top of page


Many questions can arise when considering how to reduce carbon emissions in concrete. These frequently asked questions are provided as a guide for the user’s convenience, drawing from existing ACI documents and materials.

An icon of a book with a magnifying lens

 How does concrete manufacturing affect global CO₂ emissions? 

Cement is the key ingredient in concrete that helps to achieve desired properties in concrete including strength and durability. The cement used in majority of concrete applications is portland cement, and manufacturing portland cement results in the majority of the contribution of concrete to global CO₂ emissions.


Portland cement is manufactured from raw materials, typically limestone and clay that combine at high temperatures, approximately 2640°F (1450°C) to form calcium silicates that provide the binding properties of cement. Approximately 40% of the CO₂ emitted during pyroprocessing is due to fuel combustion, with the remaining 60% driven off of the limestone during calcination (conversion of limestone to lime and CO₂). It is generally estimated that producing 1 ton (900 kg) of portland cement produces 1 ton of CO₂. However, the cement manufacturing industry has been active and innovative over the last few decades to reduce carbon footprints, and therefore, these general estimates may be higher than actual values.


References: ACI PRC-130-19; SCG1; ITG-10R; ITG-10.1R-18; E3-13

Return to Top

 What are different approaches for reducing carbon emissions in  concrete? 

There are many ways to reduce carbon emissions in concrete manufacturing and construction including,

  • Efficient use of materials and energy in cement manufacturing

  • Alternative fuels in cement manufacturing

  • Supplementary cementitious materials (SCMs) to replace portland cement content

  • Blended cements

  • Performance-based cements

  • Alternative cements or binders

  • Water-reducing admixtures to reduce cement content

  • Optimization of aggregate gradation to reduce cement content

  • High-performance concretes

  • Efficient design to reduce total volume of materials used

  • Carbon capture and sequestration technologies

  • Reduce concrete waste

  • Recycle concrete and wash water


More information can be found in the following resources.


References: SCG1; ACI PRC-130-19; ACI PRC-232.2-18ACI PRC-232.5-21; ACI PRC-233-17; ACI PRC-234-06; ACI PRC-225-19; ACI PRC-212.3-16; ITG-10R; ITG-10.1R-18; ACI PRC-325.14-17; ACI PRC-555-01; ACI CODE-318-19

Return to Top

 What are the barriers to low carbon concrete? 

The cement and concrete industry, and researchers have been working on finding ways to reduce the carbon footprint of concrete while maintaining the expected performance of concrete for the past several decades. They have come up practical solutions to produce low carbon concrete and continue to work toward producing carbon neutral concretes.

Several initiatives have been taken to resolve energy usage in cement kilns and reduce carbon emissions. Many alternative technologies, alternative cements and supplementary cementitious materials to reduce carbon footprints have been investigated and tested in laboratories. However, one of the major barriers to implement the most innovative methods is a lack of standards or acceptance criteria. Innovative approaches are sometimes expensive and require new capital equipment or processes that make them difficult to commercialize. For instance, some new technologies may require upgraded cement plants to produce low carbon cements or alternative cements as well as upgraded ready-mix plants to use the new materials. Some technologies may also require training for effective placement and curing of new types of concrete.

References: SCG1; ACI PRC-130-19; ITG-10R; Thoughts Regarding Cement, SCMs, Concrete, and CO₂

Return to Top

 What is Life-Cycle Assessment (LCA)? 

Life-Cycle Assessment (LCA) is a method to evaluate the full environmental impact associated with a process or a product over its lifetime. A credible LCA of a product or process helps to make awareness and transparency of its environmental impacts and provides the opportunity for improvements.

The requirements and guidelines for conducting an LCA are described in international standard ISO 14044. An LCA consists of four iterative steps: Goal and Scope, Life-Cycle Inventory (LCI), Life-Cycle Impact Assessment (LCIA), and Interpretation. Cradle-to-grave LCAs are the most representative accounting of environmental impact of a product or process because they include all environmental impacts over its full life cycle. For example, an LCA of a concrete product provides a broad range of environmental impacts such as greenhouse gas emissions, toxic releases, energy use, and other resource use, considering several phases of the product including raw material acquisition, cement manufacturing, concrete production and construction, transportation, utilization, and end use.


References: ISO 14044:2006; ISO 14040:2006; SCG1

Return to Top

 What is an Environmental Product Declaration (EPD)? 

An Environmental Product Declaration (EPD) is a comprehensive summary of multiple sustainability measurements associated with a process or a product based on its Life-Cycle Assessment (LCA). For example, an EPD may include (but are not limited to) global warming potential, acidification potential, eutrophication potential, ozone depletion potential, photochemical ozone creation potential, energy and other resource consumption and generation of wastes.

In order to fulfill the requirements for compiling and presenting data in the EPD, a Product Category Rule (PCR) must be established prior to developing an EPD to provide instructions on how to conduct an LCA. The principles and procedures for developing Type III environmental declaration programs and declarations are described in ISO 14025. This standard establishes the use of LCAs as per ISO 14040. An EPD must be peer reviewed and third-party verified.


An EPD provides transparent data that allows architects, engineers, building owners, and other specifiers to better understand the environmental impacts of a certain product or process, and also helps to compare different products or processes. For cement and concrete, industry wide EPDs are commonly available. These EPDs represent average environmental impacts associated with a certain type of concrete or a group of concrete products. To have a better understanding of environmental impacts associated with a concrete product, regional EPDs are essential.

References: ISO 14025:2006; ISO 14044:2006; ISO 21930:2017; ACI Concrete Sustainability Forum VI; ACI PRC-232.2-18

Return to Top

 What is a Product Category Rule (PCR)? 

A product category rule (PCR) provides instructions as to how to conduct a Life-Cycle Assessment (LCA) to develop an Environmental Product Declaration (EPD) for a specific product or product category. A product category is a defined family of products that has similar functionality.


PCRs are created or administrated by program operators. Core set of PCRs for developing EPDs of construction products or services are defined in ISO 21930:2017. This standard refers to ISO 14025:2006 for conducting a PCR review. The two key components of a PCR are to set rules for LCA and to set rules for what is reported in an EPD. Setting up rules for LCA includes specifying life-cycle stages for inclusion (e.g., cradle-to-grave), parameters to be covered and cut-off rules for data, and life-cycle impact assessment (impact categories and assessment method). Setting up rules for what is reported in an EPD includes information about program operator and verifier, required format for EPD label, impact categories to list and inventory items to list.


A better understanding of PCRs is needed in order to compare EPDs from one product to another. Practitioners can use a PCR that has been established for a specific product or process to generate consistent results within the same product category for similar applications. Conversely, a single PCR cannot be used for different product categories. For example, PCRs for ready mixed concretes differ from PCRs for precast concretes or concrete masonry units.


References: ISO 21930:2017ISO 14025:2006ITG-10.1R-18

Return to Top

 What is the NEU technology validation/verification program? 

With increasing focus on climate change, there is a corresponding increase in product manufacturers and technology development companies making claims in the marketplace to position themselves and their products/technology as possible solutions to reducing carbon emissions in the built environment. With the wealth of information out there, it is not easy for decision-makers to decipher the best products/technologies to use. Therefore, it is important that as emerging products/technologies come forward, they are vetted and assessed for their efficacy.

To facilitate that, NEU is developing a technology validation/verification program to assess and validate the claims of innovative and new products/technologies associated with low carbon cement and concrete production. The terms validation and verification are defined as below.


Validation: Process for evaluating reasonableness of the assumptions, limitations and methods that support an environmental statement about the outcome of future activities.


Verification: Process for evaluating an environmental information statement based on historical data and information to determine whether the statement is materially correct and conforms to relevant criteria.

The product/technology categories for NEU validation/verification program will include, but not be limited to the following,

  • Alternative Cements/Binders

  • Supplementary Cementitious Materials (SCMs)

  • Admixture / Additives

  • Carbon Capture, Utilization and Storage (CCUS)

  • Ultra-High-Performance Concrete (UHPC)

  • Manufactured Aggregates

  • Nanoparticles

  • Low-Carbon Reinforcing Steel

  • Reduced-Emissions Cement Manufacturing / Alternative Fuels and/or Energy Sources


The process is conducted impartially by subject matter experts (SME) based on internationally accepted standards and will consist of several phases including application and initial review, independent SME committee formation, SME review and report drafting and public release or appeal. Pilot projects for this process are being identified by NEU to help facilitate the development and protocols needed for this program.


The Validation/Verification program is a key function of NEU and plans are underway to start accepting applications in coming months.


References: ISO 14065:2020 General principles and requirements for bodies validating and verifying environmental informationISO 14066:2011 Greenhouse gases — Competence requirements for greenhouse gas validation teams and verification teamsISO 14067:2018 Greenhouse gases — Carbon footprint of products — Requirements and guidelines for quantificationISO/IEC 17029:2019 Conformity assessment — General principles and requirements for validation and verification bodies

Return to Top

 What is portland cement? 

Portland cement is the product obtained by pulverizing clinker, consisting of hydraulic calcium silicates to which some calcium sulfate has usually been provided as an interground addition. When first made and used in the early 19th century in England, it was termed portland cement because its hydration product resembled a building stone from the Isle of Portland off the British coast. The first patent for portland cement was obtained in 1824 by Joseph Aspdin, an English mason. The specific gravity of portland cement particles is about 3.15.


There are four primary phases in portland cement: tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetracalcium aluminoferrite (C4AF). The strength and other properties of concrete are mainly derived from the hydration of tricalcium and dicalcium silicates. The composition of any of these phases in a particular clinker will not be precisely in the composition indicated.

References:SP-1(02);ACI CT-23;ACI 225R-19;E3-13; ACI B-14;ACI Physical Testing of Cement Training Video

Return to Top

 What are the raw materials used in the manufacture of portland cement? 

The two principal materials from which portland cement is made are a material of high lime content, such as limestone, chalk, shells, or marl, and a material of high silica and alumina content such as clay, shale, or blast-furnace slag. A small amount of iron is also needed. Sometimes the principal materials are combined in naturally occurring deposits. The proportions of the raw materials need to be controlled to ensure a uniform product.


References: SP-1(02); ACI PRC-225-19; E3-13 

Return to Top

 What is a pozzolan? 

A pozzolan is a siliceous or siliceous and aluminous material that in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds having cementitious properties. It is therefore classified as cementitious material. There are both natural (e.g., metakaolin – ACI PRC-232.1) and artificial (e.g., fly ash - ACI PRC-232.2, and silica fume, ACI PRC-234) pozzolans. Descriptions of various kinds of pozzolans and specifications for them are given in ASTM C618 and ASTM C1240.


References:SP-1(02); E3-13; SP-221; ACI CT-23; ACI PRC-232.1-12; ACI PRC-232.2-03; ACI PRC-234-06; ASTM C618; ASTM C1240

Return to Top

 What does a pozzolan do in the concrete? 

As the definition implies, a pozzolan combines with calcium hydroxide in the concrete to form calcium silicate hydrate, similar to that produced by hydration of portland cement. This adds to the strength, impermeability, and sulfate resistance, and reduces expansion from the alkali-silica reaction that might otherwise take place. Use of pozzolans may increase or decrease water demand depending on the particle shape, surface texture, and fineness. Fly ash usually decreases water demand. Most of the other pozzolans increase the water demand. Pozzolans reduce bleeding because of fineness; reduce the maximum rise in temperature when used in large amounts (more than 15% by mass of cementitious material) because of the slower rate of chemical reactions; which reduce the rise in temperature.


References:SP-1(02); E3-13; SP-221; ACI CT-23; ACI PRC-232.1-12; ACI PRC-232.2-03; ACI PRC-234-06; ASTM C618; ASTM C1240

Return to Top

 What is fly ash? 

Fly ash is the finely divided residue that results from the combustion of ground or powdered coal and that is transported by flue gasses from the combustion zone to the particle removal system. ASTM C618 provides classification for Class F and Class C.


References:SP-1(02); ACI CT-23; SP-242; ACI PRC-232.2-03; ACI PRC-232.4-20; ACI PRC-232.5-21; ASTM C618

Return to Top

 Do all fly ashes behave in the same way? 

No, Class F fly ash has little lime and is purely pozzolanic; Class C fly ash typically has a high lime content and is self-cementing. Fly ashes may also have other chemically reactive constituents. All fly ashes possess pozzolanic properties. They have different characteristics in strength development and resistance to chemical attack.

References:SP-1(02); ACI CT-23; SP-242; ACI PRC-232.2-03; ACI PRC-232.4-20; ASTM C618

Return to Top

 What is slag cement or ground granulated blast-furnace slag (GGBFS)? 

Blast-furnace slag is the nonmetallic product consisting essentially of silicates and aluminosilicates of calcium and other bases that develop in a molten condition simultaneously with iron in a blast furnace. Granulated slag is the glassy, granular material formed when molten slag is rapidly chilled. Slag cement or GGBFS is granulated blast-furnace slag that has been finely ground and that is hydraulic cement. When slag cement is mixed with water, however, the initial hydration is much slower than portland cement mixed with water; therefore, portland cement or salts of alkali metals, principally sodium and potassium or lime, are used to increase the reaction rate of slag cement (ACI PRC-233). The ASTM C989/C989M slag-activity index is often used as a basic criterion for evaluating the relative cementitious potential of slag cements. Slags are classified into three grades (80, 100, and 120) based on their respective mortar strengths when blended with an equal mass of portland cement (ASTM C989/C989M).

References:SP-1(02); ACI PRC-233-03; E3-13; SP-221; ACI PRC-225-19; ACI CT-23; ASTM C989/C989M

Return to Top

 What are the benefits of using slag cement? 

The hydration products of slag cement are generally found to be more gel-like (less crystalline) than the products of hydration of portland cement; thus, they reduce the permeability of the cement paste (ACI PRC-233). Slags are used as a replacement of portland cement in amounts typically between 25 and 70% of the total mass of cementitious material. Slag concretes have improved long-term strengths even though their early strength development is lower than portland cement concretes. They also have lower permeability and improved durability. Slag cement generally costs less than portland cement.


References:SP-1(02); ACI PRC-233-03; E3-13; SP-221; ACI PRC-225-19; ACI CT-23; ASTM C989/C989M

Return to Top

 What is silica fume? 

Silica fume is very fine noncrystalline silica produced in electric arc furnaces as a byproduct of the production of elemental silicon or alloys containing silicon. The average size of a silica fume particle is about 100 times smaller than an average portland cement particle. It acts as a mineral filler and also as a pozzolan, leading to concrete of high strength and low permeability. Silica fume is marketed in different forms: as-produced, densified or compacted, and slurried.

References:SP-1(02); ACI CT-23; ACI 234R-06; E3-13; ASTM C1240

Return to Top

Carbon Emissions in Cement and Concrete
Validation/Verification of Technologies
Portland Cement
Cementitious Material in Concrete
bottom of page