We’ve been asked how we ensure the quality of our cement products, and how we consistently continue to achieve this quality in the cement production process. Dr. Hoda Beltagui from our Technical Team explains the process in our latest Your Questions, Answered video.
Cement is the active ingredient in concrete. It’s the glue that holds the aggregates together. Let’s look first at the cement production process.
The first stage of the process involves the fine grinding and milling of the raw materials, which are mainly composed of limestones, shales and clays. As these are ground together, this produces the raw meal. The raw meal is then introduced into the high temperature kiln at around 1450 degrees celsius during the calcination process. This produces calcium oxides (CaO), silicas (SiO2), aluminates (Al2O3) and iron oxides (Fe2O3).
After the calcination process, we are left with the hot clinker material, which is mainly composed of alite, belite, ferrite and aluminates, among other materials. This clinker material is then cooled, milled and mixed with other materials such as gypsum, limestone, supplementary cementitious materials to produce the final cement product.
At every stage of the production process, we carry out a range of tests to ensure the quality of the product is achieved. Part of this quality testing includes the chemical testing to ensure that the product adheres to EN 197, which is the cement specification standard, following the testing methods set out in EN 196. For example, some of the most important tests we carry out include X-ray fluorescence (XRF) and X-ray diffraction (XRD).
We carry out these tests hourly at every stage of the production process, right through from the raw materials to the final cement product, which ensures we have frequent and stringent testing of the product.
We use this chemical analysis to determine various control parameters which can be used to optimise the production process, e.g. the lime saturation factor (LSF), the silica ratio, the alumina ration and the free lime content. We can then use this to optimise the process in terms of the ratios of raw materials used.
This chemical analysis is important as it can affect the physical properties of the cement produced, for example if the free lime content of the cement is too high, this might affect the setting time of the product, and if the alite content is too low, this might affect the strength gain of the cement.
Another important part of the quality testing we carry out includes the physical testing of the cement itself. This includes compressive strength testing in accordance with EN 196-1, testing of the soundness and setting time in accordance with EN 196-3, and testing of the specific surface area or the fineness of the cement according to EN 196-6. From this, we regularly produce cement certificates.
For each cement type we produce, the requirement is to test two dispatch samples per week, however we go beyond this by testing a sample as frequently as every 36 hours. In addition to this, the standard specifies testing to be carried out on prisms, but we also carry out testing on 100cm cubes, so we go beyond the requirements set out in the standard.
The quality of our cement is verified by the numerous accreditations we hold. Our cement if CE marked by NSAI as a 1+ system, which is the most onerous and highest level of CE marking for factory production control. NSAI conduct six independent tests and one audit each year.
We have ISO 9001, which is quality management systems certification, certified by BSI, who carry out two audits annually. We also have ISO 14001, which is environmental management systems certification, certified by Certification Europe. And finally, we have BES 6001, which is the responsible sourcing of construction products, certified by CPC. We have also recently achieved ISO 50001, which is energy management systems certification, also certified by Certification Europe.
There is a lot more to the quality control process, but this provides a broad overview of what’s involved, and how much emphasis we place on producing consistently high quality cement products.