The Cement Manufacturing Process

Different minerals need to be mined in order to make cement. Limestone (containing the mineral calcite), clay, and gypsum make up most of it.  The US Geological Survey notes that cement raw materials, especially limestone, are geologically widespread and (luckily) abundant.  In 2022, U.S. portland cement production increased slightly to an estimated 92 million tons, and masonry cement production increased to an estimated 2.5 million tons, according to the U.S. Geological Survey 2023 Cement Mineral Commodity Summary. The overall value of sales of cement was about $14.6 billion, with an estimated 70% to 75% of sales were to ready-mixed concrete producers, 11% to concrete product manufacturers, 8% to 10% to contractors, and 5% to 12% to other customer types.

The Minerals Summary also noted that “cement is not recycled, but significant quantities of concrete are recycled for use as a construction aggregate. Cement kilns can use waste fuels, recycled cement kiln dust, and recycled raw materials such as slags and fly ash. Various secondary materials can be incorporated as supplementary cementitious materials (SCMs) in blended cements and in the cement paste in concrete.”

There are more than twenty types of cement used to make various specialty concrete, however the most common is Portland cement.

How cement is made

Cement manufacturing is a complex process that begins with mining and then grinding raw materials that include limestone and clay, to a fine powder, called raw meal, which is then heated to a sintering temperature as high as 1450 °C in a cement kiln. In this process, the chemical bonds of the raw materials are broken down and then they are recombined into new compounds. The result is called clinker, which are rounded nodules between 1mm and 25mm across. The clinker is ground to a fine powder in a cement mill and mixed with gypsum to create cement. The powdered cement is then mixed with water and aggregates to form concrete that is used in construction.

Clinker quality depends on raw material composition, which has to be closely monitored to ensure the quality of the cement. Excess free lime, for example, results in undesirable effects such as volume expansion, increased setting time or reduced strength. Several laboratory and online systems can be employed to ensure process control in each step of the cement manufacturing process, including clinker formation.

How cement manufacturers can help ensure cement quality

From the limestone mine, to crushing, blending, the raw mill, kiln and beyond, elemental analysis is a critical component to driving process control, efficiencies, and higher quality end products. Powerful analyzers can help you better understand the composition of your raw materials, and give you more control over downstream production processes.

Handheld XRF analyzers can identify material composition and contaminants or obtain geochemical data in the field, or anywhere in the plant. Laboratory X-Ray Fluorescence (XRF) systems are used by cement QC laboratories to determine major and minor oxides in clinker, cement and raw materials such as limestone, sand and bauxite.* Combination X-Ray Fluorescence (XRF) and X-Ray Diffraction (XRD) systems accomplish both chemical phase analysis for a more complete characterization of the sample. Clinker phase analysis ensures consistent clinker quality. Such instrumentation can be fitted with several XRF monochromators for major oxides analysis and a compact diffraction (XRD) system which has the capability of measuring quartz in raw meal, free lime (CaO) and clinker phases as well as calcite (CaCO₃) in cement.

Cross Belt Analyzers based on Prompt Gamma Neutron Activation Analysis (PGNAA) technology provide high frequency online elemental analysis of an entire raw material process stream. Analyzers using PGNAA/PFTNA are situated directly on the conveyor belt and penetrate the whole raw material cross-section, delivering minute-by-minute, uniform measurement of the entire material stream, not just a sample or surface reading. The key elemental components for cement are calcium, aluminum, iron, and silicon. However, sometimes unwanted elements such as magnesium oxide, and alkalis such as sodium, potassium, and sulfur exist within the limestone, clay, and sandstone that are adverse to the process. In these cases, PGNAA technology allows the end user to monitor MgO levels in the limestone and adjust accordingly.

Accurate cement production also depends on belt scale systems to monitor output and inventory or regulate product loadout, as well as tramp metal detectors to protect equipment and keep the operation running smoothly. The Cement Manufacturing Process flow chart sums up where in the process each type of technology is making a difference.

Cement operators also use these analysis systems to analyze and adjust the coal mix in-stream, which enables the plant to reduce energy consumption by using exactly the amount of coal fuel required to meet the specific heating values required to process materials in the kiln and mill. Kiln feed material with high chemistry variations requires more fuel to properly react and more energy at the finish mill to grind over-reacted clinker.

Additional Resources:

• Website resources: Technology and Solutions to Improve Cement Manufacturing
• Application Note:  Analysis of Clinker and Cement with Thermo Scientific ARL OPTIM’X WDXRF Sequential Spectrometer to learn why XRF is the technique of choice for elemental analysis in cement industry.
• Blog article: XRF/XRD Combined Instrumentation Can Provide Complete Quality Control of Clinker and Cement to learn more about technology that combines the advantages of both XRF and XRD together.
• Blog article: PGNAA Improves Process and Quality Control in Cement Production to learn what makes PGNAA particularly suited for cement analysis.

Editor’s Note:  This article was originally published 8/20/15 under the author Darrell Leetham but has been updated and refreshed with new information.

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