Since it’s Earth Day today, I thought it would be the ideal day to take a look at some of the ways companies and researchers are working to make cement more environmentally friendly. Cement, of course, is the binder that when mixed with aggregate, typically sand and gravel, and water forms concrete. Concrete is used throughout the construction industry in buildings, parking garages, bridges, runways, highways, dams and sewers. The only thing people use more of each year than concrete by volume is water.
The problem is cement is one of the largest contributors (5 – 10% annually) of man-made carbon dioxide emissions on the planet. The two biggest contributors to carbon emissions during cement production comes from the burning of fossil fuels needed to heat the kiln and the carbon dioxide driven off during the calcination of the limestone. This doesn’t even include carbon emissions from the equipment and vehicles used to harvest the raw materials and transport them to the cement plant or to transport the finished product from the plant.
First, let’s take a look at how cement is typically made.
Portland cement is usually made using limestone and clay containing silica, aluminum oxide, calcium compounds and iron oxide. Other ingredients commonly used include shale, iron ore and fly ash. The ingredient are crushed and placed in an angled, rotating kiln. The kiln is heated using fossil fuels to temperatures between 2,500° and 3,000° F. The heat drives off the oxygen and carbon dioxide and the ingredients sinter, or fuse together without melting into a liquid, to form new compounds that come out in the form of pellets called clinker. The clinker is ground into a powder and mixed with gypsum to create Portland cement.
Now we’ll take a look at some of the methods being developed to create a greener cement.
Ferrock is cement-like material that was discovered accidentally when a David Stone, a PhD student at the University of Arizona, was trying to find a way to keep iron from rusting. Ferrock uses waste steel dust from steel mills that isn’t recyclable and typically ends up in landfills, silica from ground glass and high concentrations of carbon dioxide (CO2) to create a cement substitute.
The CO2 is diffused in a wet mixture and reacts with the iron in the waste steel dust to form iron carbonate. The rusting of the iron dust causes the material to harden with the absorbed CO2 being trapped. The production of Ferrock also uses less heat than traditional Portland cement. The process absorbs more carbon than what was emitted during production, creating a carbon-negative alternative to cement. The material reportedly has a higher compression strength and flexural strength compared to concrete made from Portland cement.
Limestone Calcined Clay Cement (LC3)
LC3 is a low carbon cement solution that combines the components of Portland cement, clinker and gypsum, with large amounts of low-grade limestone and calcined clay. The aluminum oxide in the clay reacts with the ground limestone which creates a less porous material. Because the material is less porous than Portland cement it means it has strength equal to Portland cement despite using less clinker in its composition.
While LC3 is not carbon negative like the Ferrock, or even carbon neutral for that matter, it can reduce carbon emissions by 30% and uses less energy and costs less to produce. It also has the added benefit that it can be produced at existing cement plants. LC3 is currently being studied and tested in Switzerland, India and for things like shrinkage, creep, durability and sustainability.
Dry Slag Granulation
Slag is a by-product of metal smelting and can be granulated for use in cement manufacturing by using water jets to cool the molten slag. The problem with this method is that it uses lots of water and creates a lot of heat waste. Dry slag granulation uses spinning discs to generate centrifugal forces that results in granulated slag and superheated air. The granulated slag can then be used to produce cement and the superheated air can be for things like steam generation. This method conserves large volumes of water, reduces heat waste and turns the waste product of metal smelting into a usable product instead of sending it to the landfill.
The process was created by CSIRO, the Australian national science agency, and has recently teamed with a Chinese firm to commercialize the process.
Additional methods to creating a greener cement include the use of fly ash, sequestering CO2 back into concrete blocks, nanoengineering, geopolymers as well as altering the mix ratio of standard Portland cement to reduce carbon emissions.