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Geopolymer concrete is increasingly seen as the solution to substantiallyb reduce the CO2 profile of concrete. Besides a much more favourable environmental profile, it is said to perform technically equal or better than regular concrete. By far the majority of articles on geopolymer concrete, mostly from universities and research institutes, are positive about this material. Nevertheless, its application is still very limited and this is said to be mainly due to the lack of regulations. However, there are more reasons why geopolymer concrete is not widely used. This article discusses the facts regarding the environmental aspects and properties of geopolymer concrete and the bariers to scale-up.

This article ia also available as PDF: download here Whitepaper ‘Geopolymer concrete, sustainability and properties – a critical assessment’.

In brief:

• When granulated blast furnace slag and fly ash are used in geopolymer concrete, there is no environmental gain but environmental loss.
• Most of the properties of geopolymer concrete are inferior to regular concrete.
• Substantial scale-up of geopolymer concrete is not possible due to the limited and non-scalable availability of activators.
• Geopolymer concrete cannot be considered circular.

Figure 1. 24-storey apartment building from 1994 built with alkaline-activated slag in Lipetsk, Russia [8] (photo: Google Street View – July 2015)

Geopolymer is a common term for alkaline-activated binders. Materials such as pulverised coal fly ash and blast furnace slag are hereby activated not with Portland cement clinker, as in regular concrete, but with a strong base (alkali).

In the case of raw materials containing mainly silicon, such as pulverised coal fly ash, a more or less polymer-like structure may occur. With materials that also contain a lot of calcium, such as blast furnace slag, as with regular cement, mainly a so-called CSH gel (calcium silicate hydrate), the basis of hardened cement paste, is formed. The ratio of calcium/silicon in the CSH gel of geo-‘polymer’-concrete however does differ significantly from that in regular concrete, so the properties still differ. In alkaline-activated slag, this ratio is about 1 while in regular hardened cement paste, the ratio is about 1.5 to 1.8.

In the case of geopolymer concrete the binder usually consists of raw materials that are also suitable for regular cement, such as blast furnace slag and pulverised coal fly ash, and in addition, an alkaline activator is required. The activator creates a high pH, allowing the slag and fly ash to dissolve and react.

The first patent for a geopolymer binder (alkaline-activated slag) was issued in the US as early as 1908 [2]. However, large-scale application did not come about until the 1960s, particularly in Russia and Ukraine, driven by a shortage of Portland cement in the former Soviet Union (Fig. 1). In recent decades, geopolymer concrete has been the subject of extensive research worldwide, due to its theoretical potential to significantly reduce the CO2-profile of concrete. However, the application of geopolymer concrete remains very marginal worldwide.

At the project level, geopolymer concrete based on blast furnace slag and pulverised coal fly ash can yield a low MKI (environmental cost indicator), but on average, the MKI on a national scale will not decrease and, consequently, neither will CO2 emissions. This is due to using raw materials that are already fully used in the cement and concrete industry.

Blast furnace slag and pulverised coal fly ash are scarce raw materials which are in Europe (and blast furnace slag also worldwide) already almost fully applied in cement and concrete, thereby partially replacing Portland cement clinker (certainly in the Netherlands) already for many decades. The application of extra high contents of slag or fly ash in geopolymer concrete but also, for example, in the form of a blast furnace cement CEM III/C, therefore does not result in any environmental gain because it only involves a shift of raw materials (Fig. 2).

Of course, due to an increasing demand, we can import more slag and fly ash from neighbouring countries than we already do. In the Netherlands the average MKI may then decrease, but of course the average MKI of concrete in the surrounding countries will then increase, because there too slag and fly ash are already almost entirely used in concrete. Due to increased transport, total CO2 emissions will then even increase.

Activators also result in additioal CO2 emissions. On top of that, Portland cement clinker more effectively activates slag and pozzolans such as fly ash than strong bases: to achieve the same strength, more slag is generally needed in geopolymer concrete than blast furnace cement in regular concrete.

Geopolymers can only contribute to reducing CO2 emissions if materials are used that we do not currently use in concrete. After all, applying geopolymer concrete based on blast furnace slag or pulverised coal fly ash only achieves (apparent) environmental gains on the project, but increases the overall CO2 footprint of construction in the Netherlands. Alternative materials are for example, artificial slag, volcanic ash or calcined clay, but also secondary material streams other than pulverised coal fly ash and blast furnace slag.

Unfortunately, in the case of suitable alternative secondary material streams, available volumes are usually limited. However, calcined clay can be produced indefinitely and used as a raw material for geopolymer concrete. The only question is whether environmental benefits are then achieved compared to a regular cement with a low CO2 profile.

It can be roughly stated that the effectiveness of a raw material in geopolymer concrete is comparable to its effectiveness in combination with Portland cement clinker. In cement, granulated blast furnace slag can replace the largest proportion of Portland cement clinker, and blast furnace slag is also the most suitable for geopolymer concrete. With a high proportion of blast furnace slag, geopolymer concrete can be made that has good strength development even without heating. With less reactive materials, such as pulverised coal fly ash, ground brick, volcanic ash or calcined clay, very high doses of strong activator and/or heating are necessary. The production of the required strong activator, NaOH, however, as we further describe, cannot be scaled up.

As indicated earlier, the use of granulated blast furnace slag and pulverised coal fly ash in geopolymer concrete results in no environmental gain but environmental loss, but the volume of these by-products is also relatively limited. Taken together, they account for about 13% of global cement production [3]. There are other residual and waste streams suitable for making geopolymer concrete that are not yet used in cement and concrete, but their volumes are limited and significantly less than the amount of blast furnace slag and fly ash. For example, in Belgium a relatively large quantity of copper slag is released that appears suitable for the production of geopolymer concrete. However, this amounts to less than 200,000 tonnes per year, while in Belgium about 6 million tonnes of cement are used per year.

Clay is widely available and can be made suitable by calcination (heating between about 700 and 800 °C) for application in both cement and geopolymer concrete. This mainly concerns the clay type kaolin (Chinese clay), which is rich in the mineral kaolinite. By heating it creates a complex more or less amorphous structure, giving it pozzolanic properties. Mainly due to the energy required for heating, calcined clays do have a high CO2 profile compared to blast furnace slag and pulverised coal fly ash (270 – 423 kg CO2 per tonne [3][4]), which in combination with activators, little or no environmental gain is achieved in comparison with regular cement-based concrete with a low CO2 profile. Volcanic ash is also widely available and does not need to be heated, but like calcined clay, a very high dosage of strong activator is needed to achieve acceptable strength development in geopolymer concrete.

Even more problematic for scaling up geopolymer concrete is the limited availability of activators. Production of sodium hydroxide (NaOH, called caustic soda dissolved in water), the most important and effective activator, is around 60 million tonnes per year via electrolysis of a sodium chloride solution, which releases chlorine gas. The market for chlorine (Cl2) is limited, so scaling up
production of NaOH is not easy [5]. Moreover, demand for NaOH from other applications already exceeds the production. Caustic soda is used for paper production and numerous chemical processes, among others. The limited availability therefore hampers the scale-up of geopolymer concrete [3].

The global production of sodium silicate (water glass), an activator often used in combination with caustic soda, amounts to less than 10 million tonnes per year, with which at most 50 million tonnes of binder can be made [6]. That roughly corresponds to 1% of the total global cement volume. But water glass also has numerous other applications, which means that current production can only be put to limited use for geopolymer concrete.

Production of the weak base sodium carbonate is about 50 million tonnes, by quarrying and mainly by chemical reaction between limestone and sodium chloride [7].

Roughly half of this is used for the production of glass. Production of soda ash could be ramped up (and there are large reserves in the US), but this activator is not very suitable for activation of widely available raw materials such as (calcined) clay and volcanic ash.

The properties of geopolymer concrete are highly dependent on the chosen combination of filler(s) with binder function (powders) and activator(s), while even minor variations in the composition of the powder(s) have more influence on the properties than when used in regular cement and concrete. However, some general statements can be made that apply to most variants of geopolymer concrete.

Geopolymer concrete, in comparison with regular concrete (at least, as long as it is not yet carbonated) has a higher tensile strength at the same compressive strength. This is favourable for unreinforced concrete. For reinforced concrete, the higher tensile strength is generally unfavourable: to keep the crack width limited and avoid the risk of brittle fracture, more reinforcement is then often needed than for regular concrete.

Geopolymer concrete usually shows substantially more shrinkage than regular concrete. Twice as much shrinkage is common, although there are also studies and geopolymer variants that show little difference. Geopolymer concrete also shows significantly more creep than regular concrete [8][11]. That geopolymer concrete generally, compared to regular concrete, shrinks more and creeps more is usually unfavourable from a structural point of view.

With respect to various deterioration mechanisms, such as freeze-thaw cycles and penetration of chlorides, geopolymer concrete appears to perform well. However, the studies were almost without exception conducted on young geopolymer concrete. However, resistance to these degradation mechanisms deteriorates sharply by carbonation, while resistance to carbonation is certainly not a strong aspect of geopolymer concrete. Carbonation of geopolymer concrete, at least with alkaline-activated slag, directly affects the CSH gel [9]. Here, in addition to calcium carbonate, a type of silica gel is formed that is more porous and weaker than the original CSH gel. As a result, in geopolymer concrete, the hardened cement paste does not become harder and denser by carbonation, as in the case of Portland cement, but rather softer and more porous.

This has a negative effect on the density and thus the resistance, among other things, against ingress of chlorides and it also greatly reduces the resistance to freeze-thaw cycles (Figs. 5 and 6). The compressive strength also decreases as a result. Also, in the case of fly ash-based geopolymer concrete (and hence presumably also other pozzolans such as calcined clay and volcanic ash) due to the low content of calcium oxide (CaO), carbonation increases porosity and resistance to various degradation
mechanisms decreases [10]. Geopolymer concrete therefore performs much worse in practice than might be expected on the basis of laboratory tests on young (non-carbonated) concrete.

Incidentally, even laboratory tests on laboratory carbonated concrete do not appear to be representative of practical conditions either, because the practical conditions (varying wet/dry) are not simulated in the laboratory [10].

There is no doubt that geopolymer concrete can be built with. But the much-heard assertion that the properties of geopolymer concrete are generally equal or superior to those of regular concrete is incorrect.

The CROW-CUR guideline 2 ‘Assessment system for raw materials on suitability for circular concrete’ includes a criterion for the alkali content of recycled concrete aggregate of no more than 0.4% m/m. This criterion was included to prevent ASR being created in new concrete containing recycled concrete aggregate. Recycled concrete aggregate made from geopolymer concrete will generally not meet this criterion. ASR is the abbreviation for alkali-silica reaction, a reaction of alkalis and water with reactive silica. Reactive silica is often a component of natural aggregates. The reaction products of ASR can bind a lot of water and consequently swell, causing substantial cracking.

ASR can lead to substantial damage of concrete. Geopolymer concrete should therefore be reapplied in geopolymer concrete or it will have to be demonstrated that there is no increased risk of ASR when applied in regular concrete. However, this is not straightforward. The alkali content in geopolymer concrete is so high that when reused in the form of traditional recycled concrete aggregate, the so-called cement paragraph of CROW-CUR Recommendation 89 cannot be used because the ‘alkali contribution of other components’ will be too high even when 30% recycled concrete aggregate is used. So even when using a blast-furnace cement CEM III/B, ASR cannot then be ruled out. And this despite the fact that CUR- Recommendation 89 takes into account the binding of alkalis in a slag- and/or fly ash-rich matrix and therefore allows higher alkali contents in the case of high slag
contents.

The creep of geopolymer concrete can be a factor of four or more higher than the creep of regular concrete at comparable strength. It is unclear what the effects on the creep of new regular concrete are when the aggregate is partially replaced by recycled concrete aggregate of geopolymer concrete.

In smart crushing, the alkalis will mainly end up in the hardened cement paste fraction. The smart crushed gravel can then be reused, but reuse of the hardened cement paste in regular concrete then becomes problematic.

Recycling geopolymer concrete separately and reusing it in new geopolymer concrete is not easy to organise in practice, and it has not been demonstrated that there is no increased risk of ASR or other
negative effects with this reuse. Geopolymer concrete cannot therefore be considered circular for the time being.

What is clear is that, due to the limited availability of raw materials and activators, the production of geopolymer concrete cannot be scaled up to the point where it can replace a significant part of regular cement-based concrete. In addition, for most applications, the properties of geopolymer concrete are also inferior to those of regular concrete. Furthermore, geopolymer concrete cannot be considered circular for the time being.

Of course, it is good to use residual flows that are not already used in cement and concrete, such as copper slag, for geopolymer concrete, when this application offers the most environmental benefits. In addition, geopolymer concrete can be used because of specific properties such as acid resistance. But the potential contribution of geopolymers to the further sustainability of concrete is very limited.

1. Duurzaamheid van beton met alkali-geactiveerde slak uit de jaren 50 – Het Purdocement, Maarten Vanooteghem, Master thesis Universiteit Gent, 2011
2. US Patent 900,939, Slag cement and process of making the same, 1908
3. Ontwikkelingen betreffende hoofdbestanddelen voor klinker-gebaseerde cementen en geopolymeren, SGS INTRON B.V., mei 2021
4. Clay calcination technology: state-of-the-art review by the RILEM TC 282-CCL, Materials and Structures (2022) 55:3
5. One-part alkali-activated materials: A review, Tero Luukkonena, Zahra Abdollahne­ja­da, Juho Yliniemia, Paivo Kinnunena,b, Mirja Illikainena, Cement and Concrete Research 103 (2018) 21–34
6. Eco-efficient cements: Potential, economically viable solutions for a low-CO2, cement-based materials industry, Karen L. Scrivener, Vanderley M. John, Ellis M. Gartner, United Nations Environment Programme, Paris 2016.
7. The Essential Chemical Industry – online https://www.essentialchemicalindustry.org/chemicals/sodium-carbonate.html
8. Alkali-Activated Cements and Concretes, Caijun Shi, Pavel V. Krivenko, Della Roy, Taylor & Francis, USA, 2006
9. Carbonation process of alkali-activated slag mortars, F. Puertas, M. Palacios, T. Vazquez, Eduardo Torroja Institute (C.S.I.C), Madrid, 2005
10. Field and Laboratory Investigation of the Durability Performance of Geopolymer Concrete, Kirubajiny Pasupathy, thesis, Swinburne University of Technology, Melbourne, Australia, 2018
11. Kostiuchenko, A. (2024). Creep of alkali-activated fly ash and slag concrete: Unveiling multiscale dynamics. [Dissertation (TU Delft), Delft University of Technology].