The cement industry is the second-largest contributor to global CO2 emissions, lagging only behind the iron and steel industry. Over the past 60 years, cement production has grown exponentially, surpassing 4 billion metric tons annually and generating nearly 1.7 billion metric tons of CO₂ annually.
The massive CO2 emissions are primarily driven by the usage of clinker, a primary binding agent in traditional Portland cement. Therefore, the key strategy to make cement production more sustainable should focus on providing alternative Supplementary Cementitious Materials (SCMs) that can reduce reliance on clinker without significantly affecting cement structural integrity.
Clinker’s carbon problem and the search for solutions
Clinker production is carbon-intensive, emitting 0.83 tons of CO2 per ton produced. Although clinker constitutes only about 10% of the total mass of concrete, it is responsible for over 90% of its carbon footprint.
The decarbonization reaction (calcination) during clinker production, combined with the energy-intensive nature of the process, justifies why clinker must be the primary target for emission reduction during cement production.
Reducing clinker usage involves substituting it with Supplementary Cementitious Materials (SCMs), which are mineral additions used as a partial replacement for Portland cement. Conventional SCMs, such as fly ash and blast furnace slag, offer significant advantages in lowering CO2 emissions and reducing production costs. Yet, their availability remains constrained, and they can only meet 15% of the demand for cement.
Fly ash and blast furnace slag are produced as byproducts of the coal power and steel industries. These industries are transitioning to cleaner production technologies, making the availability of the byproducts scarce. Therefore, the focus shifts to developing newer, sustainable, widely available SCM alternatives.
The promise of innovative substitutes
In order to serve as an alternative, new SCMs must be abundantly available and globally distributed. The availability of these SCMs will contribute to reducing reliance on conventional and limited industrial byproducts. Studies show that incorporating alternative SCMs can reduce emissions by 6% to 13%, depending on the material and substitution rate.
Some natural and promising alternatives, such as calcined clays, limestone, and agricultural byproducts, can address the availability issues of conventional SCMs. For example, rice husk ash (RHA), produced by burning rice husks, produces 85-94% silica-rich pozzolan with high chemical reactivity if properly processed. However, its adoption has been limited due to variability in quality and challenges in consistent processing. Brazil, Thailand, and China, where rice production is abundant, have significant opportunities to leverage RHA as a low-carbon material.
Another alternative is calcined clays, which are globally available. When clays are calcined, they undergo chemical changes that enhance their pozzolanic activity, becoming a viable replacement for clinker.
Additionally, due to their environmental benefits, materials such as limestone are inexpensive and require minimal processing compared to clinker. For example, reducing clinker content by 8% at a plant in Brazil resulted in annual savings of $86,000 and a reduction of 9,000 tons of CO2 emissions.
Overcoming the challenges of clinker substitutes
Innovative materials that reduce emissions and address clinker dependency have to be capable of maintaining cement’s strength and durability.
A major drawback of lowering the clinker factor is that high-blend cement may exhibit slow or loss of early-age strength development and uncertainty about long-term durability. A 1% increase in SCM content can potentially lead to a decrease in 28-day mortar strength ranging from 0.2 to 0.8 MPa, depending on factors such as SCM type, cement fineness, grinding system, and clinker characteristics.
A promising development to counter the limitations of clinker substitution is the use of synthetic Calcium Silicate Hydrate (C-S-H) as an SCM. Here are some of the key findings:
- Synthetic C-S-H aids in speeding up the hydration process during cement production. This results in improved early strength, which is critical for construction timelines.
- Another approach that can be used in conjunction with this is to incorporate precisely balanced material combinations to optimize cement performance. These mixes ensure high early and late-stage strength, even with reduced clinker levels, and enable clinker content to be reduced to as low as 0.5 without compromising quality. The result is a significant reduction in CO2 emissions from cement production.
- Cement components can be ground to finer particle sizes to enhance chemical reactivity. This adjustment compensates for reduced clinker levels while maintaining robust structural integrity.
The road ahead
The construction industry’s adoption of SCMs requires demonstrating that engineers and builders can meet the same performance and durability standards as traditional cement. Therefore, rigorous testing protocols and industry-wide standards are essential to validate the safety and reliability of SCMs.
Organizations like ASTM International and the European Committee for Standardization have begun addressing this need by developing guidelines for alternative cementitious materials. The message is that establishing robust standards is the most relevant way to promote the adoption of low-carbon materials.
Scaling up alternative SCMs will require sustained research, policy support, and industry collaboration. Investments supporting research and development can lead to new activation methods for materials typically non-pozzolanic and improved processing technologies. Governments can also introduce tax incentives, subsidies, and mandates for green building practices as part of the process. The transition to low-carbon cement presents an opportunity for the industry to reduce emissions, achieve cost savings, and improve resource efficiency.
All research points out that the priority for innovative SCMs (e.g., calcined clays, limestone, and agricultural byproducts) in the cement sector can address its reliance on clinker, and contribute to reducing carbon footprint and ensuring long-term sustainability.
