Concrete is the most-used manufactured substance on the planet in terms of volume. It is used to build homes, schools, hospitals, workplaces, roads, railways and ports, and to create infrastructure to provide clean water, sanitation and energy [1]. Concrete is manufactured by mixing cement, water, aggregates and small quantities of chemical admixtures to improve its properties and meet specific product requirements. The direct CO2 emissions related to concrete largely come from cement production. The cement used today, called Ordinary Portland cement (OPC), is made from finely ground Portland clinker and gypsum. Clinker is produced by heating crushed limestone, clay and sand to 1450 °C [2]. Within the clinker production, there are two main sources of CO2: direct emissions (60%) stemming from the decarbonation of limestone and indirect emissions (40%) from burning fuel [3]. The European cement industry has actively worked on reducing emissions for a long time. In 2013, CEMBUREAU elaborated a Roadmap, which was revised in 2018 to ensure the objective of carbon neutrality down the cement and concrete value chain set for 2050 are met. A schematic illustration of 2050 road map is shown in Figure 1 [2].
Figure 1. CEMBUREAU 2050 roadmap. CO2 reductions along the cement value chain (5Cs: clinker, cement, concrete, construction, re-carbonation) Source: Cembureau [2]
SUSTAINABILITY
OPC typically contains 95% clinker and 5% gypsum. A very effective strategy of reducing cement footprint is to substitute part of the Portland clinker with other materials for which the footprint is low or even zero. Such substitutes are called supplementary cementitious materials (SCMs) and include: granulated blast furnace slag (GBFS), a by-product of pig-iron production in blast furnaces, fly ash (FA), a by-product from coal-fired power plants, natural pozzolanic materials obtained from volcanic compounds, sedimentary rocks (limestone), clays, agricultural residues (rice husk ash), silica fume (a by-product of silica and ferro-silica alloy production processes) and mixes thereof [3,4,5]. OPC clinkers partially substituted with one or more SCMs are referred to as blended cements. Reducing the clinker content in cement saves both energy-related and embodied CO2 [3]. Examples of projects applying this concept are LC3 and EnDurCrete.
INNOVATION/NEW MATERIALS
Alternatives to Portland cement can achieve significant emission savings as they rely on different raw material mixes and require lower firing temperatures. Belite-rich Portland clinkers are produced with the same process as OPC clinkers, but with less limestone in the clinker raw material mix, thus the CO2 generation is reduced. However, this emission reduction of around 10% is rather modest relative to OPC [3].
A promising lower-carbon alternative is calcium sulphoaluminate (CSA) clinker. It contains ye’elimite as the main constituent, which reduces direct CO2 emissions by 44% [1]. Unfortunately, their cost also increases significantly at the same time, because higher ye’elimite content requires more expensive aluminium-rich raw materials [3]. Belite calcium sulphoaluminate (BCSA) clinker is able to circumvent the high raw material costs of CSA clinkers. They are preferably referred to as “belite–ye'elimite–ferrite” (BYF) cements. Such clinkers have CO2 savings of 20% or greater per unit of clinker in the cement due to the lower limestone content with additional savings coming from the lower firing temperatures and 30-50% lower electricity demand for grinding as they are more friable [1]. Projects applying and developing these new types of materials include Ecobinder for BYF cements. Commercially available BYF clinkers Aether and Ternocem have the potential to replace OPC clinker in many major applications [3].
The carbonated calcium silicates (CACS) clinkers, primarily consist of wollastonite cured with CO2. Process CO2 emissions generated in CACS clinker making are, in principle, re-absorbed during the CO2 curing process [1]. Solidia cements (Solid life) have recently been commercialised for fabricating certain cement-based products, but the CO2 gas for curing currently comes from industrial gas suppliers. The long-term goal is to use recycled industrial CO2 from industrial flue gases, promoting emission reductions and circularity [3].
Cements based on magnesium oxides derived from magnesium silicates (MOMSs) are able to counterbalance and absorb the CO2 released in the manufacturing process while curing. If the magnesium oxides are coming from natural magnesium sources free of carbon, such as magnesium silicate rocks, they would yield net negative CO2 emissions, having a true environmental advantage [1]. CO2MIN is a good example, focusing on identifying such rocks.
Figure 2. CO2 savings with LC3 cement Source: LC3 [11]
RECYCLING and RE-CARBONATION
Concrete reabsorbs a significant amount of CO2 over its normal service lifetime and also after demolition. The recarbonation of recycled cement paste is a rapid process at normal pressure and temperature, showing a substantial CO2 sequestration potential. This concept reduces the CO2 footprint of cement, promotes recycling of end-of-life concrete and circularity as the carbonated material can be used as SCM for the production of new blended cements [2]. Examples of projects applying and developing recycling technologies are C2CA, TRACK4REUSE and RE4.
Users of such solutions can be:
-Industries, universities and research institutions investigating durability, sustainability, alternative cement materials, recycling and re-carbonation along concrete value chain
-Potential customers in the target sectors: concrete markets and manufacturers of new concrete technologies
-Recommendations and standardisation experts
Figure 3. Direct(=process) CO2 emissions generation intensity for selected clinkers. Source: IEA [1]
MATURITY:
Several solutions for low carbon concrete are already ready for commercial deployment or available on the market, for example:
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LC3 is a technology that is market-ready and it is already produced in several plants in the world [11]
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Aether has been demonstrated in two industrial trials completed in 2012 with 10,000t of Aether clinker produced [12]. Since 2014 customer testing is ongoing [13]. It is not commercially produced yet because specific norms for this type of clinkers do not exist, with the exception of China that has normalised their use in construction for more than 30 years [1].
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Solidia cement has been successfully produced in two continents and Solidia concrete has been successfully demonstrated in 10 countries worldwide [14].
Solutions are in demonstration include:
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Carbon8 has been demonstrated at scale [15]
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FastCarb produced two demonstrators for concrete walls formulated using recarbonated recycled aggregates [16].
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EnDurCrete tested full-scale demonstrator in four different locations in Europe, in working sites of tunnels and ports (Spain), bridges (Croatia) and offshore structures (Norway). After project completion, a time-to-market of 3-4 years is expected [17].
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