To fully decarbonise Portland cement production, will push its energy need from about 1.35 MWh (currently) to anything up to approaching 8 MWh per tonne.** Hence, as stated by the UNFCCC, for the foreseeable future a high level of Portland cement substitution is likely the only realistic way forward to decarbonise cement production (and hence the embodied carbon in concrete)...
Is awareness growing of the importance of reducing embodied carbon? In many leading OECD countries, yes. The approach can also incorporate regulatory (legislative) measures.
So, first: what does "embodied carbon" mean? It means the “CO2 emissions associated with energy consumption and chemical processes during the extraction, manufacture, transportation, assembly, replacement and deconstruction of construction materials or products" (PDF).
And how are legislators responding? In the EU there is no unifying legal measure requiring low embodied carbon in concrete, despite that since 1989 the EU's over-arching Construction Products Directive (CPD - here) required the protection of the environment – even if the EU waited till 2011 to expressly incorporate the term "greenhouse gasses" into the CPD's successor, Regulation EU 305/2011 (here)! Hence, it is down to EU Member States, of which for example the Netherlands has formed its "Betonakkoord" statutory body (here) to drive down the embodied carbon in concrete. In the U.S. at Federal level and in the U.K., as of 2022 there are no such equivalent measures that pertain to concrete – although in the UK attempts have been made to pass the Carbon Emissions Bill (here), while in U.S. there is state-level law such as California's SB596 (effective Sep. 2021, here). Further, both the U.S. Federal (PDF) and the U.K. Governments (here) have passed measures that cover procurement – requiring low carbon commitments in certain procurement applications.
But how is the construction sector itself responding? In several ways, including CO2 counting and CO2-ratings. Cutting the embodied CO2 means that component CO2 measurements must be understood. To do this, look-up data has been developed and also a number of assessment toolkits have been developed. The EPD Registry (here) is a limited searchable online database and by no means represents the totality. One of the most sophisticated is that published by the UK's Institute of Civil Engineering (PDF here). There are several others too including for example, the Netherlands' National Environmental Database (here).
Counting the CO2 costs. Let's take buildings as an example. A framework published by the WBCSD in 2020 has the stated aim of enabling opportunities for stakeholders to, "identify the best emissions-reduction strategies for all parts of the value chain". While the WBCSD framework is deliberately not prescriptive in its form and structure, it introduces a very important metric, expressed as the carbon intensity of the project, as a kgCO2/m2 measure (kilograms CO2 per square meter of floor space), and is, "applicable by all the companies across the life-cycle of buildings" (PDF):
But how might any sense of comparison be visualized? The simple way to do this is to use a visual rating, similar to labels stuck on EU appliances—but in concrete's case, here we use the colour-bands developed by the UK's Institute of Civil Engineering ("I.C.E"), which is now gaining prominence per its Low Carbon Concrete Route-map, the full version published in April 2022 (full version PDF here).
So below, we "rate" a cubic meter of different concretes to see how they compare in categories A1-A3 in the diagram above — but first we take a look at the underlying context...
These are now exciting times! There is now a positive requirement to seek CO2 mitigation at the earliest stage of construction in most OECD countries, whether by legal measures or voluntary project specifications. No matter the driver, the most fundamental components are environmental product declarations (EPDs).
EPDs. ISO 14025 establishes the principles and specifies the procedures needed to develop "Type III" EPDs. These Type III declarations are the most rigorous since they have detailed rules and must be verified by a third party. Since 2001, the EEA+UK has a system of EPDs, flowing from the EU's integrated product Policy (here). Like ISO 14025, these EPDs are a third-party Type III procedure — essentially across categories A1 to A3 in the diagram above.
Making EPDs count. Deriving a mechanism to standardise EPDs is only half the picture. There must also be compliance pressure. For example, in the EEA there is building standard EN15978:2011 and its progeny (e.g. EN 17472:2022) that categorize the various stages across the lifecycle of buildings (here). This allows for a standardised approach to assess the CO2 impact across a spectrum of discrete areas. Therefore, understanding how the CO2 "price" stacks-up across the various stages of a building's lifecycle-costs, has never been so important and relevant in the industry...
Our ratings below can serve A1-A3 above and will be in the range of < 30kg CO2 per tonne of EMC Volcanics (see diagram below)!
For the reasons stated below, the strength-class chosen for the diagrams above is C25/30. At that strength class, we calculate the CEM I mix at 220 kg CO2 per cu•m. The Construction Material Pyramid (here) rates the lesser stength-class C20/25 at 229 kg CO2. Hence, given the considerations that follow below, we believe the diagrams above serve as a good faith estimates.
The format and the colour-bandings are as suggested by the UK's Institute of Civil Engineering (ICE):
As independent results have shown, by increasing the concrete's total cement dose to 350kg/cu•m, at 70% replacement, EMC Volcanics hit 51.9 MPa in just 7-days (see here). Assuming 100% CEM I concrete could make the same strength at the same 350kg/cu•m cement dose, then using the same background figures used per the tables above:
To fully decarbonise Portland cement (OPC) production, will push its energy need from about 1.5 MWh (currently) to anything up to approaching 8 MWh per tonne.**
Embodied CO2. Until recently, embodied CO2 in buildings has been addressed on the EU level only with voluntary measures. Cities, regions and countries in Europe have also put in place various provisions in the form of certification systems, regulations, standards, and guidelines. Studies suggest that the number of measures addressing embodied CO2 have more than doubled over the last five years (here). However, comprehensive EU-level policy, targeting the whole-life CO2 footprint of buildings (including both embodied and operational carbon), still remains largely missing.
Low embodied-CO2 is important not just in new structures. The major CO2-producing aspect of buildings is the energy used to maintain and use them (heating, lighting etc.) during their useful life. To combat this, existing energy systems need to be updated. This is the "Renovation Wave".
The policy landscape is set to change. In the 2020 Renovation Wave strategy, the European Commission has adopted the principle of “lifecycle thinking and circularity” to make buildings “less carbon-intensive over their full life-cycle” (here). The ongoing review provides a significant opportunity for the EU to begin consistently integrating whole-life CO2 in its policy framework. This requires building-level policies to be well-coordinated and aligned with policy actions upstream on raw materials and construction products, as well as at the end-of-life.
The objective. To at least double the rate of annual energy-renovation over the next 30 years, in order to set the entire building stock on a net zero emissions pathway. However, energy efficiency renovations do not simply contribute to reducing operational CO2 emissions; they also increase embodied CO2 by adding new materials and systems into the building. Without accounting for the cumulation of all embodied and operational CO2 attributable to buildings over their entire life, there is a risk of exceeding the remaining carbon budget.
The EU’s buildings sector will only be able to keep on the 2˚C global warming scenario track if the embodied emissions produced by the renovations do not exceed around 125kg CO2 per sq•m...
i.e., to meet the "green" H2 and carbon capture need, on the basis that "1kg of H2 contains 33.33 kWh of usable energy" (here).
In the following analysis we distinguish between Portland cement's calcination as "Phase 1" and sintering as "Phase 2" :
The difference between using hydrogen over a fossil fuel as a combustible is that energy has to be expended to make the hydrogen, which is not required for coal or natural gas. Recall, OPC's calorific requirement and that Phase 2 requires coal or methane to induce sintering. This means CO2 is always produced in Phase 1 (calcination), even if H2 is used as the combustion fuelstock, and will always be an output in Phase 2 (sintering). According to a leading Swedish study, in a "net zero" setting, the Phase 1 energy overhead is 1,500 KWh per tonne (PDF here). This means if green hydrogen was used, 42 kg hydrogen is required for that phase (assuming 100% efficiency in the combustion process).
In the context of those two dynamics, the energy costs of producing 1kg of green H2 is at least 50 kWh per kg H2 (here). A 2019 U.K. Government report rejected hydrogen as a fuel stock for Phase 2 (PDF here). Hence, the stated 8MWh total computes as:
All figures stated as on a per-tonne of Portland cement (CEM I) basis.
All told, on the one hand although an energy need of 8,000 kWh/t Portland cement may prove to be an over-estimate, nonetheless the first two above bullet-points, already stand at about 3,500 kWh/t alone, when:
In the above context:
Therefore, adding-in a modest 20% energy overhead for inefficiencies, it's hard to see how the energy totals can be realistically kept to less than ~4.5 MWh/tonne as the sum total across all three domains — even if thermal recycling from clinker plants can lower the nascent energy need of capture systems.
At 4.5 MWh/t, that would still be over three-times the current energy need...