Cement, gravel and water. The building material is ready. Is more research needed?

By Tobias Hertel

Christmas day, 2015

After discussions about current issues in politics, the economy and – inevitably – complaints about the weather, my great-uncle noticed my presence at the table and started the overdue interrogation about my professional life. “Concrete research? Seriously? Even I can explain how it works. Just mix water, cement and gravel! No need for research! Do you want to end up as a cement mixer?” After his rhetorical questions, he concentrated, visibly annoyed, on his roast goose and the discussion was already over before it even started.

The most-used man-made material

It occurs nearly everywhere in daily life. We live and work in buildings, we go to work using streets. We take this as a matter of course. Isn’t it time to appreciate what glues our society literally together?

Okay, maybe a bit too epic, but yes, cement and concrete play an immense role in our life.

For those who aren’t convinced yet, a look at some data can be helpful to describe the importance. Global cement production in 2014 exceeded 4 billion metric tons and a further increase is foreseen [1]. Due to the booming building sector, China and India are the biggest manufacturers of cement [2]. The same increasing trend is seen in the use and production of concrete. Because of the excellent properties, the low production costs and the availability of raw materials, concrete is the most-used man-made material of the world [3].

The telling argument as to why research is needed is the environmental impact the cement industry is faced with. Due to the calcination of raw materials and the combustion of fuels during the production process, a large amount of CO₂ is emitted. According to different sources 5 to 7 % of global CO₂ emissions can be attributed to the cement industry [4], which leads to the cement industry is often being labelled as a climate killer.

Steps to more sustainability in the construction industry

The cement industry is already fighting against that image and a lot of research is already being done to lower CO₂ emissions. Alongside the modernisation of process technologies in cement plants and the use of alternative fuels, the reduction and partial replacement of Ordinary Portland Cement (OPC) clinker by cementitious materials like industrial by-products, e.g. fly ash (generated by coal combustion) or filler materials, e.g. limestone, is a key technique that has been practised for decades. Sometimes improved properties can even be achieved. For instance a mix of highly amorphous silica fume (a by-product of silicon production) with OPC and fly ash yields very high strengths and made the construction of the world tallest building, the Burj Khalifa in Dubai, possible [5].

Another promising example of more sustainable types of cement are calcium sulfoaluminate based cements, which are produced using limestone, bauxite and gypsum at lower firing temperatures than OPC (around 1250-1350 °C). These cements lead to significantly lower CO₂ emissions, less energy consumption and have advantages like early strength development and high sulphate resistance, however they face drawbacks like high costs due to the use of bauxite and durability issues [3].

Let’s make polymers…

Another approach, scientists are investigating, are alkali-activated binder materials. Geopolymers, for instance, are defined as Al- and Si-rich cementitious, amorphous binders which are formed by polymerization of an alkali activated (alkali hydroxide or silicate) solid aluminosilicate precursor like metakaolin (dehydroxylated clay mineral kaolinite) [6]. They consist of chains or a three dimensional framework of connected AlO₄^5- and SiO₄^4- building units. The more general term Inorganic Polymer (IP) defines a supergroup with a deviation from the tetrahedral building units of Al and Si as well as the aluminosilicate chemistry [7]. Besides aluminosilicates, inorganic polymers can be synthesised from a wide range of industrial waste materials and residues, e.g. ferrous (from steel production) or non-ferrous slags (from Ni, Pb, Zn, Cu production) and thus contribute to the reduction of CO₂ emissions as well as the saving of primary raw materials [8-11]. Often these materials can be used without modification, like thermal treatment or additions, as precursors for the synthesis of IP. Several properties such as high strength, a high resistance against acidic solutions and high thermal stability turn inorganic polymers into promising alternative binders to common building materials like ordinary Portland cements [12].

To conclude, the answer to the question posed above is definitely yes, especially concerning global climate change and the huge amount of wastes which have potential for valorisation.

References