Concrete change: Making cement carbon-negative
Cement is one of the world’s most-used building materials, with production reaching 4.3 billion tons/year in 2014 and growing 5 percent to 6 percent annually. Today, it is responsible for 5.6 percent of global carbon dioxide (CO2) emissions and a major contributor to climate change — if the cement industry were a country, it would be the world’s third-largest emitter. To stay below 2 degrees Celsius of global warming, cement’s carbon intensity must be reduced to near-zero as soon as technically feasible.
Fortunately, the right policies and technologies can make cement manufacturing a net climate benefit. During its lifetime and after demolition, cement naturally captures a significant fraction of the CO2 emitted during its manufacture. When this effect is combined with carbon capture and storage (CCS), energy efficiency technologies and biofuels or electrification, cement can remove more CO2 than it adds to the atmosphere.
In the new book "Designing Climate Solutions: A Policy Guide to Low-Carbon Energy," my co-authors and I identify a suite of policies such as carbon pricing, industry efficiency or emissions standards, and government research and development (R&D) support to help ensure the necessary technologies exist and incentivize their use.
Our research shows (PDF) that depending on the extent thermal fuel supply is decarbonized, a CO2 capture rate between 53 percent and 80 percent will make cement carbon-neutral, and higher CCS capture rates achieve net carbon-negative cement. This offers the prospect of a world where simply constructing buildings and infrastructure reduces atmospheric CO2 concentrations and contributes to the fight against climate change.
The composition of cement emissions
Cement is a constituent of concrete, and 60 percent to 70 percent (PDF) of its manufacturing emissions come from the breakdown of limestone into CO2 (which is released to the atmosphere) and lime. Most remaining CO2 emissions come from burning fuels, usually coal, to heat input materials.
Preventing dangerous global warming levels requires reducing CO2 emissions to zero later this century. In fact, most scenarios that avoid dangerous global warming involve "negative emissions" — such as strategies for removing CO2 from the atmosphere in the late 21st century.
The challenge is finding cost-effective technological approaches to carbon removal that work at a global scale. Because cement is already used in tremendous quantities worldwide, it is an overlooked and promising option for CO2 removal.
Cement can remove CO2 from the atmosphere
After manufacture, cement undergoes a "carbonation" process wherein concrete exposed to CO2 and humidity slowly bonds with the CO2, storing the carbon in mineral form. Because concrete is porous, CO2 slowly can diffuse into concrete, carbonating the cement to a depth of 60 millimeters or more over a number of years.
Although many (automatic PDF download) studies have explored carbonation, a recent study by Xi et al provides the best available estimates of the rate and magnitude of the carbonation process worldwide, finding roughly a third of cement’s process emissions (emissions from limestone breakdown) are re-absorbed within the first two years. Over the course of decades, this share rises to about 48 percent.
When natural carbonation is combined with measures to decarbonize or improve the energy efficiency of cement-making, as well as technologies to capture CO2 emitted during cement manufacturing, it can result in a net atmospheric CO2 reduction.
Policies to reduce CO2 from cement-making
To make carbon-negative cement possible, cement manufacturers must adopt new technologies. Several policies can provide incentives and assistance to industry, including:
- Carbon pricing, such as a carbon tax or a cap-and-trade system, that give cement makers financial incentives to install carbon capture equipment and other upgrades.
- Industrial energy efficiency or industrial process emissions standards that help ensure all producers meet minimum requirements, shoring up the lagging producers or facilities.
- Government research and development support that drives down the costs of new technologies including carbon capture, new biofuels and novel techniques for electrical generation of the high temperatures used in cement-making.
Measures to reduce CO2 from cement
Cement manufacturers can pursue a number of CO2 emissions reduction options:
- Thermal energy efficiency: The amount of fuel used to heat input materials can be reduced. For example, cement makers can use a dry-process kiln, which uses input materials with lower moisture content, so less energy is needed to evaporate water. Similarly, using a precalciner and a multistage preheater can dry input materials before they enter the kiln.
- Fuel switching and electrification: Worldwide, 70 percent of cement’s thermal energy demand is met with coal. Biomass and waste fuels can substitute for coal. Longer-term options may exist for electrification of heat creation, such as induction or microwave heating.
- Increased concrete strength: Some building designs can use less cement or concrete while maintaining requisite strength. For example, using curved fabric molds to shape concrete can reduce concrete use by up to 40 percent relative to standard geometries with sharp angles and corners. Other techniques include using high-strength concrete blends or injecting CO2 into concrete while it hardens.
- Improved building longevity: The more often concrete buildings must be replaced, the more concrete must be produced. Today, concrete buildings (PDF) last less than 80 years in much of the world and less than 40 years in East Asia. With high building quality and proper maintenance, concrete structures could last well over 200 years.
- Carbon capture and sequestration: Carbon capture systems must target process emissions and combustion emissions. These systems fall into two categories: Post-combustion technologies aim to separate CO2 from exhaust gases and typically rely on chemical CO2 absorption (for example, by amines). Oxyfuel technologies react fuel with pure oxygen instead of air, generating a purer stream of CO2, and also can capture process CO2.
Reaching carbon neutrality by 2050
The International Energy Agency (IEA) and the Global Cement Sustainability Initiative have estimated that 157 billion tons of cement will be produced worldwide from 2015 to 2050. We have used IEA projections as a baseline, added the effect of carbonation and investigated carbon capture rates that would achieve carbon-neutral cement. A capture rate of 80 percent is sufficient to offset all combustion and process emissions. If thermal fuel supplies can be fully decarbonized, only 53 percent of the CO2 emissions need to be captured.
Pouring concrete can reduce CO2 emissions
Many options can reduce cement manufacturing emissions and even make it a net climate benefit. The right set of policies — including carbon pricing, industry efficiency or emissions standards, and government support of R&D — can help ensure the necessary technologies are commercialized and adopted by industry. Thanks to natural cement carbonation, the challenge of decarbonizing cement is less than it may appear.
Depending on the extent thermal fuel supplies are decarbonized, a CO2 capture rate between 53 percent to 80 percent will make cement carbon-neutral, and higher CCS capture rates can help achieve net carbon-negative cement. This offers the prospect of a world where the act of simply constructing buildings and infrastructure reduces atmospheric CO2 concentrations and contributes to the fight against climate change.