How Cement is Made

Cement & Concrete

An English bricklayer made the first portland cement early in the 19th century by burning powdered limestone and clay on his kitchen stove. With this crude method, Joseph Aspdin of Leeds laid the foundation for an industry that annually processes literally mountains of limestone, clay, cement rock, and other materials into a powder so fine, it will pass through a sieve.

Portland cement is the basic ingredient of concrete. Concrete is formed when portland cement creates a paste with water that binds with sand and rock to harden.

Fast Fact

Concrete is the most widely used resource in the world after water.

What Cement Is Made From

Cement is manufactured from materials heated together at high temperatures to form a rock-like substance that is ground into a fine powder.

Main Ingredients

Calcium
Silicon
Aluminum
Iron

Limestone, marl, and clay are commonly used sources of these primary elements.

Other Ingredients

Other materials used to manufacture cement include: shells, chalk, shale, slate, silica sand, iron ore, and blast furnace slag.

Close up of powdered cement with a shovel

Quarrying

Cement manufacturers quarry raw materials including limestone, clay, and sand. They use fewer resources and contribute to a circular economy by adding raw substances called alternative raw materials, including fly ash, blast furnace slag, and other byproducts.

Crushing

After quarrying, producers break the rock into baseball-size pieces, sending samples to a laboratory for analysis. Each step of the cement production process is carefully monitored to ensure efficiency and quality.

Grinding, Mixing, and Blending

The rock fragments are finely ground and combined with other ingredients including clay, chalk, marl, shale, slate, sand, and iron ore. Recycled industrial by-products used to supplement some or all of these components include spent pot liners, slags, and fly, bottom, or incinerator ash.

Preheating

The mixed and blended materials travel through a preheater tower, which uses heat generated by the plant to start chemical reactions between the components—making the next process, kiln firing, even more efficient.

Kiln Firing

The finely ground raw material enters the higher end of the kiln—a large cylindrical steel structure lined with special firebrick. At the lower end of the kiln, a roaring flame heats the material to about 2,700 degrees Fahrenheit, making it partially molten. In addition to fossil fuels, U.S. plants coprocess alternative fuels including tire-derived fuel, reducing landfill waste while creating more than 15% of the energy used by U.S. cement plants.

Forming Clinker

As materials move through the kiln, a series of complex reactions take place resulting in a new substance called clinker. Clinker leaves the kiln as grey balls about the size of marbles.

Cooling

The clinker comes out of the kiln red-hot with a consistency like a lava flow. It is quickly cooled down to a handling temperature in a clinker cooler where heated air from the coolers is recovered and returned to the kilns. This process saves fuel and increases burning efficiency.

Controlling Emissions

Emission control devices collect dust particles and treat emissions to reduce any environmental impact. This happens throughout the manufacturing process, making it more sustainable. Manufacturers are implementing innovations including carbon capture, utilization, and storage (CCUS) technology, which may help to further reduce a cement plant’s carbon footprint.

Pulverizing

The clinker keeps cooling and a ball mill grinds it with gypsum or other ingredients into fine cement—about 150 billion grains per pound. Blending in higher ratios of limestone or SCMs can reduce a plant’s carbon footprint by up to 10%.

Preparing for Construction

The cement is now ready for use in long-lasting concrete structures and pavements. When a project is located within 15 miles of a ready-mix concrete plant, energy savings and reduced emissions associated with transportation are significant benefits.

Cement Goes Greener

The cement and concrete industry is dedicated to reaching carbon neutrality by 2050 or sooner. The use of lower-carbon cement blends, alternative fuels, re-use of waste products and other innovations is occurring and will continue to grow to lower the environmental impacts of concrete construction.

The Roadmap to Carbon Neutrality