Beyond Portland cement: Low-carbon alternatives

Clinker substitution (the problem with GGBS and PFA)

An effective way of reducing the carbon associated with concrete construction is to replace a substantial portion of clinker in the cement mix with supplementary cementitious materials (SCMs). Today these are primarily ground granulated blast furnace slag (GGBS) or pulverised fly ash (PFA).

PFA and GGBS are both industrial waste products, so their use in cement reduces the carbon emissions from clinker production. Their effect on concrete is well understood, with high strength and durable mixes being readily available.

The problem with GGBS and PFA is one of supply. The amount of slag available globally is only 5-10% of cement production², with a similar figure for PFA. As by-products of high-carbon industries, their availability will decrease further as coal power stations are shut down and more steel is recycled over the coming years.

Almost all of the GGBS and PFA currently produced in the UK is already being used, primarily in concrete, so specifying these SCMs does not necessarily help to reduce construction sector emissions. In addition, most slag production is localised to a few iron-producing countries, whereas cement demand is far more pervasive. To meet the massive demand for cement globally, new SCMs are needed which are both abundant and widespread.

Alternative SCMs

Cementitious materials are broadly made up of calcium, silicon, and aluminium oxides in various proportions. Unlike calcium, natural sources of silicon and aluminium are not present as carbonates, and therefore do not undergo decarbonation when calcined for cement.

One plentiful and readily available raw material is clay. Clays with a high kaolinite content, rich in silica and alumina, have been shown to be highly pozzolanic if calcined at 700-850°C². They exist worldwide in massive quantities, so vast as to be effectively unlimited¹. Another abundant SCM is limestone, which can be ground for use at modest ratios in cement in its uncalcined state.

Figure 2a: Diagram showing the composition range of various virgin and secondary cementitious source materials

Figure 2b: Availability of common SCMs

Limestone calcined clay cements (LC³)

A combination of calcined clay and limestone can directly replace a high proportion of Portland clinker in cement. This is known as Limestone Calcined Clay Cement, or LC3.

In European standards, ternary blends such as CEM IIB-M(Q-LL) already allow for a clinker content of just 65%, with calcined clays (Q) and limestone (LL) making up the rest. There is a proposed extension to the standard that will allow clinker content to be reduced to 50%, but this does not currently include calcined clay as an approved substitute.

A blend comprising just 50% clinker with 30% calcined clay, 15% limestone and 5% gypsum has been shown to produce cement with mechanical properties comparable to a CEM I blend from seven days, with better durability and with a 30% reduction in CO₂ emissions⁴. This technology has already been successfully trialled in in Cuba and India, including a concrete demonstration house in Jhansi, India which resulted in a 15.5t carbon saving⁵. It should be straightforward to include calcined clays in higher quantities in future technical standards.

Figure 3: House in Jhansi, India, built from 98% LC3 concrete

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.

Summary

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.

References

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