If you’ve ever baked a cake (or watched a baking show), you’ll know how important it is to use the right ingredients in the right quantities. Otherwise, your cake won’t taste very good – and it might not even look like a cake at all! (We’ve all been there.) But imagine if you were describing a list of baking ingredients to a robot, and said ‘sugar’. The robot might choose raw sugar cane, sugar syrup, or sugar lumps. They’re all sugar, after all – just in different forms!
So, the form or state of the ingredients is equally important information to know as what they’re made of. This is quite obvious when you’re working with delicious cake, but it becomes much trickier when you’re working at the microscopic level with individual elements.
Balancing the elements with XRF
In cement manufacturing, the final properties of the cement are affected by its chemical composition. The raw materials are limestone (the calcareous raw component containing CaO – typically around 85%), shale or clay (the argileous component containing SiO2, Al2O3 and Fe2O3 – around 13%), and additives (SiO2, Al2O3 and Fe2O3 at <1% each), which are crushed into powder and mixed to form raw mix. At this point, it’s vital to monitor exactly how much of each material is in the raw mix to keep the correct balance – and this careful monitoring continues throughout the production process using X-ray fluorescence, or XRF.
In fact, there is a whole array of oxides and elements to keep track of. For example, MgO should not exceed 4-5% to avoid cement expansion, alkalis (K, Na) can affect both kiln operation (build-ups) and product quality, and sulphurs can create setting issues and build-ups when present in excess (limited gypsum additions). The typical ratio of alkali to sulphur needs to be kept between 0.8-1.2 (molar). Finally, chloride content must be less than 0.02% to avoid serious build-up problems. XRF analysis is the best way to monitor all these various components, and has been used for process control in cement manufacturing for many years.
This mixture is fired in a furnace, where the heat causes the calcium carbonate of the limestone to decompose into calcium oxide. This part of the process releases carbon dioxide (CO2) along with other by-products – another good reason to ensure all the ratios are precise, as minimizing CO2 emissions is important both to reduce environmental impact and to comply with regulations. It’s also important to control the potentially hazardous elements in alternative fuels, which are increasingly being used to heat the kiln using recycled waste materials. These can cause other emissions that also have a negative impact on the environment.
Completing the picture with XRD
But while all this elemental composition data is invaluable, more information is needed. In our cake analogy, we know we need sugar, but we don’t yet know which form it should be in. That’s where X-ray diffraction (XRD) comes in.
XRD has been traditionally used to catch any instances of early hydration in the final cement product, but it can also be used for process control. Today, XRD is being used more and more often to control the mineralogy of cement clinker, as it identifies the crystalline phase of each component at key stages such as CaCO3, CaSO4, CaSO4.1/2H2O, CaSO4.2H2O, quartz, free lime, free magnesia (periclase), clinker phases and other mineral phases in conventional and alternative raw materials. The Rietveld method allows a complete quantitative picture of the crystal phases in cement, including polymorphs of alite and belite, along with free lime, calcite and calcium sulphates.
This is the key information that was missing before, giving deeper insight into the product as a whole and allowing a more holistic monitoring and control process. Increasingly, data from XRF and XRD are combined into a single report throughout the production process of cement and concrete, providing a well-rounded overview of both elemental composition, crystalline phase, and quality.
By combining XRF and XRD, it’s much easier for manufacturers to control the quality and final properties of their cement and concrete from an earlier stage in the process. This reduces waste, as any anomalies are picked up earlier, as well as making kilns more efficient and saving energy and CO2 emissions.
Smart systems for future-proof processes
A highly accurate XRF spectrometer like Malvern Panalytical’s Zetium pairs well with a compact and robust XRD instrument, such as the Aeris. The benefit of using these together is that they both support automation using the Universal Automation Interface, and Malvern Panalytical offers support and guidance from initial setup to advice on sample preparation and technical assistance. Our instruments are suitable for use from a busy production environment to a laboratory, and can also be integrated into laboratory information management systems (LIMS).
Written by: Murielle Goubard, Posted by: Malvern Panalytical (www.materails-talks.com)