Pozzolans are supplementary cementitious materials that can partially replace Portland cement in construction projects.
Pozzolans hold key to reducing emissions
The construction industry is a major contributor to global carbon dioxide (CO2) emissions, primarily in cement production. However, pozzolans can provide a sustainable solution to address this issue, says OMAR AL OTAIBI*.
01 July 2023
The construction sector accounts for a significant share of the global CO2 emissions, with cement production being a major contributor. And with cement being a fundamental ingredient in concrete – which is the most widely used construction material worldwide, it is crucial to explore sustainable alternatives that can reduce the environmental impact of cement production.
Concrete significance
Concrete finds extensive use in the construction of bridges, buildings, runways, roads, sidewalks, and dams across the globe. While being the most crucial construction material worldwide, its usage is projected to grow substantially in the future. The primary binding ingredient in concrete is ordinary Portland cement (OPC). Hence, cement production is estimated to reach a staggering 4.4 billion tons by 2050.
The production of cement involves the release of CO2 during the calcination of limestone and the combustion of fossil fuels in kilns. These processes generate significant amounts of CO2 emissions, contributing to climate change and environmental degradation. On average, the production of one ton of cement results in the release of approximately 0.8 to 1 ton of CO2 into the atmosphere. This highlights the urgent need to find innovative solutions to reduce CO2 emissions in the construction industry.
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Al Otaibi ... the use of pozzolans can result in CO2 emissions reductions of up to 30 per cent. |
Pozzolan, a Sustainable Alternative
Pozzolan is a supplementary cementitious material (SCM) that can partially replace Portland cement in construction projects. It has been used for centuries, with historical evidence of its application in ancient Roman structures. Pozzolans consist of fine particles that contain reactive silica and alumina compounds. The specific composition of pozzolans varies depending on the source material. For example, fly ash is predominantly composed of silica, alumina, and small amounts of other oxides, while blast furnace slag contains a higher percentage of calcium oxide. These materials possess inherent cementitious properties and can react with calcium hydroxide (lime) in the presence of water, forming additional cementitious compounds. The pozzolanic reaction occurs over time during the curing process of concrete. When a pozzolan is mixed with water and cement, it reacts with the calcium hydroxide released from the hydration of cement, forming additional calcium silicate hydrate (C-S-H) gel. The C-S-H gel contributes to the strength and durability of concrete structures.
The reactivity of pozzolans can be influenced by factors such as particle size, specific surface area, and chemical composition. Finely ground pozzolans with a high specific surface area tend to exhibit greater reactivity, providing better pozzolanic properties.
By partially replacing cement with a pozzolan, the CO2 emissions from construction projects can be significantly reduced. The utilisation of pozzolans reduces the need for large quantities of pure cement which, in turn, lowers the overall carbon footprint. Studies have shown that the use of pozzolans in concrete mixtures can result in CO2 emissions reductions of up to 30 per cent compared to conventional concrete made solely with Portland cement. This significant reduction in CO2 emissions demonstrates the environmental potential of incorporating pozzolan in construction projects.
History and Applications of Pozzolans
Pozzolan has a long-standing history as a beneficial ingredient in construction materials. The term “pozzolan” is derived from the town of Pozzuoli in Italy, where volcanic ash deposits were first recognised for their beneficial properties in cementitious materials.
The Romans discovered that by blending volcanic ash, known as pozzolana, with lime, they could produce a hydraulic cement that hardened underwater. This innovation led to the construction of impressive structures such as the Roman aqueducts, bridges, and the iconic Pantheon. The durability and longevity of these structures are a testament to the effectiveness of pozzolan-based concretes.
In modern times, the use of pozzolans has expanded, and various types of pozzolanic materials are now utilised in construction projects worldwide. Fly ash, a byproduct of coal-fired power plants, is one of the most used pozzolanic materials. It is incorporated in concrete production, replacing a portion of Portland cement. Silica fume, another type of pozzolan, is a byproduct of silicon and ferrosilicon alloy production. Due to its high silica content, it offers exceptional pozzolanic reactivity and is commonly used in high-performance concrete applications, such as bridge decks, marine structures and high-rise buildings. Blast furnace slag, a byproduct of iron and steel production, is also extensively used as a pozzolan. It is a glassy material that exhibits hydraulic properties when ground into a fine powder. The use of blast furnace slag not only provides environmental benefits but also enhances the workability and durability of concrete. These materials offer alternatives to traditional cementitious materials and contribute to sustainable construction practices.
Conclusion
From ancient Roman structures to modern-day infrastructure projects, the use of pozzolans has demonstrated its effectiveness in enhancing the properties of concrete while reducing the environmental impact of cement production. By incorporating a pozzolan as a SCM, construction projects can achieve significant CO2 emissions reductions. The utilisation of various types of pozzolans, such as fly ash, silica fume, and blast furnace slag, offers versatility and adaptability to meet the specific requirements of different construction applications. By embracing this sustainable solution, the construction industry can contribute to global efforts in reducing CO2 emissions, while simultaneously improving the performance, durability, and resilience of concrete structures. n
* Omar Al Otaibi is an engineer at Rua Al Madinah Holding, Saudi Arabia.
References:
• IEA Technology Roadmap – Low Carbon Transition in the Cement Industry. https://www.iea.org/reports/technology-roadmap-low-carbon-transition-in-the-cement-industry
• The GCCA 2050 Cement and Concrete Industry Roadmap for Net Zero Concrete. https://gccassociation.org/concretefuture/wp-content/uploads/2021/10/GCCA-Concrete-Future-Roadmap-Document-AW.pdf
• Carbon Dioxide Uptake by Cement-Based Materials: A Spanish Case Study. https://www.mdpi.com/2076-3417/10/1/339
• Kamal K. Kar (2022) Handbook of Fly Ash.
• F. Pacheco-Torgal et al, 2015 Handbook of Alkali-Activated Cements, Mortars and Concretes.
• National Pozzolan Association: The History of Natural Pozzolans” https://pozzolan.org/history-pozzolans.html
