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 percent of total volume in concrete, it is responsible for about 95 percent of the total emissions. For the Project 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 twofold; reduction in clinker intensity and improvements to the thermal and electricity intensity of a metric ton 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. Thermal and electricity intensity of clinker production is reduced by upgrading cement kilns to modern 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 outweighs the total demand for cement into the future, so the supply of clinker replacements does not bound the model.
Total Addressable Market
Historical data indicate 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 is at 31 percent 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. This market is developed based on estimates from the International Energy Agency, IIASA, in addition to forecasts from peer-reviewed literature.
Adoption scenarios are modeled on average total global adoption of alternative cements. Therefore, we estimate 100 percent 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 on cement standards currently adopted by the global cement industry. Impacts of increased adoption of alternative cement from 2020 to 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 percent, and the 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 percent, and the mitigation result equates to 16 gigatons of carbon dioxide abated by 2050.
The cumulative first cost of implementing the alternative cement solution over the period 2020–2050 is approximately US$5055 billion under the Scenario 2. 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.
Scenario 1 results show a mitigation impact of 8 gigatons of carbon dioxide-equivalent emissions over the period 2020–2050. Scenario 2 shows a mitigation impact of 16 gigatons of carbon dioxide-equivalent emissions over the same period.
The current study considers a variety of alternative cementitious materials that 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 percent). CEM II and CEM V see increased adoption to nearly 90 percent 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 byproducts, will diminish in supply as coal power and virgin steel production decreases—a situation already being realized on the West Coast of the United States. 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 reabsorption of carbon dioxide by cementitious materials, which have the potential to recover up to 17 percent 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, 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 percent 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.
 All monetary values are presented in 2014 US$.