The rise of calcined clays
An effective way to reduce CO2 emissions is to use supplementary cementitious materials such as calcined clays, as Mark Tyrer, Christopher Cheeseman and
Alan Maries* argue.
Considerable debate continues as to when the earliest concrete was developed. In many ways, it depends on what is meant by the word concrete, but there are a number of candidates for this honour. A villa at the Lepinski Vir site in Serbia (9,500–7,200 years old) contains concrete-like floors, the Nabataean people in what is now Jordan and southern Syria used lime-bound concrete for lining water storage structures over 6,500 years ago and the settlement of Yiftah El in modern-day Israel has a concrete floor thought to date back 7,000 years.
Examples from Egyptian and Roman times are better known and the exceptionally well-preserved examples such as the Pantheon (Rome, 27 BC–14 AD) and bathhouse at Caracalla (212–4 AD) are rightly famous. They use a cementitious binder that is familiar today – pozzolanic cements. The name pozzolanic is derived from the Pozzuoli region of Naples, which lies within the volcanic caldera Campi Flegrei, in southern Italy – the ashes of which were known as pozzolona. These materials use a source of alkali, mainly slaked lime (CaOH)2 mixed with natural volcanic ash, to provide a source of reactive silica and aluminosilicates. This is waterproof and has proved to be very durable.
Cement and concrete advanced rapidly in the Victorian era, not least through Joseph Aspdin’s 1824 patent for Portland cement and its commercial and technical developments in the 1840s by his son, William. Throughout much of the twentieth century, cements have been blended with industrial pozzolans, such as fly ash from coal combustion and blast furnace slag, a by-product of converting iron into steel. Such materials are attractive because the costs are lower than cement clinker and produce concretes that are extremely durable, often more so than Portland cement concrete. In recent decades, they have shown an important benefit in reducing the embodied CO2 associated with concrete, by diluting the quantity of cement clinker required for production.
Cement and carbon dioxide
There is a lot of misleading information concerning carbon dioxide emissions and cement production. In truth, cement manufacture is one of the most efficient manufacturing processes. The theoretical amount of energy required to make cement is around 2.8GJ per tonne and a modern cement kiln exceeds this by a very modest margin (as little as 3.06GJ/t).
The process requires a certain thermal inefficiency to prevent the kiln from melting, so it seems unlikely that this margin could be closed by a significant amount. To produce cement clinker, a source of calcium, such as limestone or chalk, is ground and calcined with a source of silicate and aluminate – clay or shale – when their oxides combine to produce clinker minerals, which are reactive in water.
The CO2 from the limestone is driven off and these emissions are of considerable environmental concern. Around 800kg of CO2 is released to produce a tonne of cement clinker, about half of which comes from decomposition of calcium carbonate, and the remainder from heating the process and grinding the clinker to a powder. Despite the efficiency of the production process, the enormous scale of use means that the industry is responsible for several percent of global anthropogenic CO2 emissions. Estimates vary between 5% and 10% of industrial CO2 emissions worldwide, but whatever the actual figure, it is the third greatest, after power generation and transport.
The issue was brought into focus by the World Business Council for Sustainable Development, whose study in 2002 recommended a major collaborative R&D effort focused on long term CO2 reduction in the cement and concrete industry. Subsequently, developments in low-energy clinkers, new and improved pozzolans and in the processes underlying clinker production have reduced the overall energy associated with the production of cement and, ultimately, concrete.
The most practical option for CO2 emission reduction is to use supplementary cementitious materials (SCMs), which, when blended with Portland cement, react to consume the excess calcium hydroxide produced during cement hydration, thereby forming a cement that is strong and durable. For many civil engineers, blended cements are the binder of choice in modern concrete, as they reduce the embodied CO2 content.
There is, however, a problem. The major industrial SCMs are cementitious slag and ash, which are diminishing resources. Also, as many countries seek to reduce combustion of coal, the future of fly ash production is much less certain than it once was. When combined with a global shift in steel production from its traditional base in Europe and North America towards countries in the east, the concern about the sustainability of supply is becoming urgent. Of course, these materials can be shipped around the world but that incurs cost and CO2 generated from transport. Given that the objective is to minimise embodied CO2 in concrete, this seems to be fundamentally the wrong approach.
The search for SCMs has come up against obstacles, as the vast scale of cement consumption (now exceeding 4 gtonnes per annum) makes many options unrealistic. One material, however, shows potential – calcined clays.
Clay minerals are layered, hydrated aluminosilicates and when heated, their structure changes markedly. At high temperatures, clays fuse to form ceramics such as bricks or pottery. At lower temperatures, however, the structure is disrupted and the minerals become dehydrated and highly chemically reactive towards alkalis. In this state, they have been shown to be effective pozzolans, reacting with Portland cement in a similar way to other SCMs.
Although the hydration kinetics of clay deposits is a little slower than some pozzolans, their wide availability offsets this disadvantage. The UN recently commissioned a report, Potential, economically viable solutions for a low-CO2, cement-based materials industry, which discusses aspects such as concrete mix-design and specification, illustrating the need for efficient use of materials in concrete construction. The report compares numerous low-energy binder systems but concluded that few are sufficiently mature to provide immediate alternatives to Portland cement. The most promising option is the use of blended cements and the case for calcined clays is presented with conviction. Why, then, has this not resulted in immediate uptake by the cement industry?
The obvious reason is that although there is provision in existing cement standards for their use, the materials are not of uniform composition and are not yet commercially available in high quantities. The construction industry is unlikely to adopt new materials promptly, at least until they have been widely used and proven in practice. It will take time to overcome this chicken-and-egg impasse. However, the availability of the raw materials, combined with increasingly stringent controls on CO2 emissions and the attention from the World Business Council for Sustainable Development and the UN Environment Programme, may force the industry to change sooner rather than later. The global move from exclusive, composition-based standards to inclusive, performance-based standards may provide the mechanism by which this transformation is made.
Developing nations are experiencing rapid urbanisation and population increase, making the demand for affordable housing and associated infrastructure greater now than ever before. If they are to meet this demand using cement and concrete along with their obligations to minimise CO2 emissions, a low-cost pozzolan may become very attractive. Interest in the field is also increasing – the number of publications in the last fifteen years far exceeds the total number produced before 2000.
More work is needed to define a compositional envelope of clay and accessory minerals that will meet the required performance of modern cements. There is much to learn, not least about the hydration and durability of these binders, but calcined clays do have a major role to play in reducing emissions associated with concrete and could well become the standard SCM of the future. To read World Business Council for Sustainable Development GNR Project Reporting CO2 (2014), visit: bit.ly/2km3dZJ
To read Eco-efficient cements: Potential, economically viable solutions for a low-CO2, cement-based materials industry. UN Environment Programme, visit: bit.ly/2zXcD5R
*Mark Tyrer is Professor of Geomaterials at Coventry University, UK.
Christopher Cheeseman is Professor of Materials Resources Engineering, Imperial College, London, UK.
Alan Maries is Principal, AMSTaR Consultancy, London, and Visiting Professor in Environmental Technology at the University of Greenwich, UK.