Cement is made by chemically combining raw materials such as calcium, silica, aluminum, iron, and other elements under strict manufacturing controls. Additionally, limestone, shells, and chalk or marl are frequently combined with shale, clay, slate, blast furnace slag, silica sand, and iron ore to make cement.
This all depends on the type of cement being made for use as concrete.
Cement is one of the essential ingredients in concrete, the most mass-produced building agent in the world. This substance is a binding agent that originates as a fine, gray powder.
Cement is a critical part of many types of construction. Without it, we wouldn’t have any of our great skyscrapers, sidewalks, bridges, roadways, and so much more.
In 2021, the US produced over 92 million tons of cement, here’s how much a yard of concrete weighs if you are interested.
For such a vastly consumed resource, cement is more challenging to create than one might think. There are many types of cement, the most common being hydraulic cement.
This article will cover the step-by-step process that leads to the widely available bags of cement we see in stores today.
Cement is an amalgamation of minerals. Cement’s composition is mainly limestone, clay, iron ore, and gypsum. Companies source and prepare these materials before the manufacturing process so that they can become high-quality cement.
Limestone is a sedimentary carbonate rock generally found in caves or near shallow marine water. Limestone miners unearth, crush, and grind the stone into a fine powder for cement production. Utah, Michigan, and West Virginia are the current producers of the majority of limestone in the US.
Clay is a fine-grained substance that can be sticky and moldable but dries with exposure to air. Like limestone, workers will grind clay into a fine powder for cement production.
Generally, clay comes from open pits near the production plants to help reduce costs. The largest clay deposits in the US are in Georgia and South Carolina.
Iron ores are rocks that contain metallic iron that machines can extract to get their pure forms. They primarily come from small, underdeveloped deposits that sell ores to cement companies.
California, Minnesota, and Michigan are currently the largest sellers of iron ore in the US.
Gypsum is a common ingredient in products like fertilizer, drywall, and plaster of Paris. It is a soft, white mineral that comes from calcium sulfate dihydrate.
Its raw form is a nearly opaque crystal that typically forms in oxidized conditions. Miners dig up gypsum in 19 states in the US, with Oklahoma, Nevada, Iowa, Texas, and California being the top producers.
Once cement companies have mined and gathered these materials, they bring them to the manufacturing plant and crush them into powders.
The first step in the process is crushing the individual minerals down to about 6 inches large. They then move on to a secondary crushing stage, bringing them down to about 3 inches or smaller.
Once the minerals are small enough, the producers combine all of the ingredients except for gypsum in exact proportions. Then they put them through the cement kiln.
The cement kiln is a large cylinder-shaped steel container lined with a specialty fire brick. It’s about 12ft in diameter on average and burns at 2700°F.
The kiln is slightly slanted. The minerals go into the kiln at the higher end and fall to the lower side during the heating. During the heating process, many minerals burn off, leaving behind gasses.
Several chemical reactions occur as the kiln heats to this incredibly high temperature, a process that breaks down into four major stages.
First, any free water in the powders evaporates once the temperature reaches 212°F. At 800°F, the dehydration and formation of silicon, iron, and aluminum oxides occur.
Around 1800°F, CO2 evolves, and calcination produces CaO. Finally, at 2750°, the remaining minerals bond together and form clinker. The clinker will exit the kiln as gray, marble-sized balls.
After exiting the kiln, a fan blows cool air over the clinker to cool it rapidly. Once it is cool enough to manage, workers will mix it with small amounts of gypsum.
The purpose of the gypsum is to increase the time before the cement sets so builders have time to manipulate it.
After combining the gypsum and clinker, the cement goes through a final grinding process. This process typically uses a ball mill, which is a machine with multiple rotating chambers housing various-sized steel grinding balls.
It combines gypsum and clinker into cement powder, which the companies will then deliver to stores and construction sites.
Gypsum and Setting Time
Gypsum has a critical role to play in the hardening of cement. It regulates the setting time and is therefore an essential ingredient.
The primary purpose of adding gypsum to the cement clinker is to slow the hydration process of cement once it mixes with water. When cement powder mixes with water, it begins reacting to the C3A and solidifies.
Solidification does not take long, which can prove problematic when mixing or transporting cement. However, if you add gypsum to the mix, the reaction instead begins to form ettringite. Ettringite is a hydrous calcium aluminum sulfate mineral.
During the reactions of cement, it becomes super fine-grained crystals that coat the C3A particles.
The ettringite slows the hardening process of cement drastically, allowing workers time to mix, transport, and place cement with no trouble. This can be critical in the composition and workability of concrete.
Different Types of Cement
Every step of the process of cement manufacturing requires frequent testing and monitoring to ensure it meets the standards of the industry. The type of cement may differ depending on the project or purpose of the cement. Here are a few different types of cement.
Portland cement is the most common cement. Because of this, it is also known as ordinary cement. It is the primary ingredient in concrete, mortar, and grout. Joseph Aspdin accidentally developed this cement in the 19th century after heating limestone in his oven.
The name comes from this cement’s similarity to Portland Stone, a limestone formation. Portland cement is standard in building bridges, buildings, walkways, and other common cement structures.
Quick-dry cement, or rapid-hardening cement, has significantly less gypsum than ordinary cement, decreasing the time it takes to harden fully. Workers typically use it in high-traffic areas to open to traffic more quickly or for framework removal projects. Contractors may also choose quick-dry cement for its strength early in the drying process.
White cement is precisely what it sounds like; ordinary cement that is white instead of gray. Generally, white cement is common for ornamental purposes such as gardens, pools, sculptures, etc. The raw materials for this cement may be more expensive than usual and will not include iron oxide.
Much like white cement, the only difference in this cement is the color. During manufacturing, pigments can join the powder mix to create different colors. This cement is primarily for ornamental purposes, such as decorative flooring.
Manufacturers can produce low-heat cement by keeping the tricalcium aluminate content to less than 6%. This cement is more sulfate resistant and less reactive than other cement. It can avoid cracking due to high heat and is a good option for mass concrete projects.
However, the setting time for low-heat cement is generally faster than that of ordinary cement, so it can be tricky to work with.
Cement manufacturing produces a large amount of carbon dioxide and other pollutants that factor into global warming. Additionally, cement production uses large amounts of energy and other natural resources during the process.
The process can also harm air quality due to the dust particles it releases into the atmosphere, which can cause respiratory issues.
The cement industry is undoubtedly struggling to reduce its significant carbon footprint. Certain companies have focused on converting their wasted carbon dioxide into something valuable, such as chemical feedstocks or fuels.
Green cement is another potential solution to cement production’s large environmental impact. Green cement takes the carbon from the kiln and adds or substitutes the mixture in the open space of the cement.
Manufacturers can also cure their products by putting them in a room with carbon rather than water, reducing carbon emissions by 70 percent. This cement version would cure faster, be more robust, and require significantly less energy.
Even though cement is everywhere, few people know about its extensive production process. The hundreds of bags on the shelf at home depot do nothing to tell us the story of how this widespread material has come to be or what it is doing to our planet.
It is crucial to understand where cement comes from to make the process more sustainable. Cement is an essential ingredient to our world as we know it, but its production has room for growth.
The mining and quarrying of precious minerals, the dust from grinding them, and the carbon emissions of the process all contribute to a nasty carbon footprint.
Hopefully, cement companies will find ways to make less of an environmental impact in the future.
Recycling waste products such as water and leftover concrete, assimilating other byproducts such as sand, ash, and other landfill materials, and reducing the use of fossil fuels are all great ways to do so without forcing us to get rid of cement entirely.