LC3 in codes and standards

Reducing the carbon footprint of the concrete industry is a priority in the face of global warming. Limestone calcined clay cements (LC³) provide one of the best options to reduce CO₂ emissions. For optimal implementation, but also to facilitate other low CO₂ solutions, standards that regulate the cement and concrete sector need be adapted quickly. In this article, two main standardisation regions are reviewed in light of the feasibility of adoption and deployment of LC³ technology. The challenges and limitations are identified, and avenues for further development are proposed. By Hisham Hafez, Franco Zunino and Karen L Scrivener, École Polytechnique Fédérale de Lausanne, Switzerland.

The need to urbanise a billion more people globally by 2030, coinciding with the pressing historic high level of CO₂, highlights the need to find sustainable construction solutions.¹ Concrete, due to its low cost, flexibility and reliability, will not be replaced as the primary building material in the near future.²

In any case the environmental impact of concrete is lower than most other materials, it is only the colossal amount used that accounts for its high overall impact (around 7-8 per cent of anthropogenic CO₂ emissions³). For this reason, making concrete more sustainable is justified because it will have a high impact on reducing global CO₂ emissions. Portland cement (PC) accounts for around 90 per cent of the emissions of concrete and the most efficient strategy is to replace as much PC as possible with supplementary cementitious materials (SCMs) in blends.⁴

The availability of commercially-established SCMs, namely fly ash and ground granulated blastfurnace slag (GGBS), is diminishing due to the industrial shift away from their traditional production processes, ie, coal combustion for electricity and blastfurnace steel manufacturing, respectively.^5,6 On the other hand, kaolinitic clay and limestone deposits can be found almost everywhere in the upper layers of the Earth.⁷ This is the reason why LC³ – a family of cements containing a combination of calcined clay (CC) and limestone as a partial replacement for clinker, a concept originated and developed mainly at École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, is the blended cement with the highest sustainability potential.⁸ LC³-50 is a cement that can achieve a comparable performance to OPC in concrete applications, while replacing 50 per cent of clinker.⁹

From seven days onwards, the compressive strength of LC³-50 concrete is equivalent to that of reference CEM I concrete.10 This is achieved with CC containing only 40-50 per cent kaolinite, which is considered the overburden or waste of the pure clays used for ceramic applications.¹¹ The main parameter influencing the strength of LC³ cements is the metakaolin (calcined kaolinite) content of the calcined clay.¹² Recent laboratory trials at EPFL suggest that even the slightly lower 1-3 day strength of concrete can be improved to match the control mixes with better grinding strategies.¹³

LC³ blends exhibit excellent resistance to chloride ion penetration, showing apparent diffusion coefficients an order of magnitude lower than PC. Among the reasons for this are the alkalinity of the pore solution, binding capacity and the reduction in porosity.^14,15 On the other hand, similar to most SCMs, the carbonation rate of LC³ concrete is known to be higher than PC.16 Nevertheless, recent life cycle analysis case studies show limiting water-to-cement ratios or increasing of concrete cover can give low-carbon concrete a satisfying service life.¹⁷

LC³ in codes and standards

Another challenge facing LC³ concrete is the current prescriptive cement and concrete codes and standards. The design and acceptance process of a concrete construction depends on up to three sets of regulations: cement standards, concrete standards and finally structural design codes. Today, most standards are ‘prescriptive’ – that is to say listing the compositions of the raw ingredients and specifying the concrete mixture proportions and construction techniques.18 “Performance-based” specifications give “in-place” performance requirements and allow for the faster and more flexible implementation of new technologies.

LC³ in cement standards

In the newest release (May 2021) of the European cement standard EN-197-5, LC³-50 is allowed with up to 50 per cent clinker replacement under the CEM II/C-M category.¹⁹ In the USA the ASTM C618 defines physical and chemical limits on three classes of pozzolans (N, F and C) whether natural or calcined. However, as seen in the comparison shown in Table 1, the limit on the maximum water demand might be an issue given the lower workability of LC³ blends.

For the EN-197-5, an LC³-50 (50 per cent clinker) formulation with a clay-to-limestone ratio of 2:1 would be classified as CEM II/C-M(Q-LL). However, the standard only allows ternary blends. This is an unnecessary limitation in situations where it may be interesting to make quaternary blends with different pozzolans. The standard also limits the CEM I replacement to 50 per cent, whereas 32.5 cement class can easily be met by blends with only 40 per cent clinker. These higher substitution levels can be important economically in countries that have to import clinker, such as Ghana, Senegal and Côte d’Ivoire. It is worth noting that given the rising urbanisation needs, countries in the global south are expected to produce more than 25 per cent of the 5bnt global cement market expected already in 2023.²⁰

In the US ASTM 1157 is a first move towards performance-based standards,²¹ it does not put a limit on the number, type or chemical additions to clinker. In addition, the ASTM C1679-17 allows for testing the reactivity of a blended cement through isothermal calorimetry, which is a quick and reliable method. It is also important to highlight that for such a standard to be used in countries of the global south, the standard testing temperature should not be 20 or 23˚C as currently stated but more realistically 30 or 40˚C.

LC³ in concrete standards

The recent development in cement standards does not yet mean that LC³ is cleared for use in concrete due to the limits on the concrete standards. In the US cement blends are prepared in ready-mix concrete batching plants. In this case, the additional sulphate required for proper sulphation of LC³ should be added to the CC, limestone or likely to a mixture of both (namely LC²). It is fortunate that ASTM C618 already allows the presence of four per cent sulphate content in natural pozzolans (N), which likely covers the need for additional gypsum in most cases. As it stands, the ACI 301 contains prescriptive limits on the mix design of concrete (minimum binder content, maximum aggregates size, etc), but more importantly only allows replacing OPC with calcined natural pozzolans complying with ASTM C618. Additionally, given the hierarchy of specifications, the tendency of engineers to be over cautious does not prioritise sustainable concrete practices such as reducing binder content or using LC³ as a binder.

In Europe the country specific EN-206 standard does not yet recognise CEM II/C (and so LC3-50) within potential constituents in concrete mixes. Moreover, the current methodology of the code specifies a minimum cement content, maximum water to binder ratio and maximum replacement of CEM I with specific SCMs to comply with the specific serviceability and durability requirements of concrete in service.²² This is detrimental to the potential savings of the environmental and economic impact of concrete that is closely related to the reduction of binder content even if the country specific EN-206 standards recognise LC³. As seen in Figure 1, the strength of different concrete formulations made with both CEM I and with LC³-50 does not decrease with a reduction of binder content, and it even increases with a maximum of around 275kg/m³. Moreover, a study showed that if prescribed limits were replaced by performance-based specifications, a high-performance bridge deck concrete element could be designed with 25-45 per cent less CO₂ emissions fulfilling the same strength and service life requirements.²³

Future perspectives

It is clear that a shift towards a performance-based standard for cement and concrete is necessary to work towards decarbonising the concrete industry, but the hierarchy of quality assurance of concrete structures dictates that structural design codes are adapted as well.

The new Eurocode 1992-1-1:2021 is adopting a performance-based approach for the assurance of the durability of concrete structures by demanding a certain concrete cover, equivalent of the calculated resistance of the specified concrete mix to the expected exposure class.²⁴

Although a rigorous country-specific experimental plan would be required to determine the values for the empirical coefficients used to predict properties such as carbonation or chloride penetration, the code is seen as a catalyst for change towards a more sustainable performance-based approach to concrete construction.

Final remarks

Adopting new technologies that will enable the concrete industry to achieve net zero requires a paradigm shift in the way we think and conceive cement and concrete standards. On the one hand, the advancement of research and development on more reliable, repeatable, fast and accessible testing methods will enable prescription requirements to be replaced by performance criteria. This change of paradigm will in turn facilitate the quicker uptake of new technologies that are needed to mitigate global warming in the next few decades.

The time required for standard modifications is seen as a limitation and time constraint for the deployment of sustainable technologies. For this reason, a shift is required towards flexible standards that can ensure quality and safety but are also locally relevant in the different regions of the world.

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This article was first published in International Cement Review in July 2022.