Recently, there is significant increase in globalisation in industrial areas especially in developed countries. Thus, their waste by-products become a major challenge because of the large quantities involved and the scarcity of disposal space. Industrial waste by-products and the use of traditional cementitious binders (e.g. lime, cement, and gypsum) in geotechnical applications are facing many challenges due to high greenhouse gas emissions, intensive use of energy and depletion of natural resources. In addition, most of the traditional binders are not readily acceptable due to stringent occupational health and safety issues as well as threats to the soil and groundwater environment. To address these challenges, new binders using industrial waste-by products are being investigated. Among these, alkaline activation is one of the most interesting methods being developed; it has equal or better performance than cement-based binders, but with lower environmental issues and costs. This paper reviews the key developments in alkali-activated materials since 2011, with a particular focus on advances in characterisation techniques, structural understanding, binder precursors, activation approaches, design and processing, and reaction mechanism. Finally, the paper proposes further research and development topics and suggests steps forward to enhance the potential application of these materials for ground improvement.
Loss of soil stiffness and strength due to liquefaction and consequent large ground deformation has caused extensive structural damage and economic losses in urban areas during major earthquakes. Therefore, wide ranges of remedial measures have been developed for treating or improving soils against liquefaction. These include densification, solidification, pore pressure relief, lowering ground water table, and restraint of shear deformation. Moreover, cementitious materials like cement have been widely utilised in different mitigation techniques such as compacting grouting, deep soil mixing, and jet grouting (Srbulov, 2009). These chemical admixtures are often used as an additive to improve the strength and stiffness of soils (Tatsuoka et al., 1995). However, the improvement in shear strength using traditional additives is often associated with decreased plasticity and enhanced brittleness attributed to pozzolanic hardening. Cement industry not only accounts for around 5-7% of global carbon dioxide emissions and environmental footprint such as global warming and soil ecotoxicity, but also has setting time challenge. One potential replacement for cementitious materials is alkali-activated materials (AAMs) which seem to yield similar mechanical properties like cementitious materials (Provis, 2014). Alkali activation is the generic term which is applied to the reaction of a solid aluminosilicate (termed the ‘precursor’) under alkaline conditions (induced by the ‘alkali activator’), to produce a hardened binder, which is based on a combination of hydrous alkali-aluminosilicate and/or alkali-alkali earth-aluminosilicate phases (Provis et al., 2015). Additional terminology often used to refer to these materials includes ‘geopolymer’ nomenclature that is used largely to describe low-calcium alkali-activated aluminosilicate binders. The defining characteristic of a geopolymer is that the binding phase comprises an alkali aluminosilicate gel, with aluminium and silicon linked in a three-dimensional tetrahedral gel framework that is relatively resistant to dissolution in water (Palomo et al., 2014). According to a recent definition by Provis (2014), alkali activated materials are produced through the reaction of an aluminosilicate, normally supplied in powder form as an industrial by-product or other inexpensive materials, with an alkaline activator which is usually a concentrated aqueous solution of alkali hydroxide, silicate, carbonate or sulphate. In recent years, geopolymer has attracted considerable attention among these binders because of its compressive strength, low permeability, good chemical resistance and excellent fire resistance behaviour (Provis, 2017). Because of these advantageous properties, geopolymer is a promising alternative to cementitious materials in addressing various geotechnical problems and waste immobilization solutions for the industries. The aim of the current paper is to review and highlight the key recent scientific enhancements in the development, characterisation, processing and environmental assessment of AAMs.
2. ALKALINE ACTIVATION AND GEOTECHNICAL APPLICATIONS
It is noticeable that using alkaline activation in geotechnical applications is still at the early stage of development. However, Table 1 summarises the existing literature about utilising alkali-activated binder for the purpose of ground improvement. In very limited attempts, some geotechnical researchers investigated the effectiveness of precursors, including fly ash (FA), metakaolin, blast furnace slag (GGBS), palm oil fuel ash (POFA) and red gypsum (RG), with soft soil in the presence of a predesigned concentrated aqueous alkaline hydroxide or silicate solution. In this respect, Cristelo et al. (2013) address the effectiveness of alkali-activated low-calcium and high-calcium FA as silica and alumina amorphous sources. Based mostly on the microstructural analysis, these authors demonstrated that a binding gel (either N–A–S–H and/or C–A–S–H) is developed inside the soil voids, helping to form more compact microstructures and, as a consequence, improved compressive strength. Moreover, the authors reported that the short-term strength gain of the stabilised soil is faster when high-calcium FA was used as a precursor. Sargent et al. (2013) carried out a research work to study the feasibility of using some alkali-activated by-products, such as GGBS, FA and RG, on some geotechnical properties of soft soil. Based on the test results, they concluded that alkali-activated GGBS, GGBS–FA and GGBS–RG exhibited significant strength regarding the untreated soil. Pourakbar et al. (2015) reported that the addition of highly alkaline solutes, including NaOH and KOH, increased the strength of the treated soil samples. According to this study, curing time and the water content of soil were also shown to have a significant strengthening effect on the treated soil.
Table 1: Summary of alkali-activated binders for ground improvement
3 ADVANCES IN CHARACTERISATION
3.1 Nanostructural Characterisation
The nanostructure of AAMs is strongly dependent on the available calcium content of precursors; a high-calcium system such as alkali-activated blast furnace slag is dominated by a calcium aluminosilicate hydrate (C–A–S–H) gel with a to bermorite-like structure (Provis et al., 2014), while low-calcium systems such as those based on meta-kaolin or fly ash tend to generate an alkali aluminosilicate (N–A–S–H) gel with a highly cross-linked, disordered pseudo-zeolitic structure (Richardson et al., 1994). These gels can coexist in binders based on blends of high-calcium and low-calcium precursors (Davidovits, 2005; Provis, 2014). Fourier transform infrared (FTIR) spectroscopy is a key technique for the analysis of AAMs, particularly for low-calcium systems where it can probe the connectivity within Si–O– (Si, Al) frameworks via shifts in the peak corresponding to the asymmetric stretch of that bond (Provis, 2014). The gel theory of an initial Al-rich binder gel (“Gel 1”) forming at the first hours, and then evolving to a more Si-rich structure (“Gel 2”), which was developed from ex-situ analysis of the gel evolution (Provis et al., 2015) has been refined. Note that FTIR spectroscopy (see Figure 1) detects bond vibrations rather than the actual nuclei. The ‘Gel 1’ stage involves a high degree of formation of Si–O–Al bonds relative to the bulk Si/Al ratio, because the formation of cross-links involving Al atoms joining between Si sites is rapid. This leads to a gel, which has a relatively high concentration of Si–O–Al bonds (Richardson et al., 1994).
Figure 1: FTIR spectra for fly ash and alkali-activated fly ash pastes (adapted from Richardson et al. 1994)
3.2 Microstructural Characterisation
Recent developments in the understanding of microstructural and the C–A–S–H gel in alkali-activated binders have been focused on the construction of a realistic structural description of the silicate chain structures in the AAM gel, which can differ significantly from those formed in the C–S–H produced by Portland cement hydration (Provis, 2014). As can be seen from the Figure 2, this is mainly attributed to the low Ca/Si ratio and high Al content of the gel produced by alkali activation of binder, which opens the possibility of cross-linking between the dreierketten chains of the tobermorite-like gel (Pelisser et al., 2013). The most widely used tool for microstructural analysis of AAMs is scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). The C-S-H was found to be intermixed on an intimate scale with an Mg/Al-rich phase, presumably hydrotalcite. Another key tool which is gaining popularity and interest is tomography. This can be applied on both nanometre (Provis, 2014) and micrometre (Provis et al., 2015) length scales using X-rays.
Figure 2: Schematic of structural features, silicate and aluminate, cross-linking (adapted from Provis et al. 2014).
4 ADVANCES IN BINDER DEVELOPMENT
Early studies and applications of AAMs mainly used precursors, such as blast furnace slag, and fly ash. The chemical and physical characteristics of these precursors are well described in the literature (e.g. Provis, 2014; 2017).
4.1 Waste Glass
The recycling of waste glasses from consumer utilisation and industrial processes poses a major problem for municipalities worldwide. Puertas and Torres-Carrasco (2014) investigated the properties and microstructure of alkali-activated glass fibre waste using NaOH and KOH solution as activators. The mortar samples showed compressive strengths of up to 77MPa after three days when NaOH solution was used as activator (Figure 3). Puertas and Torres-Carrasco (2014) used metakaolin (MK) to replace a part of the glass powder in order to introduce Al and also to stabilise alkali ions in the system. Compressive strength of the mortars increased with additional aluminium sources is common in the alkali activation of waste glasses, as commercial glass systems contain sufficient Al to produce a stable AAM. Bajare et al. (2014) investigated the use of a combination of aluminium recycling, calcined kaolin clay, and lead–silica glass (LSG) from recycled fluorescent lamps, to prepare foamed alkali activated binders. The residual aluminium metal in the dross generates hydrogen when reacting with NaOH activator, with a total porosity of 82–83%. An interesting point of novelty in this work was the use of a waste material as a new binder in alkaline activation methods (Pascual et al., 2014).
Figure 3: Reaction mechanism in an alkaline-activated recycled glass (adapted from Palomo et al. 2014).
5 COMPARISON WITH CEMENTITIOUS MATERIALS
5.1 Production Process
There does not appear to be a strong need to develop new process facilities in the short term to enable the production of alkali-activated materials. Mixing and using alkali-activated materials can generally be achieved using the same technology as used in Portland cement products, potentially with slight modifications to optimise the input of mixing energy and the dosing of the activator (Provis, 2014). The capital costs of a plant producing alkali-activated materials will therefore be significantly lower than that of a plant producing Portland cement. An increase in the scale of sodium hydroxide production to support million-tonne-scale production of alkali-activated binders will require more capital-intensive facilities, but the activator only comprises 5–10% of the total mass of the binder (Provis, 2017).
5.2 Material Processing and Application
Mix design of alkali-activated materials can broadly follow similar heuristics to those used for Portland cement products, particularly in terms of applications, but the binder must be designed and optimized on a case-by-case basis: the precursors available at each location will differ in chemistry, mineralogy and fineness, and will be combined with activators which are selected depending on both technical and commercial parameters (Jamieson et al., 2015). There is not yet a universal mix design procedure which can be applied to alkali-activated binders, due to the differences in chemistry, mineralogy and particle characteristics between different precursor sources, so optimisation of mixes needs to be carried out for each new precursor that is sourced (Provis, 2017). Accurate quality monitoring and control of the characteristics of precursors, which are sourced as wastes or by-products from other industries, is also essential to the successful production of alkali-activated products. Alkali-activated slag cements tend to harden rather rapidly, and in some cases, retarders are used to regulate the setting rates (Richardson et al., 1994). Probably the main challenge related to processing and application of alkali-activated materials in construction at present is the difficulty associated with developing or sourcing effective admixtures for rheology control in these systems (Provis, 2014).
6 SIMPLIFIED ENVIRONMENTAL ASSESSMENT
In any given production situation, factors such as the mix design, source and dose of the activator, transport requirements for aggregates and precursors, and energy mix used in production of all components (electricity, nuclear), must all be specified for each particular mix design, location and application. When comparing with Portland cement-based products, a comparable baseline for a particular location and application must also be specified (Habert & Ouellet-Plamondon, 2016). For these reasons, it is clearly impossible to provide a single value to globally describe the environmental savings which may be achieved through the use of alkali-activated materials in place of conventional cement and concrete (Provis, 2017; Provis et al., 2005). A recent detailed discussion of life-cycle analysis (LCA) of alkali-activated binder systems has been provided by Habert & Ouellet-Plamondon (2016). The importance of reliable inventory data, which has not always been the case, particularly for activator components (Pelisser et al., 2013), for instance, mortar based on blast furnace slag, activated by Na2CO3 and containing a high volume of granular limestone, CO2 and energy savings as high as 97% have been calculated (Provis, 2014). Habert and Ouellet-Plamondon (2016) used LCA methodology to analyse the environmental impacts of AAM concretes made with fly ash, blast furnace slag and metakaolin using published results. A calculation based on an average of 49 published mix designs indicated that fly ash-based AAM mixtures released 45% less CO2 than an average Portland cement concrete (McLellan et al., 2011). Figure 4 exemplifies the importance of the activator in the emission calculations for several fly ash-based AAM formulations.
Figure 4: Calculated CO2-equivalent emissions of four different fly ash-based AAMs paste mix designs using typical Australian data (adapted from McLellan et al. 2011).
One of the main advantages of alkali-activated methods can constitute interesting materials to fully eliminate traditional cementitious binder (i.e. cement and lime) usage in geotechnical projects. That is, alkaline activation binder is generally a synthetic alkali aluminosilicate material that is produced from the reaction of a solid aluminosilicate with a pre-designed concentrated aqueous alkaline hydroxide or silicate solution. There are many factors that influence the process of alkaline activation. Moreover, production of cementitious materials not only accounts for around 5-7% of global carbon dioxide emission and contribute to environmental footprint such as global warming and soil ecotoxicity, but also has setting time challenge. Another advantage of alkaline activation is the development of new binders using industrial waste by-products (geopolymer binders) with equal or better performance than cement-based binders for geotechnical applications, but with lower environmental issues and costs. Various source materials including natural clay, aluminosilicate minerals and agro-industrial by-products can be utilised through alkaline activation technique. Also, the recycling of industrial by-products, such as waste glasses from consumer utilisation and industrial processes, poses a major problem for New Zealand. In addition, since the strength of AAMs demands low energy consumption, environmentally-friendly nature of the process, and excellent engineering properties, alkali-activated binders are fast emerging as materials of choice for highly-demanding geotechnical engineering applications. Moreover, another advantage of AAMs is that the cost of production of alkali-activated binders is, in general, a closely-held trade secret and is fundamentally dependent on the degree of control of the materials supply chain which is held by the material. Industrial waste by-products, such as fly ash and slag, may be purchased at a cheaper price compared to that of Portland cement.
Despite the enhancement of AAMs using industrial by-products for geotechnical applications, there has been a concern for their adoption because of potential environmental impact. Hence, one of the major weaknesses of AAMs and activators is that they are not readily acceptable due to stringent occupational health and safety issues as well as threats to the soil environment, such as increased alkalinity (pH 11-12). Also, alkaline activation methods have difficulty in terms of field handling. There is limited study on activation of industrial wastes for geotechnical applications, such as soil stabilisation or ground improvement. Apart from extensive developments of using alkali-activated binder in civil engineering framework, using this technology in geotechnical applications is still at the early stage of development and, hence, need comprehensive research works in order to become technically and economically viable.
Based on the discussions, it is concluded that alkaline activation method has considerable potential to be used as a new method in geotechnical applications. The development and optimisation of alkali-activated binder formulations from an increasingly diverse range of waste-derived precursors has become the focus of efforts of many research teams worldwide, often with a focus on locally available or problematic materials. In New Zealand, the amount of glass available and utilisation of domestic recycled glass is a main challenge. So far, alkaline activation is the most promising method to develop new binders using recycled glass as compared to cement, with its lower environmental issues and costs. However, there are still a number of areas which require attention, from both scientific and technological perspectives, particularly the control of setting time and rheology. Hence, appropriate and meaningful testing and description of both advanced processing methodologies and environmental sustainability is essential in ensuring the future role of alkali activated binder systems in the geotechnical applications. Some key aspects that require detailed research attention include:
- experimental investigation of the effectiveness of the AAMs with different alkaline solution molarity on geotechnical parameters of soils;
- detailed definition of the links between the physico-chemical properties of precursors, and selected activators;
- investigation of appropriate method(s) for the characterisation of AAMs for ground improvement techniques;
- seeking of new information through the use of analytical tools such as Raman spectroscopy, confocal microscopy, and X-ray photoelectron spectroscopy, which have been under-utilised, or not used at all, in the study of AAMs;
- assessment of cyclic behaviour of the AAM-treated soils under seismic loading;
- study of the microstructural and mineralogical phases of soils before and after treatment with AAMs; and
- derivation of constitutive model for incorporation in user-friendly numerical (FEM) code.
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