The Important Role of XRD in Achieving Green Cement

Introducing an article published in Turning Cement Green With XRD,69,World Cement (2022).

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This article introduces the critical role of X-ray diffraction technology in achieving green cement by Matteo Pernechele and Murielle Goubard from Malvern Panalytical.

In the cement industry, sustainability is becoming an increasingly important issue. With rising fuel prices, shortages of supplementary cementitious materials (SCMs), and issues like water and electricity regulations, various efforts are being made towards sustainability.

For example, the European Union’s Emissions Trading System is helping to alleviate resource shortages and reduce carbon emissions. Additionally, deeper understanding of the effects and performance of alternative fuels and alternative raw materials has opened doors to new processes and materials.

To manufacture green cement and achieve decarbonization in the cement industry in the short to medium term, solutions include co-processing of alternative fuels and reducing clinker in cement using new SCMs.

This requires selecting appropriate raw materials and SCMs, optimizing pyroprocessing and its intermediates, controlling blends, and maximizing the amount of SCMs in the final green cement, making mineralogical analysis by X-ray diffraction indispensable.

Cement manufacturing is a complex process.

Typically, the production of Portland cement begins with mining raw materials such as limestone and clay, crushing them into fine powder known as raw meal, sintering it in a cement kiln at high temperatures of 1450°C, finely grinding it in a cement mill, and mixing it with gypsum to create cement.

This powdered cement, when mixed with water and aggregates, forms concrete used in construction.

This operation consumes a large amount of energy and resources. Depending on the quality control personnel, only the best raw materials from the limestone quarry may be used in the kiln, and only fossil fuels may be used.

However, in recent years, sustainability has become increasingly important in the cement industry. New SCMs like calcinated clay, and new alternative fuels (AF) such as biomass, refuse-derived fuel, municipal waste, tires, sawdust, and many other types of waste and byproducts are being utilized.

Mological analysis plays a role in the cement industry’s transition to a low-carbon, more circular economy.

To find solutions for manufacturing environmentally friendly cement, fast and automated complete mineralogical analysis by X-ray diffraction (XRD) enables easy selection of raw materials suitable for blending.

It also helps in decarbonization and the optimization and control of the clinker formation process. XRD is the only reliable and proven industrial technology that can quantify the amorphous content of SCMs and ensure that the composition of blended cement meets required standards.

X-ray Diffraction Enhancing Mineralogical Recognition

Current X-ray diffraction instruments (XRD) emerged around the 1970s.

Today, it is the leading technology for completely automated identification and quantification of minerals and crystalline phases. In fact, XRD is the only industrial technology capable of quantifying amorphous materials (including some SCMs), enabling the reduction of clinker in cement.

The latest industrial XRD maximizes its potential. Instead of focusing on specific minerals, it can fully identify the mineralogical composition of materials in just a few minutes. With improved mineralogical recognition, clinker quality, the manufacture of new green cement, and overall plant know-how can be enhanced.

Today, the use of XRD systems in cement plants focuses on maintaining product quality and ensuring smooth operation, while simultaneously reducing carbon dioxide emissions and the overall environmental footprint of cement production.

Alternative Fuels (AF)

Clinker kilns are very attractive for alternative fuels for several reasons.

Emissions generated by the combustion of AF are considered neutral and contribute to achieving carbon neutrality while also aiding in the removal of general waste and industrial byproducts. Inorganic ash resulting from AF combustion is incorporated into clinker along with a small amount of solid waste (e.g., cement kiln dust).

However, the variability of AF and air-fuel ratio can significantly impact preheater operation, kiln operation, and clinker quality.

An air-fuel ratio high in sulfur and chlorides can produce coatings in cyclone preheaters leading to complete blockages. XRD systems help detect the occurrence of such coatings by analyzing the mineralogical composition of hot meal.

The use of AF can alter the temperature gradient within kilns, potentially impacting clinker quality. High temperatures are essential for the reaction of belite and lime to form alite.

XRD systems can easily monitor the yield of this reaction and accurately measure free lime with a reproducibility of 0.1 Wt%.

The amounts of limestone and periclase must be kept below limit values of 2Wt% and 5Wt%, respectively, because the hydration of these minerals can cause volume expansion and impair the dimensional stability of cement.

Healing be correctly quenched, helping to stabilize high-temperature phases with excellent hydration rates.

For example, if clinker is insufficiently quenched, a phase change from beta-belite to gamma-belite can occur, with the latter lacking cementitious properties.

Alite is present in clinker as two monoclinic forms: alite M1 and alite M3. Most clinker contains these two forms, but by increasing the SO3 and magnesium oxide ratio, alite M1 can be preferentially formed. Alite M1 exhibits high compressive strength after hydration, and X-ray diffraction devices can distinguish it from alite M3.

Utilizing XRD in Grinding Plants

As the amount of clinker produced in integrated plants increases, so does the number of grinding plants importing clinker.

The mineralogical analysis of imported clinker is essential to ensure its quality and avoid potential performance issues in cement. In addition, XRD is widely used to assess the quality and appropriate addition levels of cement additives.

To optimize setting time, strength development, and dimensional stability, the mineralogical nature and amount of calcium sulfate must be adjusted relative to the aluminate content and type.

XRD analytical results can be combined with the sulfur trioxide content measured by X-ray fluorescence (XRF), adding value by distinguishing between dihydrate, hemihydrate, and anhydrite sulfate.

The most promising way to reduce carbon emissions from cement is to decrease dependence on clinker, for example, by replacing it with natural or synthetic pozzolans.

Pozzolans are substances that react with portlandite generated by the hydration of cement to improve its strength and durability. The quality of a pozzolan is defined by its mineralogical nature, as there are both highly reactive and harmful or inert phases.

Volcanic materials that are high in quartz, feldspar, pyroxene, and magnetite are unsuitable for natural pozzolans.

Pozzolans high in smectite or kaolinite need to be thermally activated before being used as SCMs. Zeolite-based minerals, such as analcime, leucite, chabazite, phillipsite, and clinoptilolite, are suitable as pozzolans.

The quality of ground granulated blast-furnace slag (GGBS) or fly ash heavily relies on their mineralogical nature and the amorphous components that can be clearly quantified by X-ray diffraction.

Slag not properly quenched contains large amounts of crystalline merwinite and melilite, leading to lower reactivity. Additionally, fly ash produced at high temperatures may include significant amounts of mullite, which does not possess pozzolanic properties.

The amorphous determination of SCMs by XRD analysis can more rapidly indicate its suitability compared to other methods. Furthermore, full automation is possible.

The amount of clinker that can be replaced with SCMs is strictly regulated by standards.

For example, the EN-197-1 standard clearly defines the ranges of clinker, limestone, slag, fly ash, pozzolan, calcinated shale, silica fume, and other additives for 27 types of cement.

The latest version of EN-197-5 adds Portland composite cement CEM II/C-M and another type of composite cement CEM VI not included in EN-197-1, aimed at formulating concrete, mortar, and grout in a more sustainable way.

XRD is widely used to confirm the proper mixing and homogeneity of products. At grinding plants, it is crucial to bring SCMs as close to permissible upper limits as possible while minimizing the amount of clinker and the total production cost of cement.

If SCM quantification is inaccurate, quality managers are forced to take large safety margins at the expense of product profitability. XRD systems can accurately quantify SCMs, making the return on investment for grinding plants manufacturing blended cement extremely attractive.

Calcinated Clays and New Cements

Recent discoveries of the synergy between calcinated clay and limestone in cement have gained interest from regulators and cement manufacturers.

In Europe, the new EN 197-5 standard has raised the clinker replacement limits from 35% for CEM II/B-M(Q-LL) to 50% for CEM II/C-M(Q-LL).

These new cements, such as LC3, have the potential to reduce carbon emissions by up to 40% without affecting cement strength. Accurate mineralogical analysis is crucial for identifying and developing suitable clay deposits, calcinating clay materials, and confirming the proper blending with clinker and other additives.

Using incorrect clay can significantly degrade cement performance. Kaolinite and smectite are common clay minerals that exhibit pozzolanic properties upon calcination.

Clays suitable for calcination and mixing with clinker need to have concentrations of 30-40 Wt% or higher. Other minerals such as quartz, hematite, calcite, feldspar, as well as clay minerals like mica and illite, act as fillers.

X-ray diffraction measurements reveal that dehydroxylation reactions during calcination break down the crystal structure of clays, leading to a loss of crystallinity.
Such changes are crucial to endow materials with pozzolanic properties.

Insufficiently high temperatures or short retention times result in residual kaolinite or smectite that do not contribute to cement strength and affect workability. The onset temperature for dehydroxylation reactions is around 550°C for kaolinite and 700°C for smectite.

Thus, optimal calcination conditions are highly dependent on the mineralogical properties of the clay.

High temperatures and long retention times induce the crystallization of unreactive phases such as mullite, cristobalite, anorthite, wollastonite, diopside, and gehlenite.

The optimal temperature range is narrow, and XRD provides the data necessary for manufacturing calcinated clays in the most optimal manner.

The lack of or low concentration of calcite combined with low kiln temperatures leads to a dramatic reduction in carbon emissions compared to clinker manufacturing, and the resultant calcinated earth is mixed with clinker, gypsum, and limestone in appropriate ratios. This ratio can be accurately quantified by XRD and made to comply with local standards.

XRD analysis also proves effective in producing many other types of clinker and cement.

Examples include geopolymers, calcium aluminate cements, Ciment Fondu, calcium sulphoaluminate cements, belite-ye’elimite-ferrite cements, wollastonite-based calcium silicate carbonate clinker, alkali-activated materials, slag and gypsum-based supersulfated cements, magnesium cements, phosphate cements (among other examples).

These cements have various applications such as low carbon cement, rapid hardening cement, refractory cement, and cements for the containment of radioactive or hazardous substances.

XRD for a Sustainable Future

X-ray diffraction is becoming a key analytical technique for managing the quality of clinker and cement, particularly as addressing net-zero targets is becoming increasingly urgent.

Hence, new green cement is being produced through the use of various alternative fuels and SCMs and the most circular approaches possible.

XRD is the only technique that can rapidly, accurately, and automatically quantify the mineral composition of these different compounds, enabling manufacturers to keep the cement process fully under control and make it as environmentally friendly and profitable as possible.

Most importantly, as the cement industry explores green cement manufacturing, XRD is a method that can pursue quality, sustainability, and profit simultaneously.

About the Authors

Dr. Matteo Pernechele holds a PhD in Materials Engineering from the University of British Columbia, Canada, and an MSc in Materials Science from the University of Padova in Italy.

He is active in a broad range of areas, from scientific research in solid chemistry to industrial projects specifically in the construction and mining sectors.

With 14 years of experience in the X-ray diffraction and Rietveld method fields, he has been working as an XRD application specialist at Malvern Panalytical since 2018.
He is part of the Competence Center in Almelo, The Netherlands.

Dr. Murielle Goubard is the Global Segment Manager for Building Materials at Malvern Panalytical. She has broad experience in materials chemistry, having worked at the Solvay Research Centre for 15 years. With a deep interest in the cement manufacturing process and aiming to enhance efficiency and product quality, she has been involved in the development of applications and solutions for France’s plants and major European cement companies. She has worked at Malvern Panalytical for 15 years and is now deeply involved in solutions for green cement plants, a circular society, and net-zero.

Reference

Turning Cement Green With XRD,69,World Cement (2022)