This is the latest in a regular series exploring early-stage technologies and scientific developments that could play a role in corporate solutions to climate change. (The previous installments are here.) Email ideas and pitches to [email protected] sources of carbon dioxide emissions are set in stone, somewhat literally.Consider that the process of creating concrete accounts for an estimated 5 percent of human-caused carbon dioxide (CO2) emissions annually, according to data from the U.S. Department of Energy and the International Energy Agency. Reducing that footprint has become a priority, especially in emerging economies such as China and the Middle East, home to some of the biggest construction projects in the world. Companies have been touting lower-carbon alternatives for years: There's even a 440-plus-page handbook dedicated to the topic.But is there a viable and profitable opportunity to use the concrete production process to reduce carbon emissions? That question has fascinated the industry for more than a decade, and it is intrinsic to the business models of two Canadian startups that are semi-finalists in the $20 million NRG COSIA XPRIZE competition to convert CO2 into valuable materials, CarbonCure Technologies and CarbiCrete. (The finalists will be announced in the spring; COSIA stands for Canada's Oil Sands Innovation Alliance.) Although the science behind their approaches is different, both are testing and touting processes that inject CO2 into the binding process, essentially replacing the Portland cement that makes traditional production so carbon-intensive. Both are focused on "ready mixed" production of preformed things such as cinder blocks and bricks: the aggregate revenue from this market could reach $8.9 billion in 2018.We are getting rid of industrial waste and sequestering CO2."It sounds too good to be true, but so was my last company," joked Chris Stern, co-founder and CEO of CarbiCrete, which was born out of project at McGill University in Montreal. "We are getting rid of industrial waste and sequestering CO2." (Disclosure: The author of this article is also a McGill graduate.) Stern previously was a development executive at Pure Energies, a Canadian solar company that sold itself to NRG back in 2014.A blueprint for disrupting cement productionCarbiCrete, which officially has existed for about a year and is raising funds to construct a pilot plant with an industry partner, builds on engineering work developed several years ago by another of its co-founders, Mehrdad Mahoutian, along with McGill professor Yixin Shao.Their patented "carbonation activation" process uses slag from steel factories, which contains calcium, oxides, hydroxide and silicates among other things, to replace the Portland cement. When those materials are reacted with CO2 during the curing process, they become hard, binding together in whatever shape the mold takes — such as bricks, paving stones, highway dividers or sewer pipes, Stern said. CarbiCrete estimated that each 18-kilogram cinderblock created with this process offsets and sequesters the emissions associated with producing 3 kilograms of Portland cement. "The 'competition' to our approach is convincing concrete companies to switch to slag," he said.One licensing model that CarbiCrete is considering involves co-locating concrete production at steel plants, helping facilities managers derive value out of their slag waste stream, Stern said. CarbiCrete also could become a broker of the slag, collecting it from steel companies and selling it to concrete companies for use in existing facilities. CarbiCrete won't be involved in the carbon capture end of the process; the CO2 will be supplied by companies that are selling it, such as PraxAir.Testing already under wayCarbonCure, based in Halifax, Nova Scotia, is further along in its development. Around for about five years, it is working with more than 80 ready mixed concrete producers across North America. One of its early advisors was legendary designer William McDonough. Founder and CEO Robert Niven is also a graduate of McGill University, with a degree in environmental engineering. CarbonCure's latest deal (disclosed this week) involves a partnership with St. Louis-based Breckenridge Material Co., a $100 million family-owned business, and a nearby producer, Eastern Missouri Concrete."A barrier for sustainable construction methods is typically higher upfront costs," said Patrick O'Brien, director of technical services for Breckenridge. "By introducing affordable sustainable solutions to the St. Louis area, we believe that we can actually push the market to go greener and help accelerate the adoption of greener construction in our community."CarbonCure sells retrofitting technology that injects CO2 into the wet concrete as it is being mixed, where they are turned into carbonate ions and then into "nano-size calcium carbontate minerals" (aka limestone). This reduces the amount of cement that must be used in the process by about 5 percent to 8 percent for ready mixed materials, while improving their strength, according to an explanation of the process on CarbonCure's web site.While CarbiCrete and CarbonCure are attracting attention because of their participation in the XPRIZE competition, a number of other noteworthy companies are commercializing processes that promise to sequester CO2 and use it in the production of building materials. Here are two other ventures we're watching:Blue Planet (Los Gatos, California) – A producer of bagged aggregate that can be used in a number of materials, including roofing tiles. The founder and CEO, Brent Constantz, is a serial entrepreneur who teaches biomineralization at Stanford University.Solidia Technologies (Piscataway, New Jersey) – The company has attracted investors including Kleiner Perkins Caulfield & Byers, BASF, BP and (most recently) the Oil and Gas Climate Initiative, a billion-dollar fund created by 10 companies that represent more than one-fifth of global oil and gas production. The current CEO, Tom Schuler, previously was president of DuPoint Building Innovations.