Substituting geopolymers for cement could lead to reductions in atmospheric carbon dioxide. These materials also may be more resistant than cement to high temperatures and chemicals.

Cement production is responsible for more than 5% of global yearly CO2 release, university researchers say. Making cement requires grinding and burning limestone, clay, and marl, and combustion accounts for some CO2 release. Grinding the limestone causes an additional release of the CO2 trapped in the stone.

Pouring a geopolymer-based substance.Pouring a geopolymer-based substance.Geopolymers are two-component systems, consisting of a reactive solid that contains silicon and aluminum 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”. Mixing the solid, which is ground stone, with the activation solution produces a rock-hard, anorganic solid.

Prof. Eddie Konders of Technische Universität Darmstadt in Germany and his research team are working to develop improved geopolymers that can be as workable as cement. Past attempts based on metakaolin proved unsatisfactory, due both to the amount of energy necessary to prepare the product and to workability problems.

One change is using a less-pure base material. For example, kaolin that contains iron oxide and other foreign materials appears to produce a more workable end product. Researchers are also studying fly ash, a waste product, or ground trass (a type of stone) as possible base materials or additives to kaolin.

The alkali concentration in fly ash provides an additional potential advantage. The alkali activation solution can contain less alkali, reducing production costs.

The research team is also working on methods to give the geopolymers specific characteristics, such as a superplasticizer. Additives that work on cement do not have the same effect on geopolymers.

One application currently under test is a sewer pipe, which exploits the tested geopolymer’s greater strength and resistance to chemicals.

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