Cement is made by a process known as calcination. By heating limestone (CaCO3) with small quantities of other materials such as clay to 800 °C in a kiln, carbon dioxide (CO2) is expelled from the calcium carbonate to form lime (CaO), which is then blended with the other materials that have been included in the mix. The resulting material, called clinker, is then ground and mixed with a small amount of gypsum into a powder to make Portland cement, the main ingredient in concrete.
When limestone is processed this way, the greenhouse gas carbon dioxide is an unfortunate by-product. Cement manufacturing is responsible for up to 7% of global greenhouse gas emissions, releasing about 9 kg of carbon dioxide for every 10 kg of cement produced. Global production of cement is about 3.3 billion tons per year which creates about three billion tons of greenhouse gasses each year.
Researchers at George Washington University have devised a greener technique: Solar Thermal Electrochemical Production (STEP), which eliminates carbon dioxide production from the process. It is also less expensive than calcination. For cement, STEP would use solar energy in two ways. The STEP method eliminates the needs for fossil fuels by instead using solar thermal energy to melt the limestone. Then it can assist with electrolysis. Electrolysis of the melt produces lime, oxygen and carbon—and no carbon dioxide.
When STEP is run at higher temperatures, it creates carbon monoxide which can be used for industrial applications, such as in fuels, plastics or pharmaceuticals. If cement were created this way, the sale of the by-products could net up to about $300 per ton of STEP-created cement. Compare that with a cost of $70 per ton to produce cement via traditional methods, and its not hard to see that STEP is a step in the right direction. The challenge now is to scale up the process for commercialization. Cement manufacturers are already looking for ways to reduce their greenhouse gas emissions, so if the process can be adapted for industrial use it could transform the industry.