth glass particles and the cement matrix. However, the incorporation of MK improved these strength properties due to the pozzolanic reaction of MK, which generated additional C-S-H gel, thereby refining the pore structure and enhancing the overall strength. Similarly, the modulus of elasticity decreased with increase in CRG content but was improved with the addition of MK. For investigating durability performance of SCC, water absorption, sorptivity, chloride ion permeability, drying shrinkage, water penetrability under pressure, and alkali-silica reaction (ASR) tests were conducted. Both CRG and MK contributed to reduce water absorption with MK showing a more pronounced effect due to its pozzolanic reactivity. In case of sorptivity testing, although no clear trend was observed by varying the CRG content, yet sorption rates of SCC (at a given MK substitution level) containing 50% CRG were found to be lower than corresponding 0% CRG mixtures. Besides, SCC mixtures with higher MK content demonstrated significantly lower water flow rates. The inclusion of MK and CRG effectively reduced the chloride ion permeability but the effect of MK was significantly better than CRG. The addition of MK also mitigated drying shrinkage and water penetrability; though, no particular trend was observed due to increase in CRG concentration in case of these two properties. In case of SCCs made without MK, ASR expansion increased as the CRG content was raised in mixtures, with mixtures containing 40% and 50% CRG exceeding the prescribed limit of 0.1% (limit is prescribed in ASTM C1260). However, the incorporation of MK significantly reduced ASR expansion, indicating its effectiveness in mitigating deleterious effects of ASR. Microstructural analyses, including X-ray diffraction (XRD), thermogravimetric (TG) analysis, and scanning electron microscopy (SEM), provided insights into the mechanisms behind the observed improvements in SCC properties. XRD results confirmed a reduction in portlandite content in SCC mixtures containing MK, which was attributed to its consumption during the pozzolanic reaction, leading to the formation of additional C-S-H gel. TG analysis further supported these findings, revealing the formation of secondary hydration products that densified the pore structure and refined the interfacial micro-cracks between CRG particles and the cement paste. SEM images highlighted the smooth texture of CRG particles, which contributed to the weaker interfacial bond and reduced strength in SCC with higher CRG content. However, the addition of MK improved the microstructural properties by refining the CRG-cement interface and reducing micro-crack width, thereby enhancing the overall durability and mechanical performance. It was evident from the results that a sustainable and robust green SCC can be developed by utilization of CRG as a sand substitute up to 50% and by incorporating MK up to 12% of the total binder content. This research underscores the potential of using waste materials like CRG in conjunction with SCM like MK to produce environmental friendly concrete with enhanced performance, thereby contributing to the preservation of natural resources and the reduction of environmental impact associated with concrete production.