The ingredients of concrete are associated with a colossal environmental footprint. The Princeton Student Climate Initiative found that each pound of concrete emits 0.93 pounds of carbon dioxide into the atmosphere. These emissions originate from both the energy used to heat the material and the chemical reaction that occurs when the mixture is exposed to heat. The process is so energy intensive that the Energy Information Administration dubbed it the “Most energy intensive industry of all”. In total, the concrete industry releases 4 billion tons of carbon dioxide every year, contributing to 8 percent of global emissions. Cement only accounts for 10 to 15 percent of concrete by mass; however, it is responsible for 90 percent of its carbon dioxide intensity. The 3 billion tons of Portland cement produced annually cause the emission of more than 2.8 billion tons of carbon dioxide.
The world’s population is growing and urbanizing. An additional 2 billion people are projected to be born within the next 30 years. In addition, further economic development is expected to help most of the world’s poorest countries join the global middle class. According to Our World in Data, as populations get richer, they also tend to urbanize. They expect that over two-thirds of the world’s population will live in urban areas by 2050, meaning that 3 billion more people will live in cities than those who do today. With the human population and its needs expanding, infrastructure development must increase in order to meet its needs; and the use of concrete will play a major role. Due to the enormous scale of this industry, innovations in cement making processes can bring about very significant economic and climate benefits.
Concrete contains two main components. The first is an aggregate of sand and gravel. The aggregate acts as filler material and absorbs the compressive stress in the concrete. The second component is cement that acts as a glue when hardened through the process of curing.
Conventional cement, called Portland cement, is manufactured through the controlled chemical combination of calcium, silicon, aluminum, iron, and other materials. During the cement-making process, ingredients are heated to form a rock-like material and then ground into an extremely fine powder.
The most common modern method of producing cement is the dry cement-making process. The step-by-step process to go from raw materials to the finished product is as follows:
Because it has such a large footprint, reducing cement industry emissions is critical for the world to meet its climate goals. Governments have begun to implement policies designed to incentivize change. For example, the Biden administration’s Buy Clean Task Force prioritizes the use of low-carbon concrete in public construction projects, supported by a direct investment into decarbonization. Even some cement companies, such as Heidelberg, Holcim, and Cemex, have set targets to address their own footprint using the Science Based Targets Initiative. This new wave of incentives and technologies is creating new options for the cement industry to reduce its emissions. However, long development cycles of large, centralized, capital intensive projects have made adoption of new technologies slow, and the low volumes typical of emerging products make it difficult for them to meet industry needs for massive scale.
With so many angles to tackle the problem of cement industry carbon emissions, multiple startups have emerged with proposed solutions. These solutions usually target one of the main areas of focus for emissions reduction.
Fossil fuels can be substituted for alternative sources, such as tires, household waste, or commercial waste, to fuel the kiln. While this does displace the usage of fossil fuels, combustion releases the pollutants and carbon that are held in these sources instead.
Furno is developing a new, high-efficiency modular kiln that uses natural gas instead of coal to decarbonize cement. Unlike traditional centralized plants that leverage size and scale to achieve profitability, Furno leverages the low-cost strategy of mini-mills and applies it to the cement industry by building small, efficient and modular cement plants. By using natural gas instead of coal or oil, their technology can better leverage carbon capture to get to net zero emissions.
Sublime Systems is developing an electrochemical process that is an alternative to traditional cement. Originally, the company separated the carbon dioxide from flue gas produced by kilns in order to store it. In January 2023, Sublime Systems closed a $40 million Series A funding round to increase production at its pilot plant, build its team, test its product, and advance its offtake commitments. Sublime Systems’ newer electrochemical system does not require a kiln, producing lime using off-peak renewable electricity and calcium sources that do not release carbon dioxide. Their technology can eliminate carbon dioxide from the cement making process.
The feedstock materials for clinker production can be swapped out for alternatives that do not require the calcification of limestone, a significant carbon dioxide emitter. Supplementary cementitious material (SCM), such as fly ash from coal-fired plants or blast furnace slag from the steel industry, can be used as building materials or substituted for clinker in cement mixtures. However, both minimum required clinker ratios and the availability of SCMs limit the amount of clinker that can be displaced in mixes. Instead of the traditional calcification process, bacteria can also be used to break down limestone, creating bio-cement that can be reconstituted into bio-concrete.
Ecocem is an Irish company, founded in 2003, that produces low-carbon cement for the construction industry. Their cement alternative is made from recycled metallurgic slag, a by-product of the metallurgic production of iron, steel, and other metals. 70 percent of the cement in concrete can be replaced by Ecocem’s product, reducing emissions by up to 70 percent.
However, as a by-product, metallurgic slag has only limited availability. There are approximately 600 million tons of iron and steel slag produced every year, a large share of which is already reprocessed and circulated back into the metal making process. However, using all slag for cement and filler alternatives would reduce global cement-related consumption emissions by about 20 percent. While this is progress, it doesn’t come near net-zero unless it is paired with other solutions to replace the remaining 80 percent.
Based out of Los Gatos, California, Blue Planet has developed scalable mineralization technology that converts carbon dioxide with calcium sourced from waste to manufacture a synthetic limestone aggregate that can be used as a substitute for the real thing in cement production. Each ton of their aggregate permanently stores 440 kilograms of carbon dioxide. Given that their process is not just carbon neutral, but carbon negative, the company projects that the storage needed to keep temperature rise below 1.5 degrees Celsius could be achieved if 16 percent of all aggregate used today were replaced with their synthetic limestone by 2050.
Carbon dioxide can be separated from other gasses in kiln flue gas and stored locally or underground, or utilized onsite to produce carbon-neutral materials and products. Although carbon capture is one method of offsetting cement production emissions, it increases operation costs and is limited by the availability of transport and storage infrastructure.
Founded in 2007 in Dartmouth, Nova Scotia, CarbonCure injects captured carbon dioxide into concrete during the mixing process. When cured, the carbon dioxide mineralizes and becomes one with the stone, where it can remain stored for millennia, even if the building is demolished. This reduces emissions by 15 kilograms of carbon dioxide per cubic meter of concrete (about 6%). The company claims that the amount of stored carbon can be tracked, and that so far, CarbonCure has removed over 260,000 tons of carbon dioxide, 105,000 of which has been sequestered in the past year. This is equivalent to 0.0027% of annual global cement-related emissions. The added carbon does unfortunately make the concrete more brittle, which makes it less ideal for construction.
In addition to their technology, CarbonCure bases their success on a strong business model using Carbon Credits. Because of the performance, verifiability, and scalability of their solution, CarbonCure sells carbon removal credits that companies and individuals who want to offset their emissions can buy. Additional revenue from these sales is shared with local concrete producers to encourage more widespread adoption of CarbonCure’s technology. The team’s investors include Bill Gates, Sustainable Development Technology Canada, Innovacorp, Pangea Ventures, Eagle Cliff Partners, BDC Capital, and Amazon’s Climate Pledge Fund.
Carbon Limit, a South Florida-based climate-tech company, produces a cement additive that gives concrete the ability to capture carbon directly from the atmosphere in the curing process. Their product, CaptureCrete, is made of non-calcined materials that are highly reactive to carbon dioxide. This allows the concrete to permanently store carbon through the process of carbonization. Their product is eligible for carbon credits. In August 2022, the Minnesota Department of Transportation used Carbon Limit’s cement in 35 new pavement mixes laid along a quarter mile of major US interstate. The company’s first major deployment will serve as a 3-year pilot and case study to measure its performance.
With the human population and its needs expanding, infrastructure development must increase in order to meet its needs. As a result, the production of concrete shows no signs of slowing, and neither does its massive carbon footprint. This stands in direct contrast to sustainability goals of a decarbonized world that has been limited to only 1.5 degrees Celsius of warming.
In order to meet emissions reductions goals, new cement alternatives are needed. Successful startups must overcome barriers to creating cost-effective and durable construction materials at scale, either by using materials with a lower environmental footprint, reducing kiln fuel emissions, or using carbon storage projects to offset emissions.