Concrete carbon reduction: Exploring innovative technologies in this space

This article is part of our concrete series. Read the main article here. Keen to know more about the market? See this article.

Credits: Uve Sanchez on Unsplash.

What is concrete made of?

Portland cement is a key ingredient in concrete; essentially like a glue. Typically about 10–15% by weight of the total concrete mixture. Portland cement includes:

• Tricalcium silicate (Ca3SiO5 or C3S): 55–65%; general strength provider.
• Dicalcium silicate (Ca2SiO4 or C2S): 15–25%
• Tricalcium aluminate (Ca3Al2O6 or C3A): 8–12%
• Tetra-calcium aluminoferrite (2(Ca2AlFeO5) or C4AF): 8–12%

Clinker is a key intermediate product in the manufacturing of Portland cement and is produced by heating the raw materials, limestone (source of CaO), clay (source of SiO2 and Al2O3), and iron ore (source of Fe2O3), in a kiln at high temperatures. To form the Portland cement, clinker is ground and a small amount of gypsum (CaSO4·xH2O) to control the setting time.

Water is essential for the hydration of cement. The chemical reaction of cement with water, and hydration, forms compounds that contribute to the strength of the concrete. The water/cement ratio determines the concrete’s final strength and durability and a common range is 0.4–0.6 by weight.

Aggregates are inert granular materials such as sand, gravel, or crushed stone that are mixed with cement and water. Aggregates do not chemically react with cement and water but provide bulk and strength to the concrete. Their composition can vary widely based on the source. Aggregates typically make up 60–75% of the concrete’s total weight and are typically a mixture of fine (sand (±40%)) and coarse (gravel (±60%)) aggregates.

Admixtures are materials that are added to the concrete mix to modify its properties. This is normally not more than 0.5–2% by weight. They can include: superplasticisers (chemical formulas vary), air-entraining agents (e.g., vinsol resin), set retarders (e.g., calcium sulphate), and set accelerators (e.g., calcium chloride).

Additional components include:

• Fly ash from coal combustion is composed primarily of silicon dioxide (SiO2), aluminium oxide (Al2O3), and calcium oxide (CaO)).
• Fumed silica is an ultrafine powder consisting mostly of silicon dioxide (SiO2)).
• Slag is a byproduct of steel manufacturing, primarily composed of calcium, silicon, aluminium, and iron oxides.

Even though cement makes up for only 10% of concrete, 90% of the emissions come from cement. Credits: Extantia.

Which raw materials do we need for cement?

Limestone
Limestone (CaCO3) is typically the main source of lime (CaO) and makes up about 78% (by weight) of the raw materials needed for cement. Limestone found in nature is rarely pure CaCO3 and always contains contaminants which can significantly impact the clinker quality and kiln operation:

• Magnesium oxide (MgO): dolomite (CaMg(CO3)2) is typically dispersed within a limestone deposit. According to EN-197 (EU standard) cement can maximum contain 5% MgO (by weight) and according to US ASTM C150–07 this can maximum be 6%.
• Chlorides: limestone near the sea is often contaminated with sodium chloride (NaCl). Chloride in cement is typically limited by most national standards and a kiln bypass may be required to remove chloride if it is present in the original limestone source.
• Fluorides: fluorite (CaF2) acts as a “mineraliser” and decreases the viscosity of the liquid phase in the sintering part of the kiln. Its presence can damage the clinker cooler as the nodular clinker becomes more like lava when CaF2 is present in excessive quantities.
• Alkalis (Na and K): found typically in the clay and the calcium-containing components. Negatively impacts the kiln system when present in large quantities.

Clay
Clay is typically the main source of silica (SiO2) and alumina (Al2O3). Clay typically makes up 17–20% by weight of the raw materials needed for cement. Found in nature, it typically contains alkali elements (Na and K) as well as iron (Fe).

Sand
Chemically speaking, sand is a crystalline form of silica (SiO2) and typically makes up 3–4% by weight of the raw materials needed for cement.

Iron ore
Iron ore can be a source of iron oxide (Fe2O3). Sometimes clays also contain iron. Other options for an iron source are mill scale or blast furnace slag.

Other raw materials
Gypsum (CaSO4·xH2O) is a raw material used to control the setting time of the cement and is only added during one of the final steps. Optionally, other raw materials, such as blast furnace slag, shale, fly ash from coal burning power plants, pozzolana, bauxite (alumina), mill scale (iron-containing by-product from the steel industry). Some of these materials are essentially waste products, such as slag, fly ash, and mill scale, and can be a cost-competitive and sustainable source of raw materials.

How is cement made?

1. Preparation of feedstock and crushing
The raw materials limestone, sand, clay, etc. are first crushed upon arrival on site. In the primary crusher for limestone, the material is crushed to pieces of about 15 cm. The secondary crusher (or hammer mill) reduces the particle size to 7.5 cm or smaller. The particle size of the raw materials input is essential for the final cement and concrete outcome.

2. Proportioning the limestone with clay, sand, and iron ore
After the initial crushing steps clay, sand, and iron ore (typically stored in silos and already in the desired particle sizes) are added to the finely ground limestone. Limestone usually makes up about 78–80% of the mixture, with the remaining 17–20% being clay, and the rest (3–4%) being sand.

3. Grinding
The limestone, clay, sand (and iron ore) mixture is then dried to reduce moisture content and ground even more finely in a grinding mill. The particle size of the raw materials is important for complete combination in the kiln, CaCO3 should be below 125 um and quartz (sand) ideally below 45 um.

4. Thermal process and chemical reactions
The finely ground raw material mixture is then ready to undergo a thermal process. To improve energy efficiency, reduce fuel consumption, and increase overall production capacity, the raw material powder mixture is first subjected to multiple preheating steps in the preheater tower. Hot gases from the kiln are typically used for the preheating steps. Chemical reactions already occur in the preheater: clay minerals are dehydrated and organic matter is broken down.

After the preheating steps, the hot raw materials move on to the calciner, which has as its main purpose the transformation of limestone (CaCO3) to lime (CaO). This occurs at high temperatures (700–900C) and the limestone loses about 44% of its mass, released as CO2, during calcination. As soon as the lime (CaO) is formed, it starts reacting with SiO2 to form dicalcium silicate (Ca2SiO4) already in the preheaters and calciner.

Then, the material moves on to the kiln, where the actual clinker forms. In the burning zone of the kiln, the temperature is typically between 1350 and 1450C. Multiple chemical reactions take place in the kiln, as the free lime (CaO) reacts with SiO2, Al2O3, Fe2O3, and Ca2SiO4.

Freshly produced clinker moving from the kiln to the clinker cooler. Image from visiontir.com.

5. Clinker cooling
After clinker formation at high temperatures, the material has to cool down. If the clinker is cooled too slowly, the following undesired chemical reaction may take place:

Ca3SiO5 → Ca2SiO4 + CaO

Clinker coolers are designed to minimise this reaction, which negatively influences the cement’s crystalline structure and early strength, by rapidly cooling down to below 1200C and down to a range of 150–300C. Air blowers are typically used and the coolers are designed to maximise heat exchange. The heated air is often reused for combustion in the kiln, which improves the system’s overall energy efficiency. When heat exchange and clinker distribution is not effective, undesired clinker deposits sometimes enriched in K2O and SO3 (also referred to as “snowmen”) can happen, which can lead to operational issues such as blockages and increased wear.

6. Addition of gypsum
Gypsum (CaSO4·xH2O) is added essentially to slow down the rate of hydration, which leads to hardening once the cement is mixed with water. Without gypsum, the cement would set almost instantly particularly due to the hydration of tricalcium aluminate(C3A or Ca3Al2O6). The hydration process is also highly exothermic and due to the addition of gypsum the generated heat is reduced/released more gradually, which prevents thermal cracking. For ordinary Portland cement, the amount of gypsum is 3–4% and in quick-setting cement it can be 2.5%. Gypsum can often be sourced sustainably, as it is a byproduct of industrial processes like flue gas desulphurisation.

7. Proportioning and final grinding mill
The final particle size distribution of the clinker is important for the cement’s quality and performance. If the particles are too large, it can lead to incomplete hydration of the cement, which reduces the concrete’s strength and durability. On the other hand, too fine particles can lead to high water demand, leading to issues like setting too quickly, shrinkage, and cracking. The final grinding step is most commonly done with a ball mill.

8. Cement storage and/or shipping
After the final grinding, the cement is stored in silos before it is packed and shipped. Proper storage is crucial; cement needs to be protected from moisture and other environmental factors that can affect its quality. Shipping normally happens in containers or bags and again exposure to moisture should be avoided.

The cement making process. From Encyclopedia Britannica.

What types of cement are there?

Generally, almost all cement types are based on Portland cement or some variation of it. The term “Portland cement” comes from its similarity to Portland stone, which was quarried on the Isle of Portland in Dorset, England. Joseph Aspdin, a bricklayer and stonemason, first patented it in 1824 and used this name as the finished cement resembled the Portland stone in colour and quality.

Cement types are typically classified based on its compressive strength (in N/mm2 or MPa), specific composition, and/or specific application. From a general perspective, the European Norm (EN) focusses more on composition in the classification system, whereas the American Society for Testing and Materials (ASTM) classifies cement rather by its strength and performance.

European classification
European cement types are classified under the EN 197–1 standard, which specifies various types of cement based on their composition and intended use. See the table below for an overview:

Each type of cement is further divided into subcategories based on its specific composition:

• CEM II is subdivided based on the type of additional constituent, such as slag, fly ash, limestone, etc. Based on the exact constituent, an additional letter is added to the abbreviation. For example, CEM II/L means limestone is added and CEM II/LL means the limestone content is higher. “D” indicates the addition of fumed silica, which enhances durability and mechanical properties. “M” stands for mix and can include various additions.
• CEM III is categorised as A, B, or C based on the percentage of blast furnace slag.
• CEM IV is categorised based on the type and amount of pozzolanic material.
• CEM V combines both pozzolanic and slag components.

US classification
The types of cement used in the US are specified by the ASTM C150 standard. This standard categorises Portland cement into eight different types, each suited for different uses. See the table below for an overview:

On top of that, the US has blended cements as per ASTM C595 and performance-specific types as per ASTM C1157. See the table below:

Compressive strength
Compressive strength after a certain period of time is a very important property typically for all cement standards (EN, British Standards (BS), ASTM). For example following European standards (EN-197), compressive strength is determined after 28 days under standard curing conditions. 32.5N (meaning 32.5 N/mm2 after 28 days of curing) is the most commonly used cement for non-structural applications. For structural applications 42.5N or 52.5N are needed. Another class of cements is the “R” category, which stands for rapid setting and hardening. A 42.5R cement, for example, has a compressive strength of 20 N/mm2 after 2 days already, while a 42.5N cement is only at 10 N/mm2 after 2 days. To achieve early strength, the clinker content is typically higher and the clinker is normally ground more finely.

US standards also test for compressive strength and typically report the values for different curing times (1 day, 3 days, 7 days, and 28 days) in MPa. See for example this resource for more specific numerical values per cement type.

How is concrete made?

When the cement is ready, we can move on to making the actual concrete. The basic ingredients for concrete are cement (10–15% by weight), water (5–10% by weight), aggregates (60–75% by weight).

Mixing
The cement, water, and aggregates are mixed together. Homogeneous mixing is important for the concrete’s final quality. Mixing can, for example, be done in a large scale industrial mixer, a transit truck mixer, or smaller on-site mixers.

Graphic of concrete mixing.

Hydration and setting
When water is added to cement hydration occurs, which is defined as a series of chemical reactions. In the presence of water, the calcium silicates (CS) form calcium silicate hydrates (CSH). This leads to hardening of the mixture.

Graphical representation of cement hydration.

The mixture must be poured, placed, and/or formed within a certain timeframe to make sure it is still malleable.

Curing and finishing
After the concrete has been placed and compacted, curing is the next step. During this process, it is important to maintain suitable moisture content and temperature to obtain desired strength and durability. Spraying water or blowing steam are common procedures during the curing process. Curing can take anywhere between 3 and 28 days and once that is done, finishing steps to achieve desired texture (texturing), surface appearance (colouring or polishing), or durability (sealing) can still be undertaken.

How can we make concrete more sustainable?

Although cement is only about 10–15% by weight of the concrete, it makes up about 90% of the total CO2e emissions related to concrete. Therefore, to reduce the emissions drastically, we should be addressing the emissions at the source: the cement. Below are some opportunities that have been investigated for CO2e reduction of the cement industry:

Low carbon fuels / energy
Clinker and cement production are generally energy-intensive processes. Clinker consumes about 945 kWh per tonne, of which 90% is currently fossil-based energy, and cement about 100 kWh per tonne. Roughly 40% of a cement plant’s emissions come from burning fuel. It is obvious that the carbon footprint of cement production can be lowered by using renewable energy instead of fossil-based energy. However, in practice this is more challenging than it seems at a first glance as most systems are based on coal or natural gas heating and will not be replaced until their end-of-life.

In most cement plants, energy and waste heat are already recycled and integrated where possible. Another approach that is being implemented in some plants already is the addition of municipal waste or biomass waste or used tires as alternative fuel. Some alternative fuels could be used as a drop-in directly in existing systems, such as wood chips or car tires, but many of these fuels suffer from low adiabatic flame temperatures and slower burning rates, which makes the process inefficient. Alternative fuels can lower the fossil fuel consumption and the ashes can be incorporated into the clinker. It is estimated that switching fuels can cut cement emissions by roughly 7%.

Process optimisation
Typically, to be on the safe side, an excess of concrete is used in buildings. Cutting down the excess of concrete used, roughly 26% of concrete’s emissions can be saved.

Implementing advanced analytics and digital technology can lead to more efficient operations and lower emissions. For example, optimising a kiln’s heat profile through self-learning models can save fuel and reduce emissions.

Alternative raw materials
The amount of cement used in the concrete mixture could be lowered by using alternative raw materials that do not reduce the mechanical characteristics of the concrete. For example, scrap rubber has been tested as an aggregate or filler for concrete. However, the more rubber particles are added, the lower the concrete’s overall compressive strength. Apart from regulations often prohibiting or limiting the use of alternative raw materials, compromising compressive strength is typically the main challenge.

While the cement composition varies slightly, 90% clinker and 10% gypsum is a common ratio in the US. Globally, the average clinker content was about 71% in 2022 and the remaining 28% typically consisted of gypsum, fly ash, and steel slag.

Reducing demand for clinker, which is carbon-intensive to produce, by substituting it with waste materials like blast furnace slag and fly ash from coal power plants, can significantly cut emissions. Up to 30–40% of clinker can be substituted without compromising cement strength. LC3 even estimated that up to 50% of clinker can be substituted by supplementing with more clay and unprocessed limestone. In this case too, regulations often limit the amount of substitutes allowed in cement and concrete.

Recycled concrete
End-of-life concrete can be used to substitute primary raw materials, reducing CO2e emissions by decreasing the need for new cement production. The concrete waste is typically broken down into smaller pieces which are then processed to separate the aggregates from the cement fines and to remove any impurities. The cement fines are more challenging to recover compared to the aggregates due to their small size.

Globally we have between 3 and 10 billion tonnes of concrete waste every year. If all of this were to be reused to replace primary raw materials, we could save up to 720 Mt CO2e annually.

In terms of regulatory approval, in Europe NEN-EN 197–6 on “cement with recycled building materials” states that the amount of recycled materials should be limited to 35% by weight to comply with current regulations.

Carbon storage in concrete
Attaching carbon capture plants to cement kilns can capture the CO2 released during the production process, i.e. blue cement. This approach has substantial potential, with numerous pilot and larger-scale demonstrations planned.

Alternatively, CO2 can be captured and mineralised typically into a calcium carbonate (CaCO3) or magnesium carbonate (MgCO3). This sequesters CO2 and can reduce the amount of (virgin) cement required. A bottleneck here is that the CO2 gas (reacting with the calcium or magnesium source) typically needs to have high purity, which means it either needs to undergo gas cleaning (e.g. biogas and flue gas (from the cement plant itself)) or it originates from direct air capture (DAC) which is energy-intensive and expensive. There are essentially two methods to store carbon in concrete:

1. Carbonation of cement or concrete: Cement powder or concrete (e.g. demolition waste, but also other materials like olivine can be used) is exposed to CO2 under controlled conditions and reacts with the calcium silicates or magnesium silicates to form calcium carbonates or magnesium carbonates.
2. Direct mineralisation: CO2 is directly injected into concrete during the mixing process and reacts with calcium-containing compounds to form carbonates. This pathway can potentially be integrated into the manufacturing of precast concrete.

Both methods could potentially increase the concrete’s strength and reduce the amount of raw materials needed, but the costs and energy input required to capture and process CO2 can be significant. Apart from costs and scaling, integration into existing infrastructure poses challenges.

In terms of regulatory approval some progress has been made. For example, in the US some states have approved the use of CarbonCure as an ASTM C494 Type S admixture. In Europe, there are no specific regulations around CO2 mineralised concrete yet.

Electrochemistry
Roughly 60% of the cement plant’s emissions come from the calcination of limestone into lime. In an attempt to curb these emissions, researchers are working on an electrochemical method to do this conversion. Limestone (CaCO3) is dissolved in the acid at one electrode and CO2 is released, while lime (CaOH) precipitates out as a solid at the other electrode. The CaOH can then be processed to produce the cement. Inventions like these ones are often still at early developments stages (TRL 1–4) and their retrofittability as well as industry acceptance is still to be determined.

Biochemistry
Calcium carbonate (CaCO3) can be synthesised via biological pathways using enzymes in a calcium-rich solution. Such technologies are often in early stages of development (TRL 1–4) and time will have to tell whether they are retrofittable and adopted within the wider industry. Although the time to maturity is likely to be at least 3–5 years, such pathways may have potential to reduce emissions for prefabricated and non-structural concrete in the long run.

In summary, the path to reducing the environmental impact of cement and concrete involves both innovative approaches and collaborative efforts. The adoption of alternative raw materials, enhanced recycling practices, and cutting-edge carbon capture technologies presents a compelling opportunity for significant reductions in CO2 emissions. These solutions not only address environmental concerns but also offer potential cost savings and efficiency improvements.

We encourage entrepreneurs, scientists, and other innovators working on green solutions for the cement and concrete industry to connect with us. By joining forces, we can accelerate the development and implementation of these technologies. If you are something exciting that could transform the industry, reach out to our team at Extantia!