How is Cement Made? A Simplified Explanation of the Process of Making Cement
When building professionals and amateurs understand how cement is made it helps them to make more informed decisions about the selection, usage, and application of cement in their projects.
The production of cement is a complex process that begins with natural resources. Manufacturers mix and heat them in such a way that it produces a chemical reaction, leading to the substance we call cement.
Once you understand the process that creates cement, this knowledge provides valuable insights into the building substance that is one of the most important components of modern building infrastructure.
How Cement is Made
Manufacturers make cement through a multi-step process that begins with raw product extraction and ends with packaging and distribution. Though each manufacturer has their own process, here are some general guidelines for the steps to produce cement.
Raw Material Extraction
The main raw materials that manufacturers use to make cement are hard rocks such as limestone and slate and soft rocks like clay and chalk. They quarry or dig out the rocks and transport them to the crushing facility. The transportation of dry materials is usually by trucks, railway cars, or conveyor belts depending on the facility. Wet materials are sometimes delivered by pipeline.
Crushing and Grinding
The raw materials go through an initial crushing process to reduce the size of the rock. Cement producers use jaw crushers, hammer mills, and cone crushers to reduce the size of the hard rock. Manufacturers then use machines like roller mills to grind the hard rock into a fine powder they call raw meal.
Soft material like clay and chalk is also processed. They break down soft material by mixing it with water in wash mills. This produces a slurry. They pass the slurry through a fine mesh to remove any remaining large particles.
Preparing the Raw Mix
Cement producers will next mix proportions of the raw ingredients together to get the exact composition of cement ingredients for the type of cement they manufacture. This composition also varies according to the desired capabilities of the cement, the local availability of raw materials, and the manufacturing process.
In most cases, the primary ingredient of the mix is limestone dust. The proportion of limestone dust is normally between 80%-95%. Clay or shale dust makes up between 5%-20% of the mixture. Silica, iron oxide, alumina, and other additives might also be added to the mix, depending on the specific type of cement. The proportion of these additives ranges between 1%-10%.
Pre-Heating and Pre-Calcination
The raw mix is next pre-heated and pre-calcined to improve the fuel efficiency of the process and enhance production capacity. The raw materials go into a machine like a tower preheater or a cyclone preheater and are heated to a temperature of 800o or 900o Celsius. This process removes any moisture or carbon dioxide from the mix. This process begins some of the chemical reactions necessary in the formation of cement.
Clinker Formation
The clinker formation process is the primary step in the formation of cement. This is the stage where the complex chemical process occurs which forms cement into a strong and durable building product.
Cement producers feed the raw cement material into a kiln lined with refractory bricks. High temperatures exceeding 1400o Celsius melt the additives which liquify the limestone to react with the silica and form the clinkers. Clinkers are nodular or lump-like material with a diameter of 3mm-24mm (.12in-.98in).
Cooling
Clinkers are rapidly cooled to a temperature of 100o-200o Celsius using air or water. This process is vital in ensuring the quality of the final cement product because it affects the microstructure of the clinker. Rapid cooling allows the clinkers to solidify their mineral composition, improves grindability, prevents rehydration, and helps prevent cracking.
Mixing and Final Grinding
Cement producers mix the cooled clinker with a quantity of additives like gypsum, fly ash, and limestone. These additives help regulate the setting time of cement, serve as fire retardants, improve workability, and help reduce shrinkage among other qualities. The mixture of clinker and additives is fed to a ball mill, or similar machine, and is ground to a fine powder.
Packaging and Distribution
The cement is transferred from the mill into storage silos. From the storage silos the cement is packaged into bags and shipped to distributors and customers.
Image from cement.org.