The commercial availability of calcined clays is currently limited compared to the sheer quantity needed, as the cement industry is currently geared towards Portland cement production, but the raw materials are sufficiently abundant. Calcined clay limestone cements have the potential to dramatically expand the use of SCMs as partial clinker replacement and make significant contributions to CO2 emission reduction.
Alkali-activated materials / geopolymers
In the long term, alternative cementitious materials have the potential to replace up to 100% of Portland clinker, GGBS and PFA in cement mixes. Some raw materials high in alumina and silica include volcanic rocks (common in southern Europe, the Andes and the Middle East), lateritic soils (common in the tropics) and clays high in kaolin (common worldwide). These raw materials are widely distributed and exist in quantities vastly exceeding global cement production6.
They can be heated and crushed to form a powdered precursor for use in cement. The resulting product is known as an alkali-activated material, or a geopolymer when minerals lacking calcium are used. When combined with a strong alkali ‘activator’, the precursor reacts to form a hardened binder. In the case of geopolymers, this binder consists of aluminosilicate phases, rather than calcium hydrates present in Portland cement, but with similar properties. This technology may play a critical role in achieving the emissions reductions required of the construction industry.
Various commercial alkali-activated cements have already been developed, such as DB Group’s Cemfree and Cemex’s Vertua which are available in the UK today, but both are reliant on GGBS as the precursor. In Northern Ireland, Banah UK Ltd developed a geopolymer cement based on calcined clay, and claimed a 75% reduction in carbon emissions, but the company shut down in 2019 for commercial reasons.
Commercially available, low-calcium geopolymer cements based on natural resources have the potential to dramatically reduce global concrete emissions, but they are still a nascent technology with significant R&D required to create a viable product.
Reducing construction industry emissions by the extent necessary to limit warming to 1.5°C will require a huge increase in the use of alternative cements and a shift away from Portland clinker, GGBS and fly ash, in addition to lowering the overall demand for cement where possible. While these new materials currently have a limited production capacity compared to the vast quantities required, it is important we educate ourselves and be prepared to incorporate these alternative cements into our designs as they become available. Engineers can help create a market for, and build acceptance of, these alternative cement products. The future of the cement industry could conceivably involve the development of a wide range of cements based on locally available, highly abundant, low carbon natural resources, with massive reductions in embodied carbon as a result.
1 Karen L. Scrivener, Vanderley M. John, Ellis M. Gartner (2018) 'Eco-efficient cements: Potential economically viable solutions for a low-CO2 cement-based materials industry', UN Environment
2 Karen Scrivener, Fernando Martirena, Shashank Bishnoi, Soumen Maity (2018) 'Calcined clay limestone cements (LC3)', Cement and Concrete Research, 114, pp49-56
3 Jannie S. J. van Deventer, Claire E. White, Rupert J. Myers (2020) 'A Roadmap for Production of Cement and Concrete with Low‑CO2 Emissions', Waste and Biomass Valorization (2020)
4 Maria C.G. Juenger, Ruben Snellings, Susan A. Bernal (2019) 'Supplementary cementitious materials: New sources, characterization, and performance insights', Cement and Concrete Research, 122, pp257-273
5 LC3 in use: Applications
6 John L. Provis (2018) 'Alkali-activated materials', Cement and Concrete Research, 114, pp40-48