Climate-friendly Cement Substitute

Researchers at TU Darmstadt are proposing geopolymers as an alternative to cement.

These mineral binders are not only more environmentally friendly, they are also more resistant to chemicals and high temperature.

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.

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.

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.

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.

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.

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.

Facts and figures

Publication:  N. Ukrainczyk, O. Vogt, E. A. B. Koenders: Reactive Transport Numerical Model for Durability of Geopolymer Materials. Advances in Chemical Engineering and Science (6) S. 355-363, 2016

Additional focuses  of the research of Professor Eddie Koenders working group:

• Mineralised foam as an insulating material, already installed in the ETA Factory at TU Darmstadt
• Integrating phase change materials in walls and floors, to reduce the energy requirement for cooling and heating
• Concrete components for the production of renewable energies
• Rubber aggregate made from old tyres as an additive for road surfaces, to improve durability

Symposium on geopolymers:  10 February 2017, TU Darmstadt, Institute of Construction andBuilding Materials, contact: Ms Aysen Cevik, ++46-6151/16-22210, cevik@wib.tu-…

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Taxonomy

• Concrete