The future of concrete is low carbon

The reference standard for concrete in Europe is EN 206:2021, which is currently under review. The new text of EN 206 (currently being reviewed by the various committees responsible for the revision) introduces the issue of the environmental sustainability of concrete, which can be measured by a percentage reduction in carbon footprint compared to a reference concrete.

The carbon footprint class of the reference concrete (a type of concrete based on obsolete technologies and therefore with a high environmental impact) is a function of the characteristic compressive strength class and the environmental exposure class. Indeed, the carbon footprint of the reference concrete increases with the characteristic compressive strength class, as higher strength concrete requires a higher dosage of cement, which is the main contributor (approximately 70-75%) to the environmental impact of the cementitious mixture.

Yes, an approach essentially equivalent to that of Europe and Italy has also been adopted in the United States, where a reference value (Baseline – Embodied Carbon from cradle to gate) has been set for the CO₂ incorporated in one cubic metre of concrete, depending on the compressive strength class of the cementitious mixture. Comparing the CO₂ values set in Italy with those in the United States, we can say that Italy has adopted values that are slightly lower (lower carbon footprint than the reference) or equal to those in the United States. This 'more virtuous' approach in Italy has historical roots, as in Italy, and, indeed, in Europe as a whole, the use of low-clinker (limestone Portland cement), pozzolanic and blast furnace cements with a lower carbon footprint than Portland cement, which has long characterised US cement production, has been widespread for decades.

In favour of the United States, however, it should be noted that some administrations (General Services Administration, Marin County) have already set the maximum carbon footprint for concrete to be used on construction sites, aiming for a 10-20% reduction compared to the baseline.  

As highlighted for the United States, it is desirable that the most important clients include a GWP limit value (or a GWR reduction class compared to the reference concrete) in their specifications based on the characteristic strength class of the concrete. A lower GWP class (or, in other words, a higher carbon footprint reduction class) could be a 'bonus requirement' when awarding contracts for the supply of concrete. As regards widespread construction (residential, tertiary, etc.), it is desirable that designers integrate the concrete specification with the GWP class (or GWR reduction class) to encourage the use of concrete with a lower environmental impact.

It is worth emphasising that a 'small steps' policy should be adopted, avoiding 'leaps into the unknown', which means being careful not to prescribe a carbon footprint reduction of more than 30% compared to the reference concrete, as this target may be inconsistent with the materials and technologies available. Therefore, the suggestion is to adopt a 'small steps' policy, requiring carbon footprint reductions of 15-30% at most to ensure that concrete can actually be produced. In the immediate future, it may be possible to aim for a carbon footprint reduction greater than these values.

As I mentioned earlier, the carbon footprint of concrete is largely attributable to the cement used in its manufacture. Therefore, decarbonisation strategies consist of both using cements with a reduced carbon footprint and reducing the cement factor, while still meeting mechanical and durability requirements. With regard to the first aspect, we are seeing a steady reduction in the consumption of Portland cement (characterised by a carbon footprint of approximately 815 kg/tonne) and an increase in the use of cements with a lower carbon footprint and low clinker/cement ratios (K/C).

This trend is set to continue with the introduction on the market of cements compliant with the European standard EN 197-5 and EN 197-6, which have a lower carbon footprint due to their lower K/C ratio. It is clear that reducing this K/C ratio is not without consequences for concrete, as it is well known that the use of these low-clinker cements reduces both short-term (1-3 days) and long-term (28 days) strength.

Therefore, if the performance of concrete is to remain unchanged when using cements with a lower environmental impact, the role of admixtures for concrete, and water reducers in particular, is fundamental. Research in this area is making a significant effort to reduce or eliminate both the slow development of initial strength and the lower long-term performance.

In this regard, a new frontier of admixtures called 'strength enhancers' offers hope for resolving the slow development of short/long-term strength typical of concrete made with cements characterised by low K/C ratios. These admixtures work by increasing the fraction of cement that hydrates. In this way, by making better use of the cement used, it will be possible to produce concrete with significantly lower cement dosages than at present, with the same mechanical performance and durability, substantially reducing the carbon footprint of the cementitious conglomerate.