Cement is a primary building material in the modern world; as a major constituent of buildings, concrete structures and roads the consistency of its composition during production is essential.
A major method of quality control (QC) for cement production is XRF spectroscopy as it can provide fast and accurate analysis in cement manufacturing using in-line methods to ensure 24/7 results. XRF analysis is used at several stages of the cement analysis process:
- Evaluation of raw materials at the quarry
- To assess intermediate products (clinker, gypsum, limestone)
- Approval of the finished product (cement) to the required standards such as ASTM C
114  and ISO/DIS 29581-2 
In all of these areas, robust and simple to use XRF instruments that are operated by non-expert production staff provide cost-effective chemical composition analysis.
XRF analyzers typically measure the concentrations of MgO, Al2O3, SiO2, SO3, K2O, CaO and Fe2O3 in cement materials to assess quality and are also used to monitor sulfate addition to gypsum. Typically, the powder sample can be analyzed directly with little or no additional sample preparation.
The constituents of cement
Cement is produced using a closely controlled chemical combination of calcium, silicon, aluminum, and iron oxides as well as sulfur trioxide. Portland cement is the most common cement in universal use around the world. This is made by roasting limestone (calcium carbonate) with materials such as clay (aluminosilicates) to 1450 °C in a kiln. This process is known as calcination and here carbon dioxide is lost from the calcium carbonate to form calcium oxide, or quicklime, which then chemically combines with the other materials in the mix to form calcium silicates and other cementitious complexes. The resulting hard 'clinker', is then ground with a small amount of sulfate rich gypsum into a powder to make 'ordinary Portland cement' (OPC).
Portland cement is a basic ingredient of concrete, which is a composite material consisting gravel and sand, cement, and water. Portland cement is an example of a hydraulic cement, which sets upon contact with water. Common raw materials used in the manufacture of cement include limestone, shells, and chalk or marl combined with shale, clay, slate, blast furnace slag, silica sand, and iron ore. It can be seen that assessment using XRF of the quality of raw materials used to produce the cement is essential for the integrity of the final product.
Different cements are produced for different applications and by virtue of the raw materials used in production. Portland cement is a good standard for construction and use in mortar and concrete. The formulation can be changed to attain quick drying, underwater non-hydraulic drying and super strength concrete.
However other cements such as Super sulfated cements which contain about 80% blast furnace slag, 15% gypsum or anhydrite and some Portland clinker or lime as an activator can also be produced for specific applications. The super sulfated cements produce strength by formation of ettringite and in a similar way to Portland cement grow in strength as they age. They display good resistance to aggressive agents, including sulfate, that could attack mortar or structural concrete from acid rain.
Calcium aluminate cements are a version of hydraulic cement produced from limestone and bauxite. The ingredients include monocalcium aluminate CaAl2O4 and mayenite Ca12Al14O33. In this case strength is attained by hydration to form calcium aluminate hydrates. This cement is adapted for use in refractory (high-temperature resistant) concretes such as in furnace linings.2,3,4
The environment and cement
The amount of CO2 emitted by the cement industry is nearly 900 kg of CO2 for every 1000 kg of cement produced and is not very environmentally friendly.
Although in Europe carbon emissions from cement production have been reduced by 30% since the 1970s and there are new formulations of cement that absorb CO2 from the air during the curing/hardening process.4,5,6
How can XRF improve cement production?
Cement manufacture was an early adopter of XRF and now it depends upon XRF for production control. The main units used are WDXRF analyzers in a centralized lab that can be automated for highest productivity.
Innovation in EDXRF has now resulted in smaller, compact instruments with an improved ability to measure light elements, such as Na2O and MgO and this has led to an improvement in quality. X-ray fluorescence (XRF) is used routinely in cement plants, and is the primary means of controlling the composition of raw materials, the raw feed, as well as clinker and cement. XRF provides rapid compositional data for controlling almost all stages of production and also assessing the final product.5,6
Sample preparation for cement XRF analysis
Specimens of cement or raw feed materials may be prepared in the form of a fused glass bead7,8 or more traditionally as a pellet of pressed powder. Beads are produced by heating the specimen together with a flux, which is typically lithium tetra borate, at about 1100 °C to make a ‘glass’. This method has the advantage that the specimen becomes a homogeneous material, allowing more accurate X-ray analysis as there are no particle effects or other interferences with the XRF.
Pressed pellets are made by grinding the specimen finely and compressing the resulting powder with an inert binder to form a pellet. Specac offer a range of hydraulic pellet presses such as the AtlasTM Series Autotouch Automatic Hydraulic Press for higher throughput sampling which allow pellets to be analyzed quickly and directly. The AtlasTM series can work with loads of up to 40 tons, allowing industrial scale XRF analyses to be easily achieved.
Preparing pressed pellets for cement XRF analysis is quicker and easier than preparing glass beads. Pellets are prepared using a cylindrical sample die, which acts as a mold in which the pellet is formed. The die is filled with powdered sample which is then compressed to give the pellet. If the sample resists compression it can be retained inside the die using an XRF cup which caps one open end of the die, leaving one face exposed for XRF analysis. However, because of particle effects using a pellet sample can make the calculation of the processes of X-ray fluorescence and absorption within the specimen more complicated and prone to some loss of accuracy. Many researchers prefer the glass bead method for new cement analysis as it is easier to produce referenceable calibration standards.7