Making Concrete a Climate Solution
The sidewalks you walk on might seem pedestrian, but they could soon be an exciting frontier for climate action.
Concrete is both ubiquitous, as the most common man-made material on the planet, and incredibly carbon-intensive. Manufacturing its core ingredient, cement, produces about 8% of global greenhouse gas emissions annually — four times more than aviation. With the number of buildings worldwide expected to double by 2060 as construction accelerates in the industrializing world, it will be virtually impossible to meet climate goals without substantially decarbonizing concrete.
Fortunately, a slew of emerging technology solutions offer several pathways to significantly reduce the emissions impact of this building block of modern life — and perhaps even make concrete a climate solution, rather than a climate problem.
Pathway 1: Change the formula
The first step in making concrete is combining primary ingredients (mostly finely-ground limestone, clay, and iron ore) in a kiln to produce cement. The method of making Ordinary Portland Cement, the most commonly used variety, has remained largely unchanged for more than a century. Altering the formula could reduce emissions in several ways.
Solidia is pioneering an alternative cement formulation that significantly lowers the temperature needed in the kiln, which the company claims can achieve a 30–40% reduction in emissions through energy savings. Engineers at Purdue University recently discovered that adding nanoscale titanium dioxide to cement nearly doubles the amount of CO2 that concrete absorbs from the atmosphere as it hardens.
An electrochemical approach could separate out CO2 before the kiln. In 2019, MIT researchers announced they had demonstrated a breakthrough process using electrochemistry to separate carbon dioxide from limestone, producing a clean lime that can be combined with clay in a kiln. The process could be powered by clean electricity, and the separated CO2 would be pure and easily captured for use or storage. Leah Ellis, who was then a post-doc at MIT and the lead researcher on the process, has since spun out an early-stage startup called Sublime Systems to commercialize the technology.
Alternatively, the formulation of concrete could be changed to reduce or eliminate cement. Carbicrete has developed a process for producing pre-cast concrete that replaces cement with steel slag, a waste product from steelmaking. Because CO2 is absorbed through their process, Carbicrete’s concrete blocks are carbon-negative.
Pathway 2: Change the kiln
Cement kilns are typically coal-fired, and the ultra-high temperatures required (reaching 2,700 degrees Fahrenheit) make it challenging to switch to an alternative fuel. There are some small gains to be made in increasing energy efficiency and shifting to lower-carbon fuels like natural gas or biomass, but major carbon reductions require a more significant change.
Decarbonizing existing cement plants could be accomplished by capturing carbon dioxide directly from flue gas. Several point-source carbon capture startups have begun partnerships with some of the largest global cement manufacturers. Carbon Clean has announced a pilot project with CEMEX in California and Svante is working with LafargeHolcim at a plant in British Columbia, both focused on developing cost-competitive carbon capture.
Fossil fuels could be replaced by hydrogen or direct electrified heat to reach high temperatures without fuel emissions. However, neither approach has been significantly tested, and using a zero-carbon fuel would not address the 65–70 percent of emissions stemming directly from the chemical process of converting the calcium carbonate in limestone to lime (CaCO3 → CaO+ CO2). The European Union has partnered with a consortium of cement industry titans on a project called LEILAC to develop a new kiln design where process emissions, which are almost pure CO2, are kept separate from the fuel exhaust, making carbon capture of the process emissions much easier.
Pathway 3: Change the aggregate
Concrete is made by combining cement with aggregate material, typically a mixture of sand and gravel. Cement only accounts for approximately 10–15 percent of concrete by volume but is responsible for virtually all the emissions in the final product (thus, a current opportunity to somewhat lower emissions is to reduce the amount of cement in the final concrete mix). One breakthrough opportunity to reduce the climate impact of concrete is to use aggregate materials with ‘negative emissions’ by trapping CO2 in the rock. By reacting carbon dioxide with minerals such as calcium or magnesium oxide, carbon dioxide is mineralized and permanently locked into stable rocks. Because the material grows larger through carbonation, the process can produce more aggregate by volume.
German startup Neustark is recycling old concrete from demolition sites into fresh concrete by binding carbon dioxide to the recycled material, trapping CO2 and reducing the amount of cement needed. They claim that 10kg of CO2 is captured and 20kg of CO2 emissions are avoided in each cubic meter of their concrete. BluePlanet, a Silicon Valley startup backed by Leonardo DiCaprio, is creating aggregates of varying sizes with a synthetic limestone coating that consists of 44 percent CO2. Their method can use flue gas containing CO2 without requiring an additional step of purifying and separating the CO2, making it a direct carbon capture solution for industrial sites and power plants.
Pathway 4: Absorb CO2
In a somewhat ironic twist, the massive carbon impact of cement production is partially balanced after it is used in construction, as concrete naturally absorbs CO2. The hardening of concrete once mixed with water and air is a process called weathering carbonation, in which the concrete absorbs CO2 from the atmosphere. A large amount of CO2 is absorbed in the initial setting period, but concrete continues to absorb CO2 slowly for several decades, reaching nearly 45 percent of the emissions from cement production over the lifetime of concrete structures exposed to air.
To maximize this uptake, several startups are pursuing applications to set, or “cure,” concrete with CO2 instead of water. Nova Scotia-based startup CarbonCure has deployed their curing process, which not only permanently binds carbon dioxide but makes the concrete stronger and uses less cement, in more than 100 sites across North America. However, scaling up carbon curing at job sites will require developing a network of carbon sources and transport. CarbonBuilt offers a similar curing opportunity for precast concrete producers using diffuse CO2 from industrial flue gas sources near to the concrete plants. CarbonCure and Carbon Built recently won the high-profile Carbon XPRIZE, a five-year global competition for technologies that converted CO2 into valuable products.
Laying the foundation for a climate breakthrough
Major hurdles remain to changing a slow-moving industry. Incumbent cement and concrete companies have already invested massive amounts of capital into long-lived facilities. Meanwhile, the construction industry and its regulators are (understandably) risk-adverse when considering changes to key material formulations to support infrastructure and buildings.
But as demand for greater sustainability rises and regulators target high-emitting sectors, low to zero-carbon cement could be not only the climate choice but the economic choice. Startups pursuing alternative approaches could make their products even more profitable if they can sell at a ‘green premium.’ New technological developments for decarbonizing cement are poised to benefit from climate-focused policies incentivizing carbon capture and utilization, early-stage technology development, infrastructure built with low-carbon materials, and building decarbonization.
The wealth of emerging solutions to decarbonize cement present an opportunity for a rapid climate transition of a vital sector. Even if no solution proves to be the perfect “silver bullet” to zero out emissions, multiple decarbonization pathways along the concrete process could be employed simultaneously and we may ultimately see low or even negative-carbon concrete created using a “silver buckshot” combination of the pathways explored above.
