NetZeroCities

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:

LC3 is a technology that is market-ready and it is already produced in several plants in the world [11]
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].
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:

Carbon8 has been demonstrated at scale [15]
FastCarb produced two demonstrators for concrete walls formulated using recarbonated recycled aggregates [16].
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].

Preconditions

Technical aspects/infrastructure: A change to a system of harmonized performance-based technical specification can accelerate the adoption and acceptance of new low carbon construction materials using different feedstock [6].
Policy and regulatory/legal framework:

Amending existing standards and codes is required because construction materials must typically meet composition-based and performance-based codes and standards for use. This will allow the use of cements based on alternative binding materials such as BYF cement (in Europe), concrete with recycled concrete waste, cement with SCMs such as carbonated recycled cement paste, concrete with synthetic aggregates.
Co-processing must be recognized in framework waste legislation [7].
Policies should facilitate waste shipment between EU countries, discourage landfill and prohibit exports of waste outside of the EU [2].

Funding and financing: Incentives for the use of recycled building materials should be created for customers [3].

Enabling conditions

Technical aspects/infrastructure

The re-carbonation of built concrete products (its ability to uptake CO2 from the atmosphere) over their life cycle should be recognised in CO2 emissions accounting, carbon footprint methodologies, and CO2 certification removal schemes [2].
The design of low carbon concrete should take into account all the embodied carbon in the material [6].
The use of digitalisation across the concrete industry: Data for cement and concrete can be available to determine the carbon footprint of the construction material or building and to show the source of the materials used in construction [2]

Policy and regulatory/legal framework:

Extending actions down the value chain to the final customer. Emission Trading System based only on clinker is not efficient as it affects only the minor part of the value chain [11]. Mandates that require buildings and infrastructure to be constructed only using cement that has a very low green house gas (GHG) footprint has the potential for large additional reduction in CO2 [8] or bans or restrictions on the use of cement with only virgin materials.
Introduction of guidelines that mandate public procurers to only purchase cement and derived products with close-to-zero CO2 emissions [1]. These regulations should be standardised beyond national borders.
Policies can incentivise the use of digitalisation across the concrete industry [2].

Funding and financing:

The production and use of quality recycled CDW must be incentivized as well as the production of building materials containing recycled content and industrial waste. This could provide an economic advantage for using recycled material steering the value-chain towards a more circular and sustainable economy.
Tax penalties on conventional products with a high GHG footprint. Taxes on virgin materials.
Access to EU/public funding and innovation procurement can further incentivise the development of R&I solutions for low carbon building materials, transforming waste into resources and promoting a circular economy.

Economic and social context:

Affordable low carbon construction materials for housing solutions must be at the heart of housing policies [7].
Industry should promote international training events with national standardisation bodies and accreditation institutes, researchers and students to educate and inform the public and key stakeholders and to build social acceptance. This includes training for architects/engineers on the applicability of lower-carbon concrete mixes and training for engineers and contractors to use different types of low carbon cement and to get a better understanding of sustainability issues related to building materials

Project governance and implementation modalities:

Carbon pricing mechanisms must be designed to target value chains and be embedded in holistic climate policy frameworks in order to create demand for low-carbon concrete innovations.
Through investment, mechanisms that use private funding for low carbon innovative technologies and through the promotion of private-public partnerships, the governments can mitigate the risks associated with the deployment of the innovative technologies. Horizon 2020 and the Innovation Fund are such examples that help attract private investment.
The use of co-creation methods: collaborations with public authorities at the design stage of projects, can find the best low carbon building material solutions with optimized costs, durability, sustainability performances
Trade and climate policy frameworks need to be mutually reinforcing [7].
Product design policies product labelling or voluntary certification systems [9]. Labels indicating the environmental characteristics or other beneficial aspects of products can help to create or increase demand for such products. Development of standardized and transparent assessment methods, such as life cycle assessments (LCA) [6].
Citizen-engagement initiatives to raise and maximise the chances of take-up. Rapid creation of necessary framework conditions that lead to increasing environmental awareness and may also lead to market acceptance.

Technical aspects/infrastructure

Lack of established standards
The integration of these new binders into existing or new standards at national or European level is a challenge, especially in terms of time
Lack of performance data
Lack or low access to material flows from concrete demolition because of "downcycling" of the material or even final disposal in landfills or export outside of the EU
Insufficiently developed supply chains

Policy and regulatory/legal framework “carbon leakage” - EU producers are at a competitive disadvantage compared to non-EU producers that are not obliged to buy Emissions Trading System allowances [8].
Funding and financing limited access to finance for small and medium sized enterprises (SMEs), start-ups is considerable and they face heavy competition from major construction companies [6].
Economic and social context

Higher costs relative to conventional benchmark products. Novel technologies for separation and carbonation naturally also require high investments in plant, accompanied by rising costs for such novel, resource and environmentally friendly products making it prohibitively expensive for some customers.
Less attractive from the economic standpoint considering the demands on speed of construction meaning low-carbon mixes are not favoured.
Small industries producing alternative low carbon solutions cannot compete against established industries economies at scale [6].

Project governance and implementation modalities

The implementation of such low carbon concrete solutions can face the very conservative attitudes towards new technologies in the construction industry
Lack of awareness and practical knowledge among specialists [10]
Negative perception held by other professionals [10]
Client acceptance [10]
Low availability of skilled labour [10]
The possibility and responsibility to implement low carbon concrete to reduce CO2 emissions is spread across different segments of the value chain with diverging incentives [10]
Too time consuming to design with, lack of design guides and tools, lack of confidence in contractor ability to deliver a new design, perceived concerns about material sourcing prevent selection [10]
Difficulties obtaining insurances for novel and reused materials [10]

EnDurCrete focuses on developing a new cost-effective sustainable reinforced concrete for durable and added value applications. The concept is based on the integration of novel low-clinker cement including high-value industrial by-products, new nano and micro technologies and hybrid systems ensuring enhanced durability of sustainable concrete structures with high mechanical properties, self-healing and self-monitoring capacities (Horizon 2020, 2018-2021).

LC3 aims to make a standard and mainstream general-use cement in the global cement market. The new type of cement that is based on 50% clinker replacement level by a blend of limestone and calcined clay. LC3 can reduce the CO2 emissions by up to 40%, is made using limestone and low-grade clays which are available in abundant quantities, is cost effective and does not require capital intensive modifications to existing cement plants. (Swiss Agency for Development and Cooperation, 2014-present).

ECO-binder aims to develop a new generation of concrete-based construction materials with more than 30% lower embodied energy, 20% improved insulation properties and 15% lower cost than the current solutions that are based on OPC. The main objective is to demonstrate that OPC can be fully replaced with new ones based on the Belite-Ye’elimite-Ferrite (BYF) class of low-CO2 binders, without compromising on quality or cost (Horizon 2020, 2015-2018).

Aether aims to develop a new, innovative class of clinkers, with a lower environmental footprint. Less limestone is required to produce Aether cements, which are also manufactured at lower temperatures and using less energy than conventional OPC. This allows a 25-30% reduction in CO2 emissions during the production process (LIFE+, 2012).

C2CA aims to develop three innovative technologies for recycling end-of-life concrete that yield fine cement paste, coarse aggregate and fine binding materials. C2CA also helped to create government mechanisms for recycling of concrete from old buildings to new ones (FP7, 2011-2015).

CO2MIN focuses on identifying minerals suitable for permanently sequestrating CO2 from flue gas. The carbonised minerals produced using these technologies could be used for various product applications in the construction industry (Federal Ministry of Education and Research, Germany, 2017-2020).

CO2WIN focuses on the areas of electrochemical conversion of CO2, chemical and biotechnological processes for the production of sustainable chemicals and fuels as well as mineralization using CO2 for the production of climate-friendly building materials. This technological diversity will be necessary for the industrial recycling of carbon to succeed in the future (Federal Ministry of Education and Research, Germany, 2020-2024).

C²inCO2 key objective is to carbonate the recycled concrete fines (paste) using exhaust gases from the cement plant containing CO2. The carbonated hardened cement paste can be used as a clinker substitute, thus decreasing the clinker content in cement, preserving natural resources, and reducing CO2 emissions (Federal Ministry of Education and Research, Germany, 2020-2023).

SOLID LIFE ( Solidia) project aims to demonstrate the feasibility of producing low-emission cement and concrete products on an industrial scale in existing industrial installations. The new products would have cost the same to make as the conventional Portland cement but have superior performance and reduce CO2 emissions by 70% (LIFE, 2016-2019).

TRACK4REUSE addresses the challenge of optimising industrial truck traffic and introducing new standards for the green construction and demolition waste (CDW) industry, with the aim of reaching the EU goal of using 70 % recycled material. This will result in reduced construction environmental impact, improved traceability and recycling and increased onsite safety (Horizon 2020, 2020-2022).

RE4 goal is to promote new technological solutions for the design and development of structural and non-structural pre-fabricated elements with high degree of recycled materials and reused structures from partial or total demolition of buildings (Horizon 2020, 2016-2020).

RECODE promotes a circular-economy-approach by reusing the CO2 produced during the cement manufacturing within the plant itself to produce better SCMs entailing less embedded energy and related CO2 emissions (CaCO3 nanoparticles to be used as concrete fillers) (Horizon 2020, 2017-2022).

FastCarb aims to combine concrete recycling and with its ability to capture CO2 in an accelerate way. This approach reduces the carbon footprint of concrete and promotes circularity (Institute for Applied Research and Experimentation in Civil Engineering, 2018-2022).

SCOT focuses on how CO2 can be used in accelerated mineralisation to create low-carbon construction materials. Industrial wastes (low-value materials) can be converted into useful higher-value products for the building sector (synthetic aggregate). This provides means that can permanently sequester CO2, divert waste from landfill, avoid landfills costs and reduce the extraction of virgin aggregate materials (FP7-Regions 2013-2016).

Carbon8 treats industrial residues from the steel, cement, pulp and energy sector (fly ashes) with CO2 captured directly from the source through accelerated carbonation to produce lightweight carbon-negative aggregates for various applications in construction. This represents an alternative to natural virgin aggregates, inherently reducing the carbon footprint of the concrete and reducing the consumption of primary raw materials, thus contributing to circularity (Carbon8 Systems Ltd. Company, UK, a spin-out from the University of Greenwich, London).

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