Fly ash bricks
Technical Summary

Alternative Cement

Project Drawdown defines alternative cement as: the partial replacement of clinker with alternative materials (such as fly ash, slag, natural pozzolans, and calcined clays) to reduce the quantity of clinker in ordinary portland cement systems. Additionally, alternative cements as a solution includes efficiency upgrades to cement plants that produce clinker, reducing its carbon intensity. The practices of making clinker more efficiently and reducing the ratio of clinker to cement reduces the emissions as compared to conventional ordinary portland cement systems.

In 2016, 1.46 gigatons of carbon dioxide was released as a result of cement production (Andrew, 2018). While cement is only about 10% of total volume in concrete, it is responsible for about 95% of the total emissions. For the Drawdown solution alternative cement, the forecasted demand of cement was collected from various global projections. Adoption of the solution within the global cement demand is two-fold; reduction in clinker intensity, and improvements to the thermal and electricity intensity of a tonne of cement. The reduction in clinker intensity is achieved by replacing ordinary portland cement with alternative cementitious materials, or pozzolans which also have binding capabilities. Such alternative materials include industrial waste products such as fly ash and slag, in addition to naturally occurring materials such as natural pozzolans, calcined clays, and limestone which all have lower carbon-dioxide emissions. The amount of clinker reduction is determined based upon the adoption of low clinker to cement ratio mix designs of which there are many. Reducing the thermal and electricity intensity of clinker production is achieved by upgrading cement kilns to modern day standards, such as the use of pre-calciners and dry-kiln technologies.


To measure the impact of alternative cement, a set of adoption scenarios was developed for low-clinker cement in the context of international cement standards (ASTM and CEM). These scenarios were then compared to a Reference Scenario that fixed the adoption of low-clinker 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 ordinary portland cement and low-clinker cement.

The availability of alternative cementitious materials to replace clinker is evaluated. A fly ash availability model is developed based upon time series supply and demand data for coal production and evaluated between 2020 and 2050 as the basis to limit future fly ash availability. The supply of other clinker-replacement materials, such as calcined clays and limestone, largely outweigh the total demand for cement into the future, thus the supply of clinker-replacements does not bound the model.

Total Addressable Market[1]

The historical data indicates that in 2018 the global cement market was about 4061 million metric tons per year with an average clinker to cement ratio of 0.69 – or current adoption of alternative cements are at 31% of market. In 2050, the total global cement market is estimated at 4147 million metric tons per year with varying adoption based upon the scenario in question. This market is developed based upon estimates from the International Energy Agency, IIASA, in addition to forecasts from peer-reviewed literature.

Adoption Scenarios[2]

The adoption scenarios are modeled on average total global adoption of alternative cements. Therefore, we estimate 100% of the TAM’s clinker to cement ratio decreasing from 0.69 in 2018 to between 0.46 and 0.61 by 2050. The varying clinker to cement ratio is based upon existing cement standards currently adopted by the global cement industry. Impacts of increased adoption of alternative cement from 2020-2050 were generated based on two growth scenarios, which were assessed in comparison to the Reference Scenario.

  • Scenario 1: In this scenario, the clinker to cement ratio is 0.61. This corresponds to an adoption of alternative cements equivalent to 39% and mitigation result equates to 8 gigatons of carbon dioxide abated by 2050.
  • Scenario 2: In this scenario, the clinker to cement ratio is 0.46 . This corresponds to an adoption of alternative cements equal to 54% and mitigation result equates to 16 gigatons of carbon dioxide abated by 2050.

Financial Model

The cumulative first cost of implementing the alternative cement solution over the period 2020-2050 is approximately US$5055 billion under the Scenario 2.[3]  Yet the marginal first cost of alternative cements as compared to traditional ordinary portland cement concrete is US$63.5 billion. Cost data for both conventional and solution technologies are collected from cement suppliers and peer-reviewed literature.


The Scenario 1 results show a mitigation impact of 8 gigatons of carbon dioxide-equivalent emissions over the period 2020-2050.  The Scenario 2 shows a mitigation impact of 16 gigatons of carbon dioxide-equivalent emissions over the same period.


The current study takes considers a variety of alternative cementitious materials which can reduce the clinker to cement ratio. This reduction amount is based upon international standards of cement production and includes cement types CEM II – CEM V. CEM III and CEM IV are typically used in specialties construction applications and were weighted based upon their base year adoption levels (combined to be about 10%). CEM II and CEM V see increased adoption to nearly 90% of the market, displacing all market share of CEM I (traditional OPC system) in 2050 by any of the scenarios. Because the model is not bounded by the availability of alternative binding materials, high adoption of the solution is feasible. It is expected the fly ash and slag, industrial by-products, will diminish in supply as coal power and virgin steel production decreases – a situation already being realized on the west coast of the USA. Other alternative materials, such as calcined clays, limestone, and natural pozzolanic materials will replace fly ash and slag. The current study for this solution does not take into account the gradual re-absorption of carbon dioxide by cementitious materials, which have the potential to recover up to 17% of their initial manufacturing, or cradle-to-gate emissions (Souto-Martinez et al. 2018). Due to the pervasiveness of concrete as a construction material around the world, between 1930 and 2013, it was estimated that nearly 18.5 gigatons of carbon dioxide emissions were sequestered over the period 1930-2013, thereby reducing the net greenhouse impact of cement (Xi et al., 2016). Alternative cements does not account for this carbonation, since the carbon uptake of existing concrete systems will occur regardless of the adoption of alternative cements. Additionally, alternative cements do not have the same ability as traditional OPC to sequester carbon dioxide due to different chemistries (less than 10% of initial emissions). 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 either the Reference Scenario or adoption forecasts.


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

[2] 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.

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