Article from the journal ‘Mineral Products Today’ published by the Mineral Product Association (PMA). Summer 2024, issue 27, pp. 16-17
Using cementitious industrial by-products as a supplement for Portland cement is a well-established way to make efficient use of resources and reduce net carbon emissions. And new research led by the MPA has revealed another potential source that could deliver even better carbon savings, writes Dr Diana Casey.
The UK has a long tradition of using what’s known as supplementary cementitious materials. The use of GGBS (ground granulated blast-furnace slag) from the steel industry dates back to the early 1900s whilst PFA (pulverised fuel ash or fly ash) from coal power generation gained acceptance in the 1980s.
However, in recent years the UK’s ambition to reach net zero carbon
emissions has resulted in the closure of coal-fired power stations and announcements that steel blast-furnaces will be replaced with electric arc furnaces.
The resulting decline in the availability of GGBS and fly ash has forced the cement industry to look more closely at other local alternatives to meet the growing demand for lower carbon cements and the concretes made with them.
One such alternative is ‘calcined’ clay, a material that has been gathering interest in Europe in the past few years, with the continent’s first calcined clay manufacturing site recently opening in France. Such cements rely on high-value virgin materials including high-kaolinite content clays, which are already in demand in other markets such as ceramics and paints.
One kaolinic clay product, Limestone Calcined Clay Cement (or LC3 for short) has gained particular attention, with claims it can reduce embodied carbon dioxide by up to 40% compared to Portland cement (CEM I) without the need for capital intensive modifications to existing production processes. Yet while kaolinic clays are well known to enhance certain properties of cement, less is understood about lower value clays, such as those from clay quarry overburden and waste from brick manufacture.
This prompted the MPA to kickstart a comprehensive research project aimed at determining if these lower value materials – both raw clays and pre-fired brick waste – could be used as a cement alternative in concrete.
Part-funded by the Government’s Industrial Strategy Challenge Fund, the project involved a consortium of the MPA itself, MPA member companies Heidelberg Materials, Tarmac, Imerys and Forterra, along with the University of Dundee and University College London.
The project looked at different types of unused raw clay from UK quarries. These clays were then calcined, a process designed to bring about thermal decomposition. This enabled researchers to identify the right temperature (around 800°C) for the optimum pozzolanic (or cementitious) reactivity of the resulting calcined clay. Also under test were two types of finely ground brick waste, clay which has already been through a firing process.
The performance of the resulting calcined clays as the active cementitious ingredient in concrete was extensively tested, with promising results across the board. Both the low-value reclaimed clays from quarries and the finely ground waste bricks demonstrated significant potential as supplementary cementitious materials.
The concretes produced with these materials have properties that may differ slightly from reference mixes using other supplementary cementitious materials but the studies show these can be addressed with relative ease. Longer term durability tests are ongoing.
Some minor revisions to the standard specification for pozzolanic materials for use with Portland cement (BS 8615:2019) will be necessary, but the calcined clays performed exceptionally well in both standard concretes and self-compacting concrete mixes.
The results of the MPA-led research provide the cement and concrete industry with the necessary confidence that calcined clays perform similarly to mainstream supplementary cementitious materials and in some cases better.
The findings of this study underscore the significant potential of reclaimed clays and finely ground bricks as cement substitutes, offering a new set of sustainable solutions for the construction industry while contributing to the circular economy objectives of the UK.
Dr Diana Casey is the MPA’s Executive Director for Energy and Climate Change, Cement and Lime.