Climate-friendly cement substitute

When discussing greenhouse gases, there is usually one aspect that comes off badly: building with concrete is affecting the climate by releasing more carbon dioxide per year than global air traffic. The reason lies in the production of the cement, the most popular binder in the construction industry. Cement is made by grinding and burning limestone, clay and marl.

Promising alternative

This requires a lot of energy and also removes the carbon dioxide from the limestone. More than 5% of global carbon dioxide emissions come from cement production. This does not have to be the case, thinks Professor Eddie Koenders, civil engineer and Head of the Institute of Construction and Building Materials at TU Darmstadt. His group is working with geopolymers as a promising alternative to cement.

Two-component systems

Geopolymers are two-component systems, comprising of a reactive solid, that contains silicon and aluminium oxide, and a basic activation solution of alkali hydroxides or alkali silicates in water. The solid is a natural stone or mineral, which is why it has the prefix “geo”. When the activation solution is mixed with the grinded solid, to which aggregates and other substances belong to, depending on the application, the result will be a rock-hard, anorganic polymer. The molecular components, the monomers, are tetrahedrons with oxygen atoms at the four corners and a silicon or aluminium atom inside.

Great international interest

The term ”geopolymer” was launched by the French chemist Joseph Davidovits in the seventies. Thus far, the materials did not made it onto the mass market, but the ongoing climate debate has now given new momentum to geopolymer research. “There is a great international interest,” says a delighted Koenders, who together with companies and research scientists from Spain, France, Austria and the UK, is currently formulating an application for an EU project.

First geopolymers based on metakaolin

The first geopolymers were based on metakaolin, a heat-treated kind of kaolin clay. When heated to about 600 degrees Celsius, kaolin changes its structure, becomes more reactive and sets quickly whenever in contact with the activation solution. The issue is, that the pre-thermal treatment also consumes a lot of energy. But as kaolin does not contain any bounded carbon dioxide that will release during heating, and that the firing temperature is much lower than when burning cement, the carbon footprint is clearly better.

Extremely fine material

However, metakaolin is an extremely fine material, and working with geopolymers made of it is very different than working with cement paste. They are, for example, thixotropic: when you stir them or shake them, they liquefy – like ketchup, that is initially reluctant to leave the bottle, but then suddenly gushes out. The activation solution also makes the geopolymers somewhat sticky, which is why it is more difficult to remove the formwork.

Proving to be more suitable

“Geopolymers will only ever replace concrete and cement on a large scale if they have the same workability”, stresses Koenders. His staff, Dr Neven Ukrainczyk and Oliver Vogt, are testing different raw materials, to try to improve the handling. Less pure kaolins, containing iron oxide and other foreign minerals, are proving to be more suitable, as well as being more cost-effective.

Possible admixture: fly ash and trass

Fly ash, a waste product from flue gases, and the natural stone trass are a possible admixture or alternative to metakaolin. The researchers are getting the trass from the Eifel, where it was formed after volcanic eruptions. After excavation, the trass is grinded and can then be used directly. A further advantage is that it is rich in alkalies.