Geopolymers – the climate-friendly concrete substitute

Cement-free binder

The term “Geopolymer” was coined by chemist Joseph Davidovits in the 1970s. The prefix “Geo” was intended to emphasize its proximity to geological materials such as natural rocks and other minerals. The polymer structures formed in this binder are non-crystalline (amorphous) Si-O-Al frameworks similar to zeolites. According to Davidovits, metakaolin and alkaline activators are required to enable their application as binders, with metakaolin’s minimal calcium content enabling the achievement of desirable qualities such as improved durability. In essence, the binder comprises two components: a non-reactive powdered solid, ideally consisting primarily of aluminium oxide and silicon dioxide, and an alkaline activator.

This binder could be referred to as a latent hydraulic, or rather pozzolanic binder. Such binders are not self-setting and therefore require a triggering agent to harden. The defining factor is the ratio of calcium to silicon. For latent hydraulic binders, the CaO/SiO₂ ratio falls between 0.5 and 1.5, while pozzolanic materials have a ratio of < 0.5. Examples of latent hydraulic binders include blast furnace slag, and for pozzolanic materials, fly ash or trass meal. In contrast, cement has a ratio of > 1.5.

The following illustration provides a simplified explanation of the system involved. It appears that hydraulic binders, symbolized here as “balls”, react consistently with water and “roll”. Latent hydraulic binders require an initiator or trigger to roll, but once the initial energy is absorbed, the subsequent reaction proceeds in a consistent manner. Pozzolanic systems, on the other hand, perpetually require a triggering agent to keep the reaction – and thus the “balls” – in motion.

Environmentally friendly due to improved carbon footprint

An omnipresent and increasingly crucial concern today is that of CO₂ emissions. Elevated environmental consciousness has led to the increased utilisation of secondary raw materials in binder production. Analysis of the aforementioned binders like blast furnace slag quickly reveals appreciably lower carbon dioxide generation compared to cement production. And indeed, blast furnace slag can be added to the raw charge in cement production so as to reduce the carbon footprint without adversely affecting the excellent application properties otherwise achieved. Where it is possible to solely employ blast furnace slag as a binder, and to activate it, CO₂ emissions can be further diminished.

MC-Bauchemie possesses the expertise and experience to combine and activate different systems and alternative raw materials for use as substitute binders. This may also involve the use of recycled materials, which again positively impacts the carbon footprint and overall environmental compatibility of the system. And in collaboration with Australian company Wagners, MC-Bauchemie has launched a new generation of activators onto the market: Wagners EFC Activator Types A, B, BK, and C.

Blast furnace slag as a resource for alternative, eco-friendly binders

The most commonly used resource for this purpose is blast furnace slag. However, not all blast furnace slags are the same. Understanding its genesis as a secondary product in steel production helps in the differentiation process. In the blast furnace process, the hot metal (commonly known as pig iron) is first obtained by smelting (and thus reducing) iron ore through the use of coke. Subsequently, the hot metal is converted into steel through various processes, often involving scrap. The smelting and subsequent processes produce molten blast furnace slag, which is cooled, granulated and then milled into slag powder. Due to variations in raw materials during production, blast furnace slag naturally exhibits a range of compositions, leading to diverse behaviours during binder activation. MC-Bauchemie possesses the expertise to tailor activator compositions not just to enable concrete curing but also to achieve specific end-product properties. This in turn facilitates the formulation of binder recipes based not just on blast furnace slag but also on combinations with fly ash, trass meal and other pozzolanic raw materials. Hence MC is capable of utilizing additional resources with similar compositions that were previously given little consideration as potential binders.

The photo below, from left to right, depicts the  action achieved with superplasticiser MC-PowerFlow 4100, a blend of blast furnace slag and fly ash, and Wagners EFC activator compound.

Admixtures to enhance application

A challenge in using alternative binders in conjunction with alkaline activators lies in the thixotropy of the material. Activation leads to rapid setting, and currently there are no processing or application aids on the market to counter this phenomenon. However, MC-Bauchemie has developed admixtures such as MC-PowerFlow 4100, 4104, 4105 and 4106 that significantly enhance ease of pouring and placement. As demonstrated in the illustrations below, these superplasticisers are capable of producing concrete consistency classes ranging from earth-moist (semi-dry) to a slump value of f5.

To work with an alternative binder in Germany, an approval is required from the German Institute for Structural Engineering (DIBt). And in collaboration with Wagners, MC-Bauchemie registered a groundbreaking achievement by obtaining the first-ever DIBt approval in Germany for Earth Friendly Concrete (EFC) – a more climate-compatible, cement-free geopolymer concrete.