Ambient-cured geopolymer mortars prepared with waste-based sands: Mechanical and durability-related properties and microstructure
Introduction
Cementitious materials are by far the most commonly used construction materials on earth [1]. The production of ordinary Portland cement (OPC), a core component of cementitious materials, releases a significant amount of greenhouse gases (essentially CO2) into the atmosphere. As reported in Ref. [2], producing one ton of OPC generates approximately one ton of CO2. It was also reported that approximately 1.35 billion tons of CO2 annually produced from the production of OPC, which accounts for ∼5–7% of the CO2 emissions globally [3]. Therefore, in an effort to reduce emissions of CO2 associated with the cement and concrete industry it is crucial to identify alternative materials that can replace OPC to produce a green concrete. Meanwhile, the high demand for natural river sand as fine aggregate in the construction industry has led to its over-exploitation, which results in harmful environmental issues such as depletion of non-renewable natural sand and negative effects on the ecosystem [4,5]. Therefore, finding an alternative material to river sand has also become imperative.
The use of waste materials as binder and fine aggregate in the concrete is a viable strategy to develop an eco-friendly construction material that contributes toward cleaner production practices. Waste materials from industrial by-products, e.g. ground granulated blast furnace slag (GGBS) and fly ash (FA), can be considered as OPC replacement to reduce the negative environmental impact of OPC [6]. As was reported previously, the annual worldwide generation of GGBS and FA is approximately 530 million [7] and 750 million tons [8], respectively. However, it is believed that only 65% [9] and 25% [10] of the total generated GGBS and FA are currently being used, respectively. Furthermore, the use of waste-based sands, such as glass sand (GS) [11,12] and lead smelter slag (LSS) [13,14], is currently being considered as a solution against the depletion of natural sand (NS), which has already become a major issue in a number of rapidly growing cities across the world (e.g. Singapore; Shezhen, China). Existing studies have shown that GS and LSS, abundant waste products that are currently being landfilled, exhibit properties that make them suitable for use in structural concretes. It was reported that the global generation of waste glass was 130 million tons annually [15]. Disposal of a large amount of waste glass in landfill is costly and results in the depletion of the landfill space [16]. As reported in Ref. [17], the worldwide generation of the lead slag was approximately 3.9 million tons in 2009 and each ton generated 100–350 kg of LSS, only 15% of which was recycled. Therefore, there is significant potential for GS and LSS to be used as sand replacement in concrete for achieving resource sustainability in the construction industry while reducing the environmental impact of both abundant waste products and concrete. As the first systematic study on the topic, the current study examined the use of these materials in geopolymers.
In recent years, geopolymer, a new type of environmentally friendly material in which the OPC is replaced with alkali-activated binders, has received significant research attention. The production of geopolymers happens through the geopolymerization process by chemical reactions between materials with high aluminosilicate constituents and alkali activators [18]. Several researchers have investigated the properties of geopolymer mortars (e.g. Refs. [[19], [20], [21], [22], [23], [24], [25]]). As has been demonstrated in previous studies, geopolymer-based materials exhibit highly desirable mechanical and durability properties that are comparable to or better than those of their OPC-based counterparts [26,27]. However, most of the existing studies have dealt with geopolymers cured under high temperatures (i.e. over 40 °C) and only a few studies focused on ambient-cured geopolymers [17,[28], [29], [30], [31], [32]]. A major challenge related to the use of FA in ambient-cured geopolymers is that the resulting material develops low strengths under such curing conditions. Owing to the relatively high calcium content in GGBS, its addition to an ambient-cured FA-based geopolymer results in significant improvements in the mechanical properties and microstructure of the geopolymer [33]. To date, no study has been reported on the properties of geopolymer mortars containing GS or FA/GGBS-based geopolymer mortars containing LSS. Therefore, new experimental studies are needed to evaluate the properties of FA/GGBS-based geopolymer mortars containing LSS and GS.
The first experimental study on the properties of FA/GGBS-based geopolymer mortars containing LSS and GS is presented in this paper. A summary of the experimental program is first given. Test results and a detailed discussion on them are then presented, followed by the microstructural analysis of different mixes. The promising technology used in this study has a significant potential for contributing toward a green construction industry through i) conserving non-renewable natural resources, ii) reducing the large CO2 footprint associated with the use of OPC, and iii) eliminating the negative impact of industrial by-product disposal on the environment.
Section snippets
Ground granulated blast furnace slag (GGBS) and Fly ash (FA)
Ground granulated blast furnace slag (GGBS) and class-F fly ash (FA) with the chemical composition shown in Table 1 were provided by Adelaide Brighton Cement Ltd. GGBS was a by-product from Birkenhead Works and FA was a by-product from Leigh Creek Coal in South Australia.
Natural sand (NS)
River sand with a 2.13-mm maximum particle size, sourced from McLaren Vale Quarry in Fleurieu Peninsula, was used as the natural sand (NS). The particle size distribution and physical properties of NS are shown in Fig. 1 and
Flowability of fresh geopolymer mortar
Fig. 5 shows the results of flowability tests for different mixes. It is evident from the figure that geopolymer mortars containing NS exhibited the highest flow for a given binder type, which was followed by mortars containing LSS and GS, respectively. The higher flowability of NS mixes than GS mixes is consistent with previous studies on conventional cement mortar [40,41] and concrete [42,43]. As discussed later in Section 3.6.1, the higher flowability of mixes containing NS is attributed to
Conclusions
The results of the first study on the properties of FA/GGBS-based geopolymer mortars containing LSS and GS have been presented. The following conclusions can be drawn based on the results and discussions presented in this study:
- 1.
An increase in GGBS% results in a decrease in the flowability of geopolymer mortars owing to the fact that GGBS reacts with the activator solution much more quickly than FA. Geopolymer mortars containing NS exhibit the highest flow, followed by those containing LSS and
Acknowledgements
The authors thank Mesdames Dinh, Hariz, Rabbah, and Yin for performing the tests reported in this paper as part of their Honour's thesis.
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