Clinker is a nodular material that is an essential component in the production of cement. Clinker is composed primarily of calcium silicate and other materials which give it exceptional binding properties that lend strength and durability to cement applications.
The unique properties of clinker also contribute to essential hardening quality in cement, which make it one of the most valuable building materials in existence.
Production Process of Clinker
Cement manufacturers create clinkers through a multi-step process that includes ingredient selection, heating, and grinding.
- Raw Mineral Preparation – The main ingredients in clinker production are limestone and clay. Producers quarry these ingredients, transport them to the production facility, and then grind and blend them to achieve a uniform mixture.
- Kiln Preheating – The cement producers feed the raw ground mix into a preheater tower or cyclone where they are exposed to hot gasses. This step removes any moisture from the raw materials and prepares them for the chemical reactions in the next stage.
- Kiln Firing – The preheated material is fed into a rotary kiln where the materials are exposed to high temperatures of up to 1400o-1500o C (2550o-2730o F). The heat creates the chemical reaction that forms the clinker.
- Clinker Formation – As the raw material moves through the kiln it undergoes complex reactions. The primary reaction is a decarbonization of the limestone. This process creates calcium oxide (lime). The lime reacts with components like silica and alumina in the clay to create calcium silicates and calcium aluminates. These are the main components of clinker.
- Clinker Cooling – After leaving the kiln, the clinker is extremely hot. Manufacturers must cool the clinker rapidly to ensure its physical properties. They use air or water in specialized cooling units called clinker coolers.
- Clinker Grinding – Cement machines such as ball mills or vertical rollers mills grind the cooled clinker into a powder. The grinding stage may also feature the addition of additives like gypsum to control the setting time of the cement.
Composition of Clinker
The exact composition of clinker depends on the type of cement the manufacturer is producing, but there are primary ingredients that are common in most clinker production.
The most prominent ingredients in clinker are calcium silicates, which consist of tricalcium silicate and dicalcium silicate. Calcium silicates contribute to the strength and the setting properties of cement.
The other most important ingredients are calcium aluminates, the most prominent of which is tricalcium aluminate. Calcium aluminates cause the cement to set faster and cause early strength development. High amounts of tricalcium aluminate will increase set times for cement but will also increase the risk of sulfate attacks.
Calcium oxide is a byproduct of heating the raw ingredients to form the clinker. The decarbonization that produces calcium oxide contributes to the cement’s binding properties and its reaction with water called hydration.
Gypsum, which contains calcium sulfate, is a common additive in clinker. It helps regulate the setting time during the hydration process.
Minor ingredients in clinker include elements like iron oxide, magnesium oxide, alkalis, and other trace elements. These elements impact the color, setting time, and resistance to other chemical reactions.
Types of Clinkers
Cement producers create different clinkers based on variations in the raw materials and the manufacturing process.
- Ordinary Portland Cement (OPC) Clinker – Portland clinker is the most common type of clinker produced. This clinker is suitable for most general construction purposes.
- Sulfate Resistant Clinker – Manufacturers create this type of clinker so that they can use it in sulfate-resistant cement mixtures. Sulfate resistant clinkers have a lower level of tricalcium aluminates which are susceptible to sulfate reactions. These resist the effects of sulfates in water and soil.
- Low-Heat Clinker – Fabricators design this clinker to generate less heat during the hydration process. This means that there is a lower level of tricalcium aluminate than in standard clinker. Low-heat clinker is a common ingredient in cement that builders use for large structures where heat generation can cause thermal cracking.
- White Clinker – White clinker is characterized by its purity and bright, light color. Manufacturers create white clinker by using raw materials with a low iron and manganese content. This is the primary material in white cement.
- High Alumina Clinker – This clinker has a higher level of alumina than other clinker types. This is essential in refractory cement because it has a high resistance to chemicals and high temperatures.
- Belite Calcium Sulfoaluminate Ternesite (BCT) Clinker – BCT clinker is an innovative variety of clinker that produces 30% less carbon dioxide emissions. The manufacturing process for BCT clinkers also requires 15% less energy to produce.
Environmental Impact of Clinker Production
The production of clinkers has a significant environmental impact including the production of greenhouse gasses (CO2), energy consumption, air pollution, extraction of materials by mining, water consumption and pollution, and waste generation. Many cement companies are actively working to mitigate these negative environmental impacts through their everyday practices.
Cement companies are investing in carbon capture, utilization, and storage (CCUS) technologies. They store CO2 for later use or sequester it for permanent storage underground. They are also investing in alternative fuel sources like biomass and used tires rather than using fossil fuels to power production.
To reduce their use of raw materials, some are using alternate materials like fly ash, slag, and silica fumes. They are also seeking to reuse their waste as part of their process to reduce their waste disposal and create a more circular system. In addition, cement companies are implementing better water management and conservation strategies.