Alkali Activated Materials

This report has been prepared by the RILEM Technical Committee on Alkali Activated Materials (TC 224-AAM). The objectives of this Technical Committee are threefold: to analyse the state of the art in alkali activation technology, to develop recommendations for national Standards bodies based on the current state of understanding of alkali-activated materials, and to develop appropriate testing methods to be incorporated into the recommended Standards. TC 224-AAM was formed in 2007, and was the first international Technical Committee in the area of alkaline activation. The focus of the TC has been specifically in applications related to construction (concretes, mortars, grouts and related materials), and thus does not encompass the secondary field of application of alkali-activated binders as low-cost ceramic-type materials for high-temperature applications at a comparable level of detail.

Cement and concrete are critical to the world economic system; the construction sector as a whole contributed US$3.3 trillion to the global economy in 2008 [1]. The fraction of this figure which is directly attributable to materials costs varies markedly from country to country – particularly between developing and developed countries. Worldwide production of cement in 2008 was around 2.9 billion tonnes [2], making it one of the highest-volume commodities produced worldwide. Concrete is thus the second-most used commodity in the world, behind only water [3]. It is noted that there are certainly applications for cement-like binders beyond concrete production, including tiling grouts, adhesives, sealants, waste immobilisation matrices, ceramics, and other related areas; these will be discussed in more detail in Chaps. 12 and 13, while the main focus of this chapter will be large-scale concrete production.

As mentioned in Chap. 2, the development and assessment of alkali-activated binders based on calcium-rich precursors such as blast furnace slag (BFS) and other Ca-rich industrial by-products have been conducted for over a century [1–3]. However, an increase in interest in the understanding of the microstructure of alkali-activated binders has taken place in the past decades. This has been driven by the need for scientific methods to optimise the activation conditions which give a strong, stable binder from a particular raw material, and consequently a high-performance alkali-activated material (AAM) concrete, while achieving acceptable workability and a low environmental footprint. A detailed scientific understanding of the structure of these materials is required to generate the technical underpinnings for standards which will facilitate their wider commercial adoption [4, 5].

Early developments in the developments of low-calcium (including calcium-free) alkali-activated binders were led by the work of Davidovits in France, as noted in Chap. 2. These materials were initially envisaged as a fire-resistant replacement for organic polymeric materials, with identification of potential applications as a possible binder for concrete production following relatively soon afterwards [1]. However, developments in the area of concrete production soon led back to more calcium-rich systems, including the hybrid Pyrament binders, leaving work based on the use of low-calcium systems predominantly aimed at high-temperature applications and other scenarios where the ceramic-like nature of clay-derived alkali-activated pastes was beneficial. Early work in this area was conducted with an almost solely commercial focus, meaning that little scientific information was made available with the exception of a conference proceedings volume [2], several scattered publications in other conferences, and an initial journal publication [3]. Academic research into the alkaline activation of metakaolin to form a binder material led to initial publications in the early 1990s [4, 5], and the first description of the formation of a strong and durable binder by alkaline activation of fly ash was published by Wastiels et al. [6–8]. With ongoing developments in fly ash activation, which offers more favourable rheology than is observed in clay-based binders, interest in low-calcium AAM concrete production was reignited, and work since that time in industry and academia has led to the development of a number of different approaches to this problem. A review of the binder chemistry of low-calcium AAM binder systems published in 2007 [9] has since received more than 350 citations in the scientific literature, indicating the high current level of interest in understanding and utilisation of these types of gels.

Following the discussion in the two preceding chapters, which addressed high-calcium and low-calcium alkali-activated binder systems respectively, this chapter will provide a brief discussion of the progress which has been made in the development and characterisation of hybrid binders derived from intermediate-Ca precursors and mixtures of precursors. The need for durable, high-performance, low-CO₂ alternative binder systems, along with the good existing understanding of the chemical mechanisms of mechanical strength development and durability of high-calcium and low-calcium alkali-activated materials (AAMs) as outlined in Chaps. 3 and 4, has given motivation for an increasing focus on hybrid systems over the past years. These binders are expected to provide a good synergy between mechanical strength and durability, making use of the stable coexistence of the hydration-reaction products characteristic of hydration of Portland clinker or alkali-activated BFS (mainly C-S-H gels) and alkali-activated aluminosilicates (geopolymeric gel) [1–3]. Blending of aluminosilicate-rich materials with more reactive calcium sources (including Portland cement clinker) and with the use of a source of alkalis also opens the possibility for the use of aluminosilicate wastes or by-products which may be insufficiently reactive to provide good strength development when activated alone, providing a pathway to valorisation for these materials.

To commence the discussion of admixtures in alkali-activated binders, it is necessary to first give a brief discussion on the definition of the word ‘admixture’. Many of the components which are essential to the formulation of alkali-activated binders are sometimes described as admixtures (mineral or chemical) in Portland cement systems. In the context of alkali activation chemistry, neither the alkaline activator nor the solid (alumino)silicates should be considered to be an admixture; these are binder components. Similarly, the addition of clinker compounds, or related materials such as cement kiln dust, is beyond the scope of this review. The discussion to follow will predominantly address organic admixtures, although a brief discussion of inorganic components used as accelerators or retarders will also be presented.

RILEM TC AAM was initiated in 2007 to bring together leading alkali activation practitioners from academia, government laboratories and industry in an international forum, to develop recommendations for the future drafting of standards that are specifically applicable to alkali activated materials.

This chapter, and the two that follow, are structured to provide an overview of the available test methods for assessment of the performance of construction materials under a wide variety of modes of attack. These are divided, broadly, into ‘chemical’ (Chap. 8), ‘transport’ (Chap. 9) and ‘physical’ (Chap. 10) – and it is noted that this classification is to some extent arbitrary, with a significant degree of crossover between the three categories which is difficult to take explicitly into consideration in a format such as this. Some areas are discussed in far more detail than others, either because they are critical points related to certain areas of alkali-activation technology, or sometimes simply because limited information is available regarding some forms of attack on alkali-activated materials (AAMs); biologically-induced corrosion is one such case, where very little information is available in the open literature. These chapters will in general raise questions for future consideration rather than providing detailed answers, due to the limited state of understanding of AAM degradation mechanisms at present, although recommendations will be drawn wherever possible.

In most applications of reinforced concrete, the predominant modes of structural failure of the material are actually related more to degradation of the embedded steel reinforcing rather than of the binder itself. Thus, a key role played by any structural concrete is the provision of sufficient cover depth, and alkalinity, to hold the steel in a passive state for an extended period of time. The loss of passivation usually takes place due to the ingress of aggressive species such as chloride, and/or the loss of alkalinity by processes such as carbonation. This means that the mass transport properties of the hardened binder material are essential in determining the durability of concrete, and thus the analysis and testing of the transport-related properties of alkali-activated materials will be the focus of this chapter. Sections dedicated to steel corrosion chemistry within alkali-activated binders, and to efflorescence (which is a phenomenon observed in the case of excessive alkali mobility), are also incorporated into the discussion due to their close connections to transport properties.

Concrete is well known to be strong in compression but weak in flexion and tension. However, by the use of steel reinforcing, often in combination with techniques such as pretensioning, and through appropriate structural engineering design methodologies, it is possible to compensate for this weakness by ensuring that the binder and aggregate of the concrete are subjected to minimal tensile load. This means that the relationship between compressive strength, flexural strength and other mechanical properties of concrete is used as an essential basis for civil and structural engineering design purposes. In practice, and with the current almost-universal use of Portland cement-based concretes in civil infrastructure applications, many of these relationships are codified in standards as empirical power-law relationships involving the 28-day compressive strength of the material, sometimes as the sole property used in the predictive equations or sometimes along with a small number of additional physical parameters. For example, the American Concrete Institute [1] specifies the prediction of elastic modulus as a function of compressive strength and concrete density, but an equation solely based on compressive strength is also provided, and is probably more widely used in practice. More sophisticated and more detailed theoretical models, or empirical correlations involving larger numbers of parameters, are often published in the academic literature, but are not in widespread application. An excellent discussion of phenomena and models for Portland cement concrete is presented by Neville [2], and the reader is referred to that text for further information.

In the context of a Report such as this, it is of immense value to be able to provide tangible examples of structures and applications in which alkali-activated concretes have been utilised throughout the past decades. A detailed outline of the utilisation of AAM concretes in the former Soviet Union and in China is given in Chap. 12 of the book by Shi, Krivenko and Roy [1], and this chapter will briefly describe some of the applications mentioned in that (more extensive) document, along with applications elsewhere in the world where AAMs have been utilised on a significant scale in the construction of buildings and other civil infrastructure components. An overview of developments and applications in the former USSR has also been presented by Brodko [2] and by Krivenko [3]. Each project reported in this chapter involves at least pilot-scale, and in some cases full commercial-scale, production of alkali-activated concretes utilising largely standard concrete mixing and placement equipment and labour, indicating that these materials are both accessible and useful on this length scale, given sufficient expertise in mix design based on locally available precursors. In the former USSR in particular, slags obtained from local iron production facilities were used in each of the different locations in which the concretes were produced, and activators were sourced in large part from locally available alkaline industrial waste streams.

The focus of this chapter is the discussion of a variety of niche applications (other than as a large-scale civil infrastructure material) in which alkali-activated binders and concretes have shown potential for commercial-scale development. The majority of these applications have not yet seen large-scale AAM utilisation, except as noted in the various sections of the chapter. However, there have been at least pilot-scale or demonstration projects in each of the areas listed, and each provides scope for future development and potentially profitable advances in science and technology. In addition to the applications specifically discussed in this chapter, there are also commercial and academic developments in alkali-activation for specific applications including a commercial product which is being marketed as a domestic tiling grout showing some self-cleaning properties [1], as well as alkali-activated metakaolin binders as a vehicle for controlled-release drug delivery [2, 3]. Although undoubtedly promising and of commercial interest, these are rather specialised applications, and so the focus of this chapter is instead on broader categories of research and development rather than in providing detailed analysis of specific products. The areas to be discussed will include lightweight materials, well cements, fire-resistant materials, and fibre-reinforced composites.

The key outcome of RILEM TC 224-AAM has been the development of a conceptual framework from which the discussion of standardisation of alkali-activated binders and concretes can proceed. There has been agreement from the members of the TC that a performance-based approach to both materials formulation (as described in Chap. 7 of this report) and testing (as outlined in Chaps. 8,9, and 10) is essential in enabling the scale-up of alkali-activation as a method of concrete production in the global context. However, it is essential to proceed with a degree of conservatism, to avoid becoming ‘the next high-alumina cement problem’ through use of a material in environments and/or systems where it is not fit for purpose. So, it is critical that standards development is conservative to ensure that due care is taken, and to make sure that poor-quality AAM products, and/or products used in unsuitable applications, do not ruin the global reputation of the technology. A key discussion which occupied much of the time of the TC was the issue of how to achieve this – and the conclusion reached was that the use of strict performance criteria (and maybe even criteria which seem excessively strict until a higher degree of certainty regarding performance levels can be reached), and with good scientific foundations, must underpin any testing method applied to these materials.