The first step is to quarry the raw materials necessary to make cement, which is mainly limestone coupled with clay and sand. Using fly ash or other byproducts to partially replace the raw materials keeps things like fly ash and industrial byproducts out of landfills and contributes to a circular economy.

After quarrying the rock, it is crushed into baseball sized pieces. Once crushed, the raw materials are then sent to a laboratory and analyzed – each step of the cement production process is carefully monitored to ensure efficiency and quality.

The crushed rock is then ground finer (in this case, in a vertical roller mill) and combined with other ingredients. Common materials used to manufacture cement include limestone, chalk, and marl combined with shale, clay, slate, blast-furnace slag, silica sand, and iron ore. Numerous industrial byproducts are used to supplement some or all of these components. Examples of recycled materials that are used include spent pot liners, slags, fly ash or bottom ashes, and various incinerator ashes.

Inside the vertical roller mill

The material then moves through the preheater tower, starting at the top and moving to the bottom. The pre-heater tower helps make the kiln process more efficient by adding heat generated from the plant to the raw materials, starting the chemical reaction.

Doing this makes the cement process more efficient. Cement production is one of the most efficient in heavy industry, at approximately 80% – for comparison, a typical car runs at about 20% efficiency.

Promising innovations have been developed over the years, such as carbon capture and storage technology, which are being evaluated and can help cement plants reduce their overall carbon footprint.

The cement kiln heats all the ingredients to about 2,700 degrees Fahrenheit in huge cylindrical steel rotary kilns lined with special firebrick and the material becomes partially molten. The finely ground raw material is fed into the higher end of the cement kiln. At the lower end is a roaring blast of flame, produced by the precisely controlled burning of powdered coal, oil, alternative fuels (such as scrap vehicle tires, for example), or natural gas under forced draft. During this process, cement manufacturers have been a leader in the use of alternative fuels to reduce waste going to landfills. In fact, alternatives fuels now represent more than 15% of total cement plant energy consumption in the U.S.

As materials move through the kiln, certain elements are driven off in the form of gases. The remaining elements chemically unite to form a new substance called clinker, which comes out of the kiln as grey balls about the size of marbles.

The clinker comes out of the kiln red-hot and has a consistency similar to a lava flow. It is brought down to handling temperature in a clinker cooler. Heated air from the coolers is returned to the kilns, a process that saves fuel and increases efficiency.

Inside the clinker cooler

Emission control devices exist throughout the process – these are called bag houses. Bag houses collect dust particles and other emissions.

After the clinker is cooled, it is ground with small amounts of gypsum, limestone, and other ingredients to make cement. In this case a ball mill with steel balls is used to reach the fineness needed to achieve desired properties. To reduce the carbon footprint by an additional 10% or more, higher limestone contents or SCMs can be blended in. This permits cements and concrete to achieve the same levels of strength, resiliency, and durability.

Inside the ball mill

The cement is ready to be used for construction projects to build long-lasting concrete structures and pavements.
Many construction projects are conveniently located within 15 miles of a ready-mix concrete plant, again limiting energy use and emissions associated with transportation.
Cement is the main ingredient in concrete. Concrete pound for pound is a low carbon footprint material, lower than wood, steel, plastic, or asphalt.
As a versatile and durable material, it is the most widely produced material on earth, providing housing, transportation, water processing and storage facilities, hospitals, and schools. It is a foundational material for modern civilization.