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Credit: Jamie Bennett


Alternative Cement

The Pantheon was a Roman temple commissioned during the consulship of Marcus Agrippa 2,000 years ago and completed by the emperor Hadrian about 128 AD. After nearly two millennia, the dome remains the largest unreinforced concrete dome in the world. What is more remarkable is that the concrete remains intact, strong and almost ageless. Standing in what is now a church, the oculus at the center of the dome rises 142 feet. Six million people visit it every year.

Cement is a vital source of strength in infrastructure, second only to water as one of the most used substances in the world. It is also a source of emissions, generating 5 to 6 percent annually.

To produce Portland cement, the most common form, a mixture of crushed limestone and aluminosilicate clay is roasted in a kiln. At high heat, limestone’s calcium carbonate splits into calcium oxide (the desired lime content) and carbon dioxide (the waste). Decarbonizing limestone causes roughly 60 percent of cement’s emissions. The rest result from energy use.

To reduce emissions from the decarbonization process, the crucial strategy is to change the composition of cement. Conventional clinker can be partially substituted for alternative materials that include volcanic ash, certain clays, finely ground limestone, ground bottle glass, and industrial waste products—namely blast furnace slag (from manufacturing iron) and fly ash (from burning coal). These materials leapfrog the most carbon-emitting, energy-intensive step in the cement production process.  

The average global rate of clinker substitution could realistically reach 40 percent and avoid up to 440 million tons of carbon dioxide emissions annually. Standards and product scales will be key for realizing the opportunity of alternative cements.


Pantheon temple in Rome: Moore, David. The Roman Pantheon: The Triumph of Concrete. Mangilao, Guam: MARC/CCEOP, University of Guam Station, 1995.

cement one of the most used substances: Scrivener, Karen L., Vanderley M. John, and Ellis M. Gartner. Eco-Efficient Cements: Potential, Economically Viable Solutions for a Low-CO2, Cement-Based Materials Industry. Nairobi: United Nations Environment Programme, 2016.

Decarbonizing limestone…emissions: WBCSD and IEA. Cement Technology Roadmap 2009. World Business Council for Sustainable Development & International Energy Agency, 2009; Amato, Ivan. “Concrete Solutions.” Nature 494, no. 7437 (2013): 300-301. 

4.6 billion tons of cement: USGS. Mineral Commodity Summaries 2015. Reston, VA: U.S. Geological Survey, 2015.

5 to 6 percent of society’s…emissions: Amato, “Green Cement”; Scrivener et al, Eco-Efficient Cements.

blast furnace slag [and] fly ash: Scrivener et al, Eco-Efficient Cements.

clinker substitution…avoid…emissions: Scrivener et al, Eco-Efficient Cements.

strength can…be higher: Amato, “Green Cement”; Crow, James Mitchell. “The Concrete Conundrum.” Chemistry World, March 2008: 62-66; WBCSD and IEA, Cement.

European Union reuses…fly ash: Moon, Steven T. “Regulatory and Legal Application: Fly Ash Use in Cement and Cementatious Products.” World of Coal Ash (WOCA) Conference, Lexington, KY, April 22-25, 2013.

New York City…ground bottle glass: Ellen Macarthur Foundation. The Circular Economy and the Promise of Glass in Concrete. Isle of Wight, UK: Ellen Macarthur Foundation, 2016.

view all book references

Technical Summary

Alternative Cement

Project Drawdown defines alternative cement as: the use of an increased percentage of fly ash instead of Portland cement in concrete. This practice replaces the use of conventional concrete made primarily of Portland cement.

During 2010, 1.6 gigatons of carbon dioxide was released as a result of cement production (Boden, Andres & Marland, 2013). At least 70% of the carbon dioxide emitted in the manufacture of concrete derives from the manufacture of Ordinary Portland Cement: each ton of that cement creates 0.75-1.0 ton of carbon dioxide. For the Drawdown solution alternative cement, a material availability analysis of cement and coal markets was undertaken to assess the potential of replacing Portland cement with fly ash, a byproduct of coal combustion that would otherwise be disposed of in landfills or put to other uses. This mixture is called High Volume Fly Ash cement, and is based on a policy of increasing the mix to an upper limit of 45% fly ash by weight. High Volume Fly Ash cement is currently considered “novel” with a relatively small share of the market (CEMBUREAU, 2013); however, blended cements account for over 70% of market share because of cost savings and environmental benefits (Freedonia Group, 2011). Blending is possible with a variety of waste materials such as iron slag, but this solution is focused on and limited to fly ash because it represents about 80% of suitable waste materials (Karim et al., 2014).


To measure the impact of alternative cement, first a material availability assessment was made for fly ash. Next, a set of adoption scenarios was developed for High Volume Fly Ash cement in concrete formulations, bounded by material availability. These scenarios were then compared to a Reference Scenario that fixed the adoption of High Volume Fly Ash cement at its current percentage of the market. Finally, emissions mitigation and financial results were drawn by comparing scenarios using an analyzed set of variables to describe the relative emissions and costs of Portland cement and High Volume Fly Ash cement.

The availability of High Volume Fly Ash cement material in various regions and countries was estimated with specially developed Material Availability Models. These models were populated with historical and forecast times-series supply and demand data for coal, fly ash, and cement for the period 1980-2050. A scenario of significantly reduced coal consumption was assumed for a 30-year period from 2020-2050 as the basis to limit future fly ash availability.

Given that the models assume that future coal combustion will be significantly curtailed, the alternative cement solution relies on storing fly ash for reuse, diverting it from other uses, and mining fly ash that was previously deposited in landfill.

Total Addressable Market [1]

The historical data indicates that in 2014 the global cement market was about 4100 million metric tons per year, with only about 100 million metric tons (2.7%) of Portland cement replaced by fly ash. This rate of substitution – the current adoption [2] of alternative cement – avoids about 79 metric tons of carbon dioxide emissions per year. The High Volume Fly Ash cement models indicate that if this cement is vigorously applied worldwide, assuming a 10-year phase-in period, the technology could significantly reduce carbon dioxide emissions. 

Adoption Scenarios [3]

Impacts of increased adoption of bioplastic from 2020-2050 were generated based on three growth scenarios, which were assessed in comparison to the Reference Scenario mentioned above.

  • Plausible Scenario: In this scenario, the mitigation result equates to 6.7 gigatons of carbon dioxide abated by 2050.
  • Drawdown Scenario: This scenario keeps the same assumptions for material availability and corresponding impact on emissions related to cement (6.7 gigatons by 2050), because it also considers the integrated perspective that coal power generation and coal incineration in general will be phased out over that time period.
  • Optimum Scenario: This scenario is identical to the Drawdown Scenario, and also results in 6.7 gigatons of carbon dioxide abated by 2050.

Financial Model

The first cost of implementing the alternative cement solution over the period 2020-2050 is approximately US$930 billion. [4] This cost is offset by savings (total savings: US$274 billion), because fly ash is less expensive to produce than Portland cement. The High Volume Fly Ash cement solution could possibly save US$191 billion over 30 years, and provides additional environmental benefits.  


The Plausible Scenario results show a mitigation impact of 6.7 gigatons of carbon dioxide-equivalent emissions over the period 2020-2050.  The Drawdown and Optimum Scenarios have the same results, as this solution is bounded by material availability and so adoption in the Plausible Scenario is already optimal.


As a general guide, the alternative cement solution can only abate around 5% of coal’s original carbon dioxide footprint, so it is much more beneficial to avoid burning coal in the first place - assuming that affordable Carbon Capture and Storage (CCS) is unavailable. The current study for this solution does not take into account the gradual re-absorption of carbon dioxide by cement products, which appears to have resulted in 43% of the original carbon dioxide pollution being sequestered over the period 1930-2013, thereby reducing the net greenhouse impact of cement (Xi et al., 2016). Vulnerability of concrete structures to the corrosive effects of global warming and sea level rise (Saha & Eckelman, 2014) might create more demand for cement in the coming decades; this, too, was not considered in the Reference Scenario or adoption forecasts. If more coal is consumed than expected under any of the forecast scenarios, then the alternative cement solution could have a larger mitigation benefit because of a larger supply of fly ash.

[1] For more on the Total Addressable Market for the Materials Sector, click the Sector Summary: Materials link below.

[2] Current adoption is defined as the amount of functional demand supplied by the solution in the base year of study. This study uses 2014 as the base year due to the availability of global adoption data for all Project Drawdown solutions evaluated.

[3] For more on Project Drawdown’s three growth scenarios, click the Scenarios link below. For information on Materials Sector-specific scenarios, click the Sector Summary: Materials link.

[4] All monetary values are presented in US2014$.

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