What Is Blended Cement and Why Is It Helping the Planet?

Blended cements have been around since the 1960s, but they have never managed to completely or even substantively replace standard Portland cement despite the environmental advantages.

As the binding element in concrete, the single most important construction material in the world, the importance of and need for cement will not diminish in coming years.

But, are there alternatives that are safer for the planet and still provide the highly important building material of concrete?

Yes.

This article highlights the virtues of blended cement and its role in revolutionizing the construction industry through its reduced environmental impact.

What Is Blended Cement?

Blended cement is a composite material that is made from mixing a certain amount of Portland cement or Portland clinker (the pre-grinding phase of cement) with supplementary cementitious materials (SCMs).²² In theory, the greater the proportion of SCMs to OPC, the more sustainable the cement is, and this usually holds true in practice, as well.

To be considered a blended cement, the cement mixture need only contain one SCM in addition to the Portland cement. However, many blended cements contain two or more SCMs alongside the OPC.

Blended cements are viewed as more durable, less vulnerable to chemical corrosion, and less susceptible to shrinking and thermal cracking. The most commonly used SCMs in blended cement are fly ash and slag cement, though limestone and silica fume are also sometimes mixed into the process.²⁶

In a report published by the Global Cement and Concrete Association of India, three types of blended cements are compared and contrasted against OPC in terms of quality, function, and environmental impact.

Those three types are:⁵

• Portland Pozzolan Cement (PPC): Made from 64% Portland clinker, 31% fly ash, and 5% gypsum
• Portland Slag Cement (PSC): Made from 38% Portland clinker, 57% slag cement, and 5% gypsum
• Composite Cement (CC): Made from 45% Portland clinker, 25% slag cement, 25% fly ash, and 5% gypsum

More Information on the Supplementary Cementitious Materials Used in Blended Cement

As mentioned above, supplementary cementitious materials (SCMs) are often industrial by-products that can be combined with standard cement to create an improved, blended cement with a lower carbon footprint.

The three most common SCMs are fly ash, silica fume, and blast-furnace slag.

The following section examines each of these by-products individually.

What Is Fly Ash?

When coal is burned, different types of residue may be produced.

One type, which is produced with the burning of crushed coal, is a fine, particulate matter known as fly ash.⁷ Unlike the coarse, bottom ash that collects in the combustion chamber, the finer fly ash is carried away with the flue gases.

Before the exhaust fumes are released into the atmosphere, the fly ash is captured and removed.⁷

Fly ash is considered a major pollutant, and it is estimated that 1.2 billion tons are produced every year during coal combustion.¹⁸ In the U.S. alone, approximately 40.8 million tons of fly ash are generated each year at coal-fired power plants, according to the U.S. Energy Information Administration (EIA).⁴

Fly ash was first used in concrete during the Great Depression of the 1930s, and to this day, the production of blended cements and concrete is the most practical and advantageous use of this material. In 2016, it was estimated that just under half of the fly ash produced per annum in the U.S. was recycled into a usable, beneficial product.²²

What Is Silica Fume?

Silica fume is a by-product of silicon alloy production in electric arc furnaces.It contains above 90% silicon dioxide with minute proportions of iron, alkali oxides, and magnesium.⁹

It is an extremely fine powder also known as microsilica. It is highly pozzolanic, producing a hardening effect when exposed to certain compounds.

Blended cements made with silica fume are stronger and less porous than standard cement but require more water in the mixture. Like fly ash, silica fume must be captured before it is vented into the atmosphere.²⁰

Silica fume is an environmental pollutant and is considered a hazardous substance posing significant health risk. Long-term exposure has been shown to cause damage to the lungs.⁸

The application of silica fume in cement and concrete production was discovered after the institution of air pollution controls regulating the release of fine particles into the air.¹⁷

What Is Slag Cement?

Ground-granulated blast-furnace slag, or slag cement, is produced when iron ore is smelted in a blasting furnace to produce iron and steel. The slag is separated from the iron at various stages in the process.

As it is cooled with water, it hardens into a glass-like substance. As it is hydraulic, slag cement can be used to replace OPC in cement mixtures, but it has a slower reaction time than OPC and is usually combined with standard cement.²²

Slag cement can make up anywhere between 30 and 70% of a concrete mixture, and it has the advantages of increased strength, resistance to chemical corrosion, and decreased porosity. It is considered a sustainable choice for concrete production as it requires barely a tenth of the manufacturing energy as OPC, significantly reducing carbon emissions.²¹

What Is ASTM C595?

ASTM International is an organization that develops global standards for materials and products, and so forth, enforcing product quality and safety.^1,2 There are more than 12,000 international standards active today.

ASTM C595 is the standard that applies to blended hydraulic cements.

Specifically, this standard describes the approved composition of various blended cements. It further describes how blended cements should be named to identify their component parts.¹²

Under ASTM C595, two types of blended hydraulic cements are designated: Type IS and Type IP.^3,26

• Type IS: Portland cement/slag cement blend with up to 70% slag cement
• Type IP: Portland cement/pozzolan cement blend with up to 50% pozzolan, typically fly ash

How Does Blended Cement Help the Environment?

According to MIT News, the cement industry is responsible for approximately 8% of total CO₂e emissions worldwide,¹⁵ and 2% of the U.S. emissions, per the DOE.²³ Remember, also, that the production of 1000 kg of standard Portland cement emits 900 kg of CO₂e, and cement accounts for 88% of concrete’s carbon footprint!^16,25

It is evident that the need for radical change in the industry is necessary, and many people consider blended cements to be the first big step in significantly reducing the vast carbon footprint of the cement and concrete industry. The use of blended cement in construction reduces the carbon footprint of building materials AND the carbon footprint of buildings in general by:²⁴

• Recycling demolished concrete materials, thus avoiding additional waste
• Reducing fossil fuel combustion by reducing the amount of natural resources mined and materials manufactured to produce OPC
• Producing concrete that will endure longer, again decreasing waste
• Reducing toxic emissions and leached contaminants from industrial by-products by capturing and repurposing those by-products into usable ingredients

What Is Cement?

Cement, a binding substance used in concrete, mortar, plaster, and so on, is probably the most common material used in construction. Manufactured as a fine, dry powder, the cement remains such until it is mixed with water.

Once introduced to water, powdered cement undergoes a series of chemical reactions which cause it to set and harden, even in wet conditions or underwater. At least, this is how hydraulic cement works and hydraulic cement will be the focus of this article.⁶

As a brief aside, non-hydraulic cements have also been used throughout history. These types of cement do not harden when exposed to water; rather, they begin to set when exposed to carbon dioxide.

Unfortunately, the drying process can be lengthy for non-hydraulic cements, and thus, they are much less common than their hydraulic counterparts.¹⁰

Cement production in the United States increases year over year, with approximately 95 million tons produced in the country in 2022. As such, the U.S. produced more cement in the year 2022 than most other countries, with the following exceptions:¹³

• China produced 2.1 trillion tons of cement
• India produced 370 million tons of cement
• Vietnam produced 120 million tons of cement

The worldwide production of cement in 2022 totaled 4.1 trillion tons (see the 2023 U.S. Geological Survey)!¹³ This number is particularly concerning when held against the shocking statistic that production of one ton of standard cement equates to 900 kg of carbon dioxide equivalent (CO₂e) emissions, nearly a 1:1 ratio!^16,25

This is much of the reason why blended cements are garnering so much attention.

How Is Standard Cement (Portland Cement) Made?

The majority of the cement manufactured today is ordinary Portland cement, or OPC, which came into popularity in the 1850s.

OPC is a hydraulic cement that is durable and versatile and can be mixed with sand or gravel to produce concrete. The primary ingredients in OPC are limestone, shales, marl, and gypsum.¹⁹

The first step in producing Portland cement is to mine the raw materials: mainly sedimentary rock. The rock is then crushed and ground into a coarse meal.

Next, these raw materials are added to a kiln and heated to high temperatures to remove carbon and fuse the materials into clinker: small lumps of aggregate. The marble-sized aggregate is then ground into a fine, hydraulic powder which hardens upon introduction to water and is water-resistant once set (see U.S. Department of Energy (DOE) Cement and Concrete Manufacturing).^14,27

What Are Some Other Types of Cement?

There are many types of cement in use today, and most, though not all, of these are formed by combining OPC clinker with other materials to produce blended cements.

The following is a list of the most well-known types of cement.^6,11

Portland Cement Blends	Other Cements
Colored cement: OPC mixed with pigment	Calcium aluminate: Limestone and bauxite mixture
Expansive cement: OPC mixed with an expanding agent	Calcium sulfoaluminate: Ye’elimite in mixture
Hydrographic cement: OPC mixed with/ hydrating additives	Electric: From recycled cement
Masonry cement: OPC mixed with various materials	Fiber mesh: Fibrous materials
Portland blast-furnace slag cement: OPC mixed with blast furnace slag	Geopolymer: Water-soluble alkali metal silicates and mineral powders mixture
Portland fly ash cement: OPC mixed with fly ash	Natural: Formed from burning clay minerals
Portland limestone cement: OPC mixed with additional limestone	Polymer: Made from organic chemicals such as thermoset
Portland pozzolan cement: OPC mixed with fly ash or volcanic ash	Pozzolan lime: Pozzolan ash and lime mixture
Portland silica fume cement: OPC mixed with up to 20% silica fume	Slag lime: Blast-furnace slag and lime mixture
Rapid hardening cement: OPC mixed with higher content calcium chloride	Sorel: Magnesium oxide and magnesium chloride mixture
White cement: OPC mixed with metakaolin or like materials	Supersulfated: 80% blast-furnace slag, 15% gypsum/anhydrite, and lime mixture

Differentiating Cement vs Concrete

Sometimes, people will confuse cement and concrete and use the terms interchangeably, but while they are closely associated, they are not one in the same. In thinking about cement vs concrete, one important thing to remember to differentiate the two is that cement is an ingredient while concrete is a final product.

According to MIT News, concrete is the second most consumed material, surpassed only by water, as it is a critical construction material for any large building project.¹⁵ Note, however, that you can’t make concrete without cement, and cement is the most important component of concrete.²⁷

Cement acts as a glue which holds the constituent ingredients of the concrete (sand and stone) together when water is added.

Cement is an indispensable material in essentially every big construction project, and through the years, people have discovered that they can make cement blends by combining standard cement with the refuse from blast and smelting furnaces.

These blended cements have many merits and advantages over standard cements, and they are a critical piece to making the cement and concrete industry more sustainable.

Frequently Asked Questions About Blended Cement