Editorial Feature

Geopolymers as an Alternative to Cement

For many construction industry leaders, including researchers, the fact that up to 7% of global carbon dioxide emissions (more than global air traffic each year) comes from cement production means that alternatives to cement must be explored. Given that cement is the most used construction material in the world, this is no small feat.

According to Professor Eddie Koenders, Head of the Institute of Construction and Building Materials at Technische Universität Darmstadt in Germany, geopolymers may be able to replace concrete in the near future. They are made of two usual components: a silicon and aluminum oxide mixture and a solution of water and either alkali hydroxides or alkali silicates. Unlike cement that is made via burning materials at 1500 degrees Celsius and then mixing limestone, clay and marl, a process from which carbon dioxide emits, geopolymer production has no excessive harmful gas release.

Considered inorganic, geopolymers are hard as rocks however. Subsequently, they are hard to work with before setting. Thus, Researchers are working on varying mixes so that the pre-set geopolymer does not get too liquid or sticky which they now have too much of a tendency to do. One ingredient swap: fly ash, a byproduct of thermal power plants that has no other use, for metakaolin. The former is not only cheaper to add (although geopolymers as a whole component is more expensive to make than cement) but also makes the material easier to handle.

Other advantages of geopolymers that are comparable to cement include being:

  • Quicker to set and re-mold,
  • Resistant to binding water like concrete within which water can cause a pressure build that then leads to cracking, and
  • Able in one day to develop compressive strengths that are similar to high performance concretes

Importantly, too, geopolymers are more resistant than cement to chemicals, as they do not contain hydration products. For example, Koenders’ group is making bio-chemical-resistant sewer pipes out of geopolymers. Yes, they are more expensive than cement pipes but they are also more durable so they do not have to be replaced frequently.

Another replacement material possibility is ferrock, the name for steel dust that can combine after the manufacture of steel. In tests, ferrock showed exponentially more tensile and compressive strength than concrete. This aspect means it would not need the steel rebar that concrete does to prevent its rupture. Questions do remain how to make ferrock at scale, but such questions are likely to be addressed, given that it actually uses the carbon dioxide it makes rather than emitting it like standard cement.  

When industry representatives for cement weigh in understandably blended cements are promoted.

A compromise from the Australian Cement Industry Foundation is that clinker, a substance made before cement is cemented, be diluted with other materials such as fly ash, ground limestone, ground granulated blast-furnace slag or silica fume. Adding these elements reduces emissions and often production costs.

Ultimately, what Material Developers and Builders choose depends on availability and cost but also on the applications themselves. Blended cements can display different chemical reactions relative to standard cements and thus become more application specific.

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