Enhancing the performance and environmental impact of alkali-activated binder-based composites containing graphene oxide and industrial by-products
Introduction
Ordinary Portland cement (OPC), which is the most common binder of cementitious materials [1], is one of the major contributors to greenhouse gases that accounts for 7% of the carbon dioxide (CO2) emissions globally [2], [3], [4]. Each ton of OPC production can emit approximately one ton of carbon dioxide due to the high energy demand in OPC production [5]. As a result, much effort has been made in recent years to reduce CO2 emissions associated with OPC production, such as improving cement plant efficiency [3] and using supplementary cementitious materials to replace OPC [3], [6], [7], [8]. Meanwhile, over-exploitation of the natural sand (NS), which is the most commonly used fine aggregate in infrastructure, has been causing harmful environmental consequences on the ecosystem [9], [10]. Therefore, finding materials to alter NS has become imperative.
The annual global generation of fly ash (FA) is approximately 750 million tons [11]; however, its global utilization rate is only nearly 25% [12]. FA is a very fine industrial by-product and its disposal can lead to harmful impact on the environment, e.g. the contamination of soil and groundwater sources and the reduction of the quality of air [11], [13]. Furthermore, the annual global generation of ground granulated blast furnace slag (GGBS) is approximately 530 million tons [14], and its utilization rate is nearly 65% [15]. In addition, lead is an important material used in a variety of practical applications from industrial manufacturing to medical applications. Nevertheless, the process of lead production generates a large amount of harmful waste. According to Pan et al. [16], the worldwide generation of lead smelter slag (LSS) from lead smelting in 2016 was approximately 5.5 million tons, but mostly dumped in landfill. LSS is a toxic by-product due to containing toxic elements (e.g. lead, zinc). Therefore, LSS disposal in landfill leads to the pollution of soil, groundwater, plants, and animals [17], [18], [19]. As a result, it is necessary to find effective methods to recycle these waste materials. This not only reduces their negative impact on the environment but also ensures sustainable development of the manufacturing industry for important products of coal, steel, and lead for the benefit of the society. Replacement of cement and NS with FA [20], [21], GGBS [2], [22], and LSS [23], [24], as industrial by-products, in concrete production leads to the development of cleaner and more sustainable construction materials compared to conventional concrete. The use of these waste materials in the construction industry can possibly benefit human health and the environment [17], [19].
Alkali-activated binder (AAB) composites are a new type of eco-friendly material that can be produced with a low calcium material such as FA (geopolymer), a high calcium material such as GGBS, or a combination of FA and GGBS. AAB composites have exhibited many advantages over conventional cement-based composites, such as more sustainable materials and superior mechanical and durability properties, higher resistance to corrosion and freeze–thaw cycles, and higher stability under chemical exposure [25], [26], [27], [28]. Nevertheless, a major issue of FA used in AAB composites is developing low strengths under ambient curing condition. For overcoming this limitation, researchers investigated combining FA with materials that have a high calcium oxide (CaO) content, such as GGBS, to significantly improve their mechanical properties at the early stage [29], [30], [31]. Although FA/GGBS-based AAB composites can develop high mechanical strengths at early and later ages under ambient temperatures, they still exhibit brittle behavior and low tensile strength, flexural toughness, and resistance to crack propagation similar to OPC-based composites [32]. These lead to durability problems, costly maintenance, and negative impacts on the longevity of construction products. To address these limitations, researchers have explored the use of different additives to reinforce AAB composites, such as carbon fibers [33] or carbon nanotubes [26]. These additive materials have shown the ability to enhance the flexural toughness, and flexural and tensile strength of AAB composites by mitigating the propagation and opening of nano/microcracks [34]. However, carbon fibers and nanotubes have low specific surface areas, aspect ratios and limited interfacial connections in the gel matrix, resulting in weak bonding and arresting crack propagations in concrete [35].
Graphene and its derivatives, i.e. graphene oxide (GO), reduced graphene oxide (rGO), and pristine graphene (PRG), have recently shown great potential for improving microstructures of cementitious materials. This is because of their outstanding properties, such as high mechanical properties, high aspect ratios and large specific surface areas [36], [37]. GO containing abundant oxygen functional groups on its surface is highly dispersive in polar liquids [38]; therefore, its use to improve the properties of organic composites [39] or cementitious composites [40] has received significant attention. However, the exploration of using GO in AAB-based materials has been very limited and only few studies have been worked on the properties of GO-based geopolymers [32], [41]. Yan et al. [41] studied the influence of different GO dosages on mechanical properties and microstructures of geopolymer pastes cured at 60 °C in 7 days. They showed that the mixes with 0.3% and 0.5% of GO increased the flexural strength and fracture toughness of geopolymer pastes by 45.5% and 61.5%, respectively. Saafi et al. [32] reported that 0.35% GO increased the flexural strength, Young’s modulus, and flexural toughness of geopolymer pastes cured at 60 °C for 24 h by 134%, 376%, and 56%, respectively. They also revealed that the alkaline environment in geopolymers reduced oxygen-containing functional groups in GO. This resulted in the formation of rGO sheets and the creation of the cross-link between rGO sheets and geopolymer matrix, leading to improvements in the mechanical properties of geopolymer composites. In addition, few studies have shown that using LSS to replace NS as fine aggregate can create composites with better compressive strength, durability properties and gamma radiation shielding compared to the NS-based composites [42], [43]. However, there is very limited research on using LSS as NS replacement in AAB composites. Moreover, the existing studies on the use of GO in AAB composites have only dealt with geopolymer pastes cured at elevated temperatures, which is challenging in practical applications due to the difficulty in the generation of heat curing conditions in construction sites. As a result, the combination of GO and ambient-cured AAB-based composites is more practical in the construction industry. However, to date, no research has been conducted on the influence of GO addition on mechanical and durability properties of ambient-cured AAB composites prepared with NS or its alternative LSS sand.
To address the research gaps, this paper presents the first systematic study on the mechanical, durability and microstructural properties of ambient-cured AAB mortars containing NS and LSS reinforced with GO. The results of this study show the great potential of using GO additives in AAB composites containing NS and LSS to improve the mechanical and durability performance of the composites. This study not only contributes toward achieving resource sustainability and cleaner production in the construction industry by reducing the use of NS and cement, but it also presents a promising pathway for the development of eco-friendly construction materials based on industrial by-products that exhibit similar or better performance compared to conventional concrete.
Section snippets
Materials
GGBS and FA class F were provided by Adelaide Brighton Cement Ltd, and were a by-product from Birkenhead Works and Leigh Creek Coal in South Australia, respectively. The specific gravity of GGBS and FA was 2.91 and 2.54, respectively, and their chemical compositions are shown in Table 1.
NS had a maximum particle size of 2.13 mm, and it was obtained from McLaren Vale Quarry in Fleurieu Peninsula. LSS with a maximum particle size of 1.94 mm was sourced from Port Pirie in South Australia. NS and
Characterization of some key ingredients used for AAB mortars
Fig. 2(a) presents XRD pattern of NS. It is shown in the figure that the XRD pattern of NS has no amorphous phases, and sharp peaks are detected at the positions from 21.2° to 77.9°, denoting all the crystalline phases are quartz (SiO2) of NS [52], [53]. Fig. 2(b) shows the XRD pattern of LSS. As shown in the figure, LSS has mostly amorphous phases, and some peaks of the crystalline phases of Wustite (FeO) are detected at the positions of 37.02°, 42.86°, and 61.74°, which is consistent with
Conclusions
This paper has presented the first study on the mechanical and durability properties of fly ash (FA)/ground granulated blast furnace slag (GGBS)-based alkali-activated binder (AAB) mortars prepared with natural sand (NS) and lead smelter slag (LSS) sand and with graphene oxide (GO) addition. The following conclusions can be drawn based on the presented results:
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The flowability of AAB mortars containing NS and LSS decreases with increasing GGBS content due to the faster chemical reaction of GGBS
CRediT authorship contribution statement
Van Dac Ho: Formal analysis, Validation, Writing - original draft, Writing - review & editing. Aliakbar Gholampour: Formal analysis, Validation, Writing - original draft, Writing - review & editing. Dusan Losic: Formal analysis, Methodology, Supervision, Writing - review & editing. Togay Ozbakkaloglu: Conceptualization, Formal analysis, Methodology, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge Mesdames Rabbah, Hariz, Yin, and Dinh for completing the experimental work, as part of their Honour’s thesis at the University of Adelaide under the supervision of the last author. The authors thank the Schools of Civil, Environmental and Mining Engineering and Chemical Engineering at the University of Adelaide for supporting this work.
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