Multicomponent mixes for low-carbon cements

The concrete industry is leaving no stone unturned in its quest for lower-carbon cements. A new set of ternary blends offer specifiers a much greater range of possibilities, says MPA Cement’s Colum McCague

Cement is responsible for many of the characteristics that make concrete such a versatile and widely used building material. It gives the material its plasticity when fresh and its strength when hardened, as well as its distinctive appearance. Modern cements are more than just grey powder – they contain other, secondary components which each make their own contribution to the resulting concrete’s plasticity, strength and colour. But the grey powder component, known as Portland cement clinker, remains the key ingredient.

Cement is also responsible for by far the greatest proportion of concrete’s embodied carbon emissions, which is why it is such a significant focus of the industry’s decarbonisation activity. Its relative carbon intensity is due partly to the energy required to operate cement kilns at high temperatures, and there is intensive research and experimentation underway into alternative fuels. Harder to abate will be the process emissions resulting from the chemical reaction taking place inside the kiln, which is why secondary components play another important role by replacing a proportion of clinker to lower the embodied carbon of the mix.

The UK traditionally produces cements with a maximum of one secondary component alongside clinker, with small quantities of gypsum to regulate setting. But research currently underway into lower-carbon formulations has demonstrated the potential benefits of multiple cementitious components, particularly when limestone powder is used in combination with ground granulated blast-furnace slag (GGBS), fly ash or calcined clay.

Fly ash, GGBS and finely ground limestone are already commonly used as secondary components in cement. In optimised quantities, each of these can improve concrete’s strength while altering its appearance – making it lighter in the case of GGBS andlimestone, and darker in the case of fly ash. Concrete that contains cements based on fly ash or GGBS gives off less heat as it sets and is more resistant to chemical attack once hardened.

However, as early strengths tend to be lower – particularly at high levels of clinker substitution – it is necessary to strike the right balance for the application.

Limestone with GGBS or fly ash

Limestone powder is abundantly available in the UK but due to its limited chemical activity it is used to substitute clinker in lower quantities than fly ash or GGBS. When used alongside other components, however, it may be a different story. In 2017, the Mineral Products Association conducted a review and found that limestone powder could be better utilised if mixed with other common secondary components such as GGBS or fly ash.

In 2018, it formed a research consortium, part-funded by the government under the BEIS Industrial Energy Efficiency Accelerator programme, which is managed by the Carbon Trust and Jacobs, to manufacture, test and demonstrate new multi-component cements containing limestone powder. Hanson Cement manufactured a series of cements that used it in combination with clinker and fly ash,and with clinker and GGBS, at its National Technical Centre in Scunthorpe, and these were subjected to extensive strength and durability testing by BRE at its Watford laboratory.

The project successfully proved that multi-component cements, when optimised, work more efficiently in concrete than their traditional analogues, allowing up to 65% clinker substitution. Each of the low-carbon formulations is covered by the European cement standard,EN 197-5:2021. After completion of the concrete testing in 2021, it was recommended to the British Standards Institution that the new cements be recognised in BS 8500 as general-purpose cements. This will give specifiers more low-carbon cement options, offering savings of up to 60% eCO2 compared with Portland cement CEM I.

Limestone with calcined clay

Calcined clay refers to clay that is thermally treated to unlock its pozzolanic properties (pozzolanic refers to a material’s ability to react with lime and water to produce a hardened cement, with lime arising from the reaction between Portland cement clinker and water). Metakaolin is the most wellknown type of calcined clay used in cement. It is produced from kaolin, a high-grade clay, and is generally limited to around 10-15% clinker substitution due to economic cost and workability considerations.

Its use in the UK is however currently limited to specialist applications, as most metakaolin is produced elsewhere in Europe or from Asia and the Americas and local kaolin is predominantly used in the paint, ceramics and paper industries. Lower-grade clays are abundantly available, but they have yet to be tested or trialled in the UK. Experience in other countries shows that the reactivity of calcined lower-grade clays ranges widely from relatively inert to relatively reactive (somewhere between the reactivity of fly ash andmetakaolin). This presents a significant challenge to their use, when little is known about the reactivity of low-grade UK clays.

Fly ash and GGBS are by-products of industrial processes that are in decline in the UK – respectively, coal-fired power generation, and the manufacture of iron and steel via the blast furnace route – and this trend may eventually be seen globally. While they will continueto be a viable, proven way to reduce embodied carbon in the short to medium term, they are unlikely to remain the dominant solution in the coming decades. So research is under way into alternatives.

Limestone with calcined clay is one potential alternative. According to BS EN 197-5:2021, it is possible to achieve as high as 50% clinker substitution when limestone powder is added alongside calcined clay in a multi-component cement. An MPA-led consortium, co funded by the UK’s innovation agency, Innovate UK, has begun to investigate the potential of new cements containing UK-sourced calcined clays. The project partners will assess the feasibility of producing calcined clays from lower-grade clays, specifically those reclaimed from extraction or other manufacturing processes. Reclaimed clay sources include waste bricks, which do not require heating as they have already undergone thermal treatment, as well as large reserves of overburden clay materials at quarry sites.

A total of 10 clay sources will be rigorously tested by Imerys Aluminates and University College London, with parameters such as kiln temperature and particle size optimised to allow the highest-possible clinker substitution in cement formulations. Low-carbon multi component cements will be formulated and tested for conformity against current standards. Following this, the cements will be sent to the University of Dundee for rigorous strength and durability testing in concrete, with the aim of establishing their suitability as general purpose cements in BS 8500.

It has been well documented that the reduced workability of concrete containing calcined clays restricts the level of clinker replacement. It is therefore necessary to develop and investigate new water-reducing admixtures which are compatible with calcined clays, and the project will include working closely with the Cement Admixtures Association to achieve this.

New colours

The evolution of secondary components in cements will likely yield a greater range of options for concrete finishes and colours. Following the update to BS 8500, up to 20% limestone powder will be included in most cements, resulting in concretes of a slightly lighter shade of grey. This is especially the case when used in combination with GGBS, and will also result in the colour of the fine aggregate being more evident in the final concrete. The colour of raw clays vary, ranging from almost white to dark grey, as well as reddish hues.

This depends on iron content – with iron-containing minerals causing a darker tone. The thermal treatment of the clays can change their colour, raw grey clay turning to reddish calcined clay for example. This can be controlled depending on the process used. Just as with GGBS and fly ash, concrete using calcined clay based cements will take up some of their colour, the impact depending on amounts used.

Much greater choice for specifiers

The 2006 version of BS 8500 included a total of 15 generalpurpose cements. In the 2015 version, this increased by one to 16. But by the time of the 2019 amendment, there were 48. This reflects demand from specifiers for a greater choice of lower-carbon cements, and the willingness of cement producers and concrete suppliers to respond. A further update of BS 8500 is planned in 2022. As it stands, the MPA proposal to include the new multicomponent cements will more than double the number of general-purpose cements to 112. Multi-component cements will not only be factory-produced in fixed proportions.

For example, concrete suppliers will be able to purchase factorymade Portland limestone cement (ie. containing up to 20% limestone powder alongside clinker) and blend additional secondary components in the desired proportions. These changes, with careful consideration of performance aspects of the concrete, such as strength gain or the heat of hydration, will allow much greater optimisation of embodied carbon.

Photos: Giorgio Marafioti, Iwan Baan, Colum McCague, Hufton + Crow, Elaine Toogood

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A demonstration precast project by the MPA, trialling the use of limestone powder in multicomponent cements. The panel on the far left contains a cement comprising 40% clinker, 45% GGBS and 15% limestone powder. The panel on the far right contains a cement with 55% clinker, 30% fly ash and 15% limestone powder.

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The in-situ external walls of the V&A Dundee by Kengo Kuma were cast using a 27% fly ash mix to reduce embodied carbon and darken the tone, contrasting with the lighter precast-concrete panels in front.

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Chart comparing strength gain of a traditional concrete mix (CEM III/A) with two new multicomponent cement concretes (MCC1 and MCC2). The results for MCC1 show that identical performance to CEM III/A is achieved when 15% GGBS is subsituted for limestone powder. MCC2 shows a slight improvement at early age and identical performance at later ages, even with 5% less clinker.