• The production of clinker—the material that comprises most of cement—is the process in cement-making that contributes the overwhelming majority of emissions. One of the most straightforward ways to reduce cement’s carbon amount is to reduce the proportion of clinker in the final cement mixture, replacing it with supplemental cementitious material (SCM). Several existing SCMs are in fact by-products of other industrial processes, namely fly and bottom ash and ground granulated blast-furnace slag (GGBFS), reduce emissions from the cement process while also using by-products of other industrial processes.
  • Fly ash and bottom ash are a by-product of coal burning that can be added to cement as a substitute for clinker, while GGBFS, also known as “slag,” is a by-product of iron production, created when iron slag is rapidly cooled. Ash can replace up to 30 percent of clinker, while GGBFS can replace as much as 85 percent of clinker content, reducing the carbon account of cement significantly.
  • California’s cement industry faces challenges in supply for both of these materials. California, and most of the American West, lacks coal power plants which produce fly ash, and therefore would have to import fly ash from the eastern half of the country. Furthermore, as the country continues to shift away from coal as a source of electrical power, the supply of fly ash nationally will dwindle, and will be unable to meet the demands of the cement industry. GGBFS are only produced in certain types of iron and steel furnaces, of which the United States has only two such furnaces, and they are often idled. While the market price of GGBFS mean that it is still economical to ship it, nearly all of it is imported from abroad, and supply constraints may similarly exist there too.