1. Introduction and context
Enormous emphasis is currently being paid to the decarbonization of the global built environment as a leading priority for the engineering community and related industrial sectors [
1]. One of the main contributors to the overall emissions footprint of the built environment—and thus a cornerstone of efforts to achieve decarbonization—is the emissions profile of construction materials during their production and utilization. The cement and concrete sector is the largest-volume contributor to the emissions incurred in meeting the world’s construction material needs and is therefore targeted in the discussion of the deep, rapid decarbonization that must be achieved in order to minimize irreversible damage to the Earth and its ecosystems.
This brief article will not explore all aspects of cement and concrete decarbonization in detail; that topic has already been covered in detail in important recent reviews [
2], [
3] and in numerous roadmaps, including global [
4], [
5] and many other assessments with national or regional scopes. However, it is important to note that there is significant bias—not always on technical grounds—in any presentation of the different technologies available to achieve decarbonization, as well as in the selection of which technologies should be favored or how efforts can be best invested at a cross-sectoral level to drive transformation. Geographical, economical, and societal considerations are often overlooked when discussing the challenges or opportunities in adopting such strategies and technologies, which realistically are some of the main drivers for the widespread uptake of technical solutions.
The very high sectoral emissions footprint of cement and concrete construction is closely linked to the extremely high quantities of these materials that are used: Global concrete consumption is calculated to be 14 billion tonnes per annum, from 4.2 billion tonnes of cement [
4]. Although the per-unit-mass (or volume) emissions footprint of concrete is much lower than that of many other engineering materials, even when scaled to account for mechanical performance [
6], the enormous production volumes mean that the overall contribution to global emissions and other environmental impacts is unacceptably high. However, this situation provides important impetus for research and development in improving the environmental profile of cement and concrete, as even a modest percentage improvement in emissions per tonne of material—if implemented broadly—can lead to very valuable reductions in the overall global emission levels of atmospheric pollutants [
7].
This possibility leads to the core purpose of this short article, which is to present and discuss one widely implementable pathway by which the emissions attributed to the global cement and concrete sector can be reduced: alkali activation as a route to lower-carbon cement production and as a technology enabling the valorization and safe management of wastes and byproducts from different industries. This is not by any means to say that alkali activation is the only possible pathway—quite the opposite, in fact. The deep and rapid decarbonization of construction materials will require a “toolkit”-type approach, with locally applicable solutions and a broad and diverse set of materials being brought to bear on this very complex set of problems [
8]. Alkali activation is not being presented here as a panacea or universal solution, but rather as a contribution toward the development of the toolkit of many innovations in cement production [
9], concrete technology [
10], and structural design [
11] that can be adopted to achieve a decarbonized construction materials sector [
2].
2. Alkali activation as a contributor to cement and concrete decarbonization
As shown in
Fig. 1, the fundamental principle of alkali activation is that the conversion of a moderately reactive aluminosilicate powder to a solid cementitious binder can be accelerated and mediated (hence, “activated”) by combining the powder with an aqueous (or rapidly soluble) source of alkalis [
12]. The aluminosilicate dissolution is enhanced by the high pH conditions that are generated, and a chemical reaction process that is often described as “dissolution-reorientation-precipitation” leads to the formation of a strength-giving alkali-aluminosilicate binding phase [
13]. This can also be achieved by blending the aluminosilicate powders with a solid alkaline source and just adding water, in what is also referred to as “one-part” alkali activation [
14].
In many ways, alkali activation can be viewed as the ultimate extension of the concept of a cement blended with supplementary cementitious materials, since largely the same aluminosilicate powders are used in alkali-activated cements as in blends with Portland cement: metallurgical slags, coal ashes, and an ever-increasing array of natural and artificial pozzolans [
15], [
16], [
17]. However, it may be more productive to view the problem in an inverted way. The development of blended cements for a reduced emissions footprint usually tends to focus on the question, “How much of this material can I blend into Portland cement and still achieve the necessary performance?” But the development of an alkali-activated binder breaks away from the assumption that Portland cement is a necessary part of the binder. Instead, the question should be framed as, “What is the most efficient way to convert this powder into a cement that will meet the requirements?”—which opens the way to selecting between a much more diverse set of blending options among different silicate (and other) powders, sources of alkalis (activators), and other admixtures. In short, there are many degrees of freedom in material design for efficiency and high performance that can be unlocked by moving away from the assumption that the word “cement” implicitly means “Portland cement.” Of course, it is very possible that Portland cement will actually be a useful component of an alkali-activated binder mixture, yielding what is often called a “hybrid” alkaline binder [
18], [
19], particularly in cases where the available aluminosilicate powder is not sufficiently reactive to provide enough early strength in its own right.
Alkali- activated binders can be—and have been—used to produce concretes at industrial scale in numerous countries for the construction of infrastructure, residential and commercial buildings, and concrete elements such as blocks, pavers, and pipes. A recent overview of the real-scale application of these materials as commercialized alkali-activated materials has been provided by Rossi et al. [
20]. Because of the diversity of materials and formulations that exist, as indicated above, the standardization of alkali-activated concretes must follow a performance-based approach, describing materials according to what they can do rather than what they are made from [
21]. Such an approach has already been fruitful in the United Kingdom, where a performance-based specification for this class of materials has been published by the British Standards Institute [
22]. Various end users and asset owners have also introduced their own tailored specifications for alkali-activated concretes in designated applications, some of the earliest being state roads authorities in Australia [
23], and initiatives intended to lead to the publication of national standards are now in motion in multiple countries. Analysis of the durability tests that essentially underpin the performance basis of these standards has also been carried out through the International Union of Laboratories and Experts in Construction Materials, Systems, and Structures (RILEM) and its technical committees [
24], [
25], [
26], and further work in this area is ongoing.
Through these testing and validation campaigns, it has been made very clear that an advanced understanding of the fundamental materials science of the binders and concretes is essential to success in performance-based design and specification, because test methods need to be selected and adapted in order to be appropriate for the chemistry and microstructure of the materials at hand. This partnership between engineering tests and scientific foundations is relatively well established for alkali-activated materials. However, as is the case for Portland cement-based materials, the complexity of the physical and chemical phenomena being tested in even a conceptually straightforward test of cementitious material means that further material-based research is certainly needed. In particular, the sensitivity of alkali-activated binders to their curing environment needs to be better analyzed and understood, and factors such as autogenous shrinkage [
27] must be controlled more effectively.
The availability of durability tests validated for application to alkali-activated materials is essential to truly unlock the emissions savings that can be offered by these materials—or, by analogy, any other class of innovative cement and concrete materials. If longevity or durability in service cannot be predicted sufficiently to give confidence to the specifier that the material will be able to serve its intended purpose over the necessary timeframe, there is no chance that it would be put into use. In fact, not only chemical durability but also resistance to mechanical and thermo-mechanical loading, moisture movement, dimensional stability, and other aspects of long-term behavior must be predictable. If the durability performance is probable but not quite sufficiently certain, it could be that materials or elements would end up being designed to be over-specified (in performance and/or dimensions), such that the targeted level of emissions savings would be compromised. So, the availability of validated testing methods and materials design protocols is essential if the potential savings in emissions from any new construction material are to be achieved. In the case of alkali-activated materials, it has been demonstrated that existing durability testing standards developed for Portland cement often produce results that have limited resemblance to what can be identified for those materials when tested under natural exposure conditions [
28]. A good example of this is carbonation, often perceived as the “Achilles heel” of alkali-activated materials. When the performance of alkali-activated materials is tested under natural carbonation conditions, a comparable or superior resistance can be identified in comparison with that of the blended Portland cement materials often used in modern infrastructure development [
29]; however, these results are quite distinct from the rather discouraging results that are often obtained from accelerated testing.
It is also important to consider the applications in which the proposed new construction materials are to be used and to select applications that match the (real or perceived) risk profile associated with the introduction of a new material into larger scale engineering usage. This is sometimes a difficult balance to strike, because applications requiring higher responsibility (and therefore bearing higher risk) are often those with greater added value, and thus those that can accept a new material that may be slightly more costly than the established options. However, proof of performance in lower risk applications is usually a prerequisite for the use of a material in elements with higher requirement. Thus, a hurdle must be crossed when using an innovative material that has not yet reached economies of scale in its production or qualification testing in lower risk applications where the least expensive solutions (which are usually the most “conventional”) would more generally be preferred. This puts further emphasis on accurate and full accounting of the price of emissions—whether embodied in a formal carbon price or through other mechanisms—when calculating the cost of a project and the materials used in it.
It is also possible that an alkali-activated binder derived from a waste or byproduct powder, in combination with an inexpensive alkali activator, may be the most cost-effective solution in a given scenario, even aside from carbon-pricing considerations. However, the economics of producing low-cost materials from wastes bring additional considerations: Once a waste generator realizes that its wastes are valuable to a potential end user, these wastes may cease being accounted as “wastes” and may become “byproducts” sold at an elevated cost (which end users may or may not be willing or able to pay). In such circumstances, the “byproducts” are then required to carry an attribution of some of the carbon dioxide (CO
2) and other emissions from the process that generates them. This is already the case for blast furnace slag in many jurisdictions [
30]; this material has moved from being a waste to being a product, as the iron producers operating blast furnaces (and associated granulation facilities) view the slag as a profitable secondary product—and therefore as one that must carry a non-zero emissions allocation. The allocation of emissions to industrial wastes or secondary products is a complex and nuanced issue, with important national, societal, and jurisdictional influences that merit further specialist analysis beyond what can be accommodated here [
31]. Nevertheless, it becomes an important consideration in any discussion of alkali-activated binders that may be derived from waste aluminosilicate powders. When considering an entire life cycle and circularity potential, such an analysis becomes far more complex, as the aspects of durability and structural integrity enter to play a key role in concrete design, particularly regarding the type and quantity of cements that must be used to achieve a certain performance. The existing standardized approach of in-service environmental conditions being classified as exposure classes, with corresponding prescriptive concrete formulations, is unlikely to be directly applicable when utilizing alkali-activated materials as binders, as they respond differently from Portland cements when exposed to different environments. There is a need to develop an equivalent set of guidelines to ensure that these materials are used appropriately in terms of sustainability potential, optimized mechanical resistance, and longevity.
3. Concluding remarks
It is eminently clear that the global construction sector must decarbonize its operations—but this is a sector that is fundamental to human wellbeing and societal development, providing safe and dignifying living conditions, safe water and food supply, and opportunities for healthy and more comfortable lifestyles to billions of people worldwide. The sector must therefore be supported in its green transition—by policymakers, regulators, and others—and thus enabled to find and deploy the materials and other technologies that will allow the necessary improvements to be found and exercised. There should also be willingness from practitioners and other stakeholders across the supply chain to facilitate in every way possible the adoption of those technologies with great potential to enable the transition to more sustainable practices. This requires the availability of a toolkit of sustainable materials, which will include alkali-activated cements and concretes as an important constituent, along with other innovative cements and non-cement solutions for the production of infrastructure materials. Long-term validation of material properties and their stability under different forms of physical, chemical, and mechanical loading for extended periods is essential, although this must be complemented by scientifically based accelerated testing so that timeframes of decades or more are not needed before new materials can enter service. New materials must be tested and validated at both the material and engineering levels, and sufficient confidence must be built in their supply chains, performance, durability, and life-cycle emissions footprints to enable them to be used according to their full potential and to bring benefit to the global society.
Acknowledgments
The contribution of John L. Provis to this work was funded by the Engineering and Physical Sciences Research Council (EPSRC), UK (EP/S019650/1). The participation of Susan A. Bernal in this work was funded by EPSRC via an Early Career Fellowship grant (EP/R001642/1), and the Transforming Foundation Industries: Network+ Towards Value by Innovation (EP/V026402/1). The participation of Zuhua Zhang was funded by the National Nature Science Foundation of China (U2001225), Fundamental Research Funds for the Central Universities (22120230174 at Tongji University), and Geopoly® Fundamental Genomic Research project.
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