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GEOPOLYMERS AS ECOLOGICAL ALTERNATIVE TO PORTLAND CEMENT
Conventional concrete consists of four elements: cement, water, sand and bricks. Only cement, the crucial binding agent of concrete, makes concrete production enormously polluting. In recent decades, there has been much research on geopolymers as an ecological alternative to Portland cement. Jasper Vitse, PhD student at KU Leuven-Bruges Campus, is investigating how these geopolymers can be better optimized.
Jasper bases his research on two ecological challenges for the construction industry. On the one hand, the construction industry is struggling with very high CO2 emissions, especially from cement production. “Cement is the core of your concrete, but for every ton of cement produced, nearly a ton of CO2 is released. So that is a big problem”, Jasper explains. Additionally, more and more countries are also introducing CO2 taxes to ensure that less of the polluting cement is used. “Therefore, the cost price for cement becomes very high for companies, but of course they cannot immediately abandon the material. Hence, there is a significant demand from the industry for a more environmentally friendly binder for concrete”, Jasper adds.
On the other hand, a lot of construction and demolition waste is generated on construction sites. “That waste often ends up in a landfill without being reused. In Europe, fortunately, we are now seeing a shift towards recycling those materials”, Jasper says.
Geopolymers
To produce cement, limestone is mined from limestone rocks. “That limestone is then burned at 1350 to 1500 °C, decomposing calcium carbonate into calcium oxide and CO2”, Jasper explains. “Not only during that combustion is CO2 released, but there are also additional emissions due to the high temperatures that must be achieved.” To limit those CO 2 emissions, Jasper is studying whether geopolymers can offer a viable alternative to cement in conventional concrete production. While the production of conventional cement requires very high temperatures, geopolymers can be formed at room temperature. “A geopolymer is an inorganic polymeric cementitious binder. It consists of an aluminosilicate precursor, a granular material rich in alumina and silica. Waste products from industry, such as blast-furnace slag from the steel industry and fly ash from coal combustion, are often used as precursors. Additionally, you have the alkaline activator, often sodium hydroxide and sodium silicate, which is added to it. This creates a chemical poly- condensation reaction, and you get a geopolymer”, Jasper continues.
Using waste
“Using by-products from industrial processes, such as fly ash and blast-furnace slag, for geopolymers is already a step in the right direction,” Jasper believes. “Otherwise, precious raw materials have to be mined and thermally treated again, putting us in the same situation as the high-temperature treatment of limestone for cement production. Moreover, the industrial by-products have already been heated to very high temperatures, making them amorphous and reactive, and thus suitable as precursors”, Jasper explains. However, these by-products are also at risk of running out at some point. “Blast-furnace slag, for example, is already being used to partially replace cement. Moreover, steel production is increasingly occurring electrically instead of with fossil fuels, which means there are fewer by-products left anyway”.
Therefore, Jasper is investigating whether certain materials within con- struction and demolition waste are sufficiently reactive to serve as precursors. In this way, he addresses two issues at once. “Besides replacing cement with a more environmentally friendly alternative, I am also looking for a new use for construction and demolition waste”, Jasper explains. When buildings are demolished, the various materials are sorted. “That allows us to finely grind a concrete wall or a traditional brick into powder less than 125 micrometers in size. The finer the powder, the more reactive”, Jasper says.
That concrete or masonry powder can be activated and partially replace blast-furnace slag as a precursor. “Concrete or masonry powder by itself is not reactive enough as a precursor. Therefore, we still need some industrial by-products. I am investigating the right proportions to achieve good workability and strength properties of the synthesized geopolymers”, concludes Jasper.
Pauline Van Springel
