Decarbonizing Concrete

While the two terms are casually used interchangeably, cement is in fact a component in concrete; it is the material that is used to bind particulate materials, such as gravel, together to form concrete’s signature durability and strength. Most cement used today is hydraulic cement—a water-activated reaction in the cement that allows it to bind, set, and harden. Contemporary concrete typically employs Portland cement, a process utilizing limestone and other additives developed in the United Kingdom in the 19th century.

Concrete is the most widely used material in construction around the world, and California is no different. Between its durability, cost, adaptability, and availability, concrete has become ubiquitous. As a consequence, the production lifecycle of concrete is also one of the largest single sources of carbon emissions globally.36 The process of manufacturing traditional cement and concrete is a highly carbon-intensive process. Every pound of cement results in the emission of approximately 0.9 pounds of CO2. In California, the production of cement accounted for about two percent (7.5 MMTCO2e) of statewide carbon emissions in 2020, and just below 10 percent of total industrial sector carbon emissions. While increasingly ambitious targets towards emissions reductions had long focused on switching over electricity and transportation to more sustainable systems, concrete’s significant contribution toward CO2 emissions has now led a spotlight toward innovations, methods, and policies which will reduce its impact.

The pursuit of decarbonized concrete should not come at the expense of a healthy pace of construction in the state. As many of the emerging green cement technologies promise a similar or even lower per-ton cost compared to ordinary Portland cement, the state should seek to facilitate the transition to green concrete processes rather than punish manufacturers. As California must be cognizant of housing goals and affordability, as well as the necessity of other infrastructure projects, ensuring a proper supply of concrete for those new constructions should be one of the priorities. Both the industry and the state ought to be committed to decarbonization.

History of Concrete in California
  • The manufacture of traditional cement and concrete is a highly carbon-intensive process. Every pound of cement results in the emission of approximately 0.9 pounds of CO2, with approximately half of that resulting from the heating of kilns and the other half released by the chemical reaction of calcination.

  • California’s cement plants account for two percent of total statewide carbon emissions and almost 10 percent of industrial emissions, and have done so consistently over the past two decades.

  • While concrete’s end uses are diverse in purpose, it is almost entirely sourced from seven plants in California, though a plant in Cupertino closed in 2020. Cement plant-associated emissions at the eight plants in California have grown by 26 percent from 2011 to 2021 in California.

  • Without including the plant that closed, the emissions from the remaining seven operational plants have increased by 46 percent between over the same time frame.

  • Cement is used in almost every sector of construction, with no single use accounting for more than 20 percent of statewide consumption.

  • Although California’s cement plants are marginally more emissions-efficient on a per-ton basis than the average American plant, they emit more CO2e per ton of cement than plants in the rest of the world. For example, they emit about 33 percent more than plants in China and India.

The State of Green Concrete Today
  • 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. California’s cement industry faces challenges in supply for both of these materials.

  • The most evident issue with many existing SCMs is that while they can reduce a significant amount of Portland cement produced for construction, they cannot replace it entirely—thereby preventing the possibility of a net zero-emission concrete.

Emerging Technologies and Processes
  • Constraints on traditional clinker replacements have spurred on the development of alternative SCMs which could similarly shrink the carbon output of cement while avoiding supply issues.

  • Some of these emerging SCMs are non-hydraulic; in that, unlike ordinary Portland cement, they are not cured by adding water to the mix, but usually by using CO2. This process greatly reduces the carbon emissions from cement and can even be a method of carbon capture.

  • Electrification of cement kilns and other methods of power use reduction have been a point of interest as a method of reducing process emissions. The process of creating ordinary Portland cement requires a temperature of nearly 1500 °C at some stages, and this high heat requires the burning of coal or other fossil fuels.

  • However, many emerging SCMs—especially those that can substitute for Portland cement wholly—may not require the same high temperatures as current conventional processes do.

Modeling Adoption of Green Concrete Technologies
  • To develop a model for quantifying green cement technology’s potential in reducing carbon emissions from the production of concrete, the analysis projects California’s cement consumption through 2035 using a combination of expected housing construction and linear projections, based on the previous decade of use, of other uses for cement.

  • This chapter explores three scenarios—business as usual (BAU), moderate adoption, and rapid adoption. Several assumptions shape the base case scenario.

  • With the most aggressive adoption model, per-ton emissions of cement can reach below 30 percent of business-as-usual by 2035—leading the industry to reduce approximately 6.9 million metric tons of CO2e in that year compared to BAU.

  • The moderate adoption model projects a 35 percent saving on emissions over the business-as-usual model, with approximately 3.4 million metric tons of CO2e saved. This amount equivalent to 760,000 gas vehicles’ annual emissions, or 430,000 homes’ annual energy usage.

  • Under the rapid adoption model over the full timeframe of 2023 to 2035, California stands to avoid almost 29.5 million metric tons of CO2e emissions compared to the business-as-usual model. This is more than the annual emissions of Denmark, 6.5 million passenger vehicles’ annual emissions, or the annual energy usage of 3.7 million homes.

  • As a percentage, the rapid model represents a 24 percent reduction in emissions over the 2023-2035 period, and the moderate case only a 9 percent saving, when compared to the business-as-usual scenario.

Policy Options to Accelerate Decarbonization of the California Cement Industry
  • The policy options for reducing emissions from concrete fall into three buckets—regulatory, fiscal, and other.

  • In order to achieve decarbonization, California should pursue several policies to facilitate a transition toward green cement, such as proactive regulations for building materials approval, utilizing the purchasing power of state agencies such as CalTrans, and providing financial incentives for producers and investors to invest and switch over to emerging SCM technologies.

  • California can also look to the European Union’s policies which have spurned greater interest in green concrete technology and adoption.


36 IEA (2018), Technology roadmap – low-carbon transition in the cement industry, IEA, Paris