1/2022

Sustainable buildings have to meet ecological, economic and socio-cultural requirements. At the same time they have to offer a high technical quality and have to be aligned with the processes of construction. An immediate and significant reduction in CO₂ emissions as a measure for climate protection is undisputable, and to meet all demands at the same time is a big challenge. Already known methods have to be applied even more ambitiously, and usable new approaches have to be put into practice as quickly as possible.

Roadmaps on decarbonisation and resource efficiency worldwide are addressing the measures and levers to reach a decarbonised concrete sector by 2050 at the latest. These roadmaps have different regional scopes and therefore take into account the respective regional aspects.

Despite these regional approaches, all roadmaps draw similar conclusions from their main decarbonisation pathways:

• Depending on the regional conditions and building traditions, low carbon cement and concrete have the potential to contribute up to one quarter to the overall decarbonisation needs
• A similar potential is seen in innovating design and construction techniques, the ranges of which are highly dependent on regional conditions and are seen between approx. 5 and 25 %
• Carbon capture and use/storage (CCUS) will be the missing link to fully decarbonise the cement and concrete value chain by 2050

Given the importance of low carbon cements and concretes in decarbonising the full construction value chain, cement and concrete producers worldwide put a strong focus on developing the respective cements and concrete. At the same time, they reach out to the downstream users who will have to request these cements and concretes to generate the market pull needed.

Low-carbon construction

Resource efficiency and climate protection must be taken into account more strongly than before in the design of concrete structures. It can already be determined today whether a concrete based on a more CO₂-efficient cement has comparable technical properties for the respective application. At the same time, however, the question of which type of cement with comparable technical performance is used in a ready-mixed concrete plant, a precast plant or another application also depends on the availability of the necessary raw materials.

In order to show which CO₂ reduction potentials exist, the Concrete Sustainability Council (CSC) recently introduced the CO₂ module for concrete. This module is used to label CO₂-efficient concrete based on its global warming potential. In addition to their CSC certificate, concrete plants can obtain a label with one to four stars (Level 1 to Level 4) for CO₂-efficient concrete which can be presented to customers. Industry reference values are based on the use of Portland cement CEM I in a typical concrete mix used for the respective strength class. The global warming potential GWP is calculated as net values, i.e. the greenhouse gas emissions associated with the use of alternative fuels in cement clinker production are not included.

The production of concrete is addressed in the life cycle modules A1 to A3. Orientation values for greenhouse gas emissions of concretes depending on the selected concrete compressive strength class are given on a regional basis.

The GWP per cubic meter of concrete does not allow a direct link between the ecological performance and the technical performance of the concrete. Therefore, in various publications performance-related parameters are also found, such as the binder or clinker intensity in kg/(m3 x MPa). Figure 1 shows the CO₂ intensity of concretes of different strength classes (data collection provided by VDZ).

Figure 1: CO₂ intensity in relation to concrete compressive strength (data collection provided by VDZ)

This illustrates the following:

• Particularly in the area of conventional concrete compressive strengths (e.g. C20/25 - C30/37), the range of CO₂ intensities which these concretes exhibit is considerable
• In the higher strength classes, the performance-related greenhouse gas emissions are lower than in the lower strength classes
• This consideration makes sense when the higher strength is utilised by reducing the component dimensions, i.e. when building in a material-saving way

In this respect, depending on the technical requirements, it is already possible today to build in a more climate-friendly way by using CO₂-efficient cements in concrete or by using concrete in a material-saving way. At the same time, it is crucial to create the necessary legal and normative conditions to enable and accelerate the widespread use of new CO₂-efficient cements.

Optimising the binder volume

In order to optimise the binder content in concrete and in doing so to lower CO₂ emissions, several measures can be taken:

• Packing density optimisation of concrete and optimisation of aggregate grading
• Optimisation of the mix design using efficient concrete admixtures
• Appropriate balance between performance/descriptive approach
• Proactive quality control using digital tools (e. g. machine learning)
• Ultrasound technology in concrete production

While the first three points are familiar approaches which could be further intensified, the last two methods are in the development stage.

Lowering of the clinker factor

Research findings have shown that CEM II/C cements are widely applicable in concrete without adapting the limit compositions of concrete as they are today. With the exception of components with high water saturation and frost (XF3) as well as exposure to frost and deicing salts (XF2, XF4), CEM II/C-M (S-LL) cements can be used in all exposure classes. With corresponding evidence via tests, the frost classes can also be opened up.

Figure 2 shows test results of con-crete with clinker-efficient cements against the evaluation background as given in CEN/TR 16563. This test must currently be carried out with a constant water-cement ratio of w/c = 0.50.

Figure 2: Evaluation of carbonation acc. to CEN/TR 16563, “Principles of the equivalent durability procedure”- Annex B

While, for example, a CEM III/A or also a CEM II/C-M (S-LL) fit well into the evaluation background in this test with the required limit composition, the carbonation depth of a cement with 20 mass % clinker, 30 mass % blast furnace slag and 50 mass % limestone is clearly outside of this background. If the water/cement ratio is lowered to w/c = 0.40, the test result is already in the upper range of the evaluation background. A further reduction to w/c = 0.35 leads to a result in the order of magnitude of the reference cements.

In conclusion, a further significant reduction of the clinker content in cements with high proportions of unburnt limestone is possible if the concretes are composed accordingly. Technical assessments and standards should support these approaches in the best possible way.