Dr. Hakan Arden KAHRAMAN – Technical Director, DMT Türkiye
Prof. Dr. Hakan BENZER – Hacettepe University Mining Engineering Department
Introduction
The process of making cement has been a part of human history for thousands of years. However, modern cement composition has only significantly evolved in the last couple of centuries. Since the main ingredient of cement is limestone and other raw materials, the quality of cement is greatly affected by the geological and mineralogical properties of the raw materials extracted and used.
This paper aims to lay the groundwork for the concept of “geocementing,” which will be the focus of a future paper. The geology and mineralogy of these materials directly determine their chemical composition, which in turn impacts the quality of the cement produced. Therefore, the goal here is to gather and review the fundamentals of the raw materials used in cement production, as geological and mineralogical differences in materials such as limestone and clay can affect their chemical characteristics and subsequently influence product quality, material handling, and processing in cement production.
Geological Factors Affecting Product Quality
Many factors may influence the final product which originated from the raw material extracted from limestone quarries. These factors are briefly given below.
Layering and Bedding
Limestone deposits are often stratified, with layers (beds) of limestone alternating with other sedimentary rocks like shales or clays. The continuity of these layers can vary, depending on depositional environments. In some deposits, limestone beds are continuous over large areas, while in others, they may be tapered, pinched out, wedged, interrupted, or replaced by other sediments. The presence of certain fossils or sedimentary structures, such as crossbedding or ripple marks, can help geologists correlate limestone layers across different parts of a quarry. These markers assist in understanding the lateral continuity of the deposit, which is critical for predicting where highquality limestone is likely to be found.
Facies Changes
Lateral changes in the sedimentary environment can result in facies changes within the limestone deposits. For instance, a limestone facies rich in shell fragments might transition laterally into a micritic limestone facies with different chemical properties. Understanding these lateral variations is essential for maintaining consistency in the raw material.
Homogeneity vs. Heterogeneity
Vertical continuity in a limestone deposit refers to the uniformity of the deposit with depth. In some deposits, limestone quality remains relatively homogeneous with depth, making it easier to manage. In other cases, the quality may vary, with alternating bands of pure and impure limestone, impacting how the deposit is mined and processed.
Diagenetic Processes Processes like dolomitization, recrystallisation, or the introduction of secondary minerals (e.g., silica, siderite) can occur at specific horizons within a limestone deposit. The recrystallisation process can reduce porosity and increase the density of the limestone, making it more suitable for cement production. Dolomitization is a process where calcium is replaced by magnesium, forming dolomite. This process can significantly alter the chemical and mineralogical composition of the deposit. These diagenetic changes can reduce the vertical continuity of quality limestone, requiring more detailed geological modelling to predict and manage the variability.
Lithological Variations
Limestone deposits, even those that appear massive and homogeneous, often contain subtle lithological variations. These variations can include differences in texture, colour, grain size, and the presence of impurities. For instance, a limestone deposit might have alternating layers of pure calcite-rich limestone and argillaceous limestone with a higher amount of clay content. Silica in limestone can originate from quartz, chert, or clay minerals. While a certain amount of silica is necessary for clinker formation, excess silica can lead to the formation of high-silica phases that are difficult to grind and affect the strength of the cement. These variations can lead to significant differences in the chemical composition of the raw material feed, affecting clinker formation and cement quality. For example, on a limestone deposit, a zone with finer-grained limestone might have higher clay content, contributing additional silica (SiO2) and alumina (Al2O3) to the raw mix. While these elements are essential for clinker formation, their excess can lead to challenges in controlling the burnability of the mix and the resulting cement’s hydraulic properties.
Structural Features
Geological structures like faults, fractures, and bedding planes can impact how minerals are distributed within a deposit. For instance, a fault zone may contribute to the formation of secondary minerals, leading to localised variations in mineral composition, such as an increase in dolomite or clay content. Understanding these structures is important for predicting these variations, which is crucial for planning extraction and blending strategies. For example, a fault intersecting a limestone deposit could introduce hydrothermal fluids, resulting in the formation of dolomite-rich zones and other mineralization near the fault. These zones would contain higher MgO content, which must be carefully managed in the cement raw mix to prevent issues with clinker quality.
Mineralogical Variations Affecting the Chemistry and Handling
Several minerals, natural or man-made may participate the composition of the cement. A summary is given below.
Limestone Mineralogy
Limestone, the primary source of calcium oxide (CaO) in cement production, is composed of mainly consists of calcite but other various minerals that can significantly influence its chemical characteristics and, in turn, the quality of the cement, be present. The primary component of limestone is calcium carbonate. High-purity limestones, often referred to as “high-calcium limestone,” contain over 95% CaCO3. However, impurities such as silica (SiO2), alumina (Al2O3), and iron oxides (Fe2O3) can reduce the calcium content, impacting the quality of the cement produced. Dolomitic limestones contain significant amounts of MgCO3. While small amounts of MgCO3 can be beneficial, high levels can lead to the formation of periclase (MgO) during clinker production, which can cause expansion and cracking in concrete.
Calcite (CaCO3): Calcite is the purest form of calcium carbonate and the most desirable mineral in limestone for cement making. High calcite content translates to a high CaO content in the clinker, which is crucial for forming key cement compounds like alite (tricalcium silicate, Ca3SiO5 or C3S) and belite (dicalcium silicate, Ca2SiO4 or C2S). Calcite-rich limestone is relatively easy to grind and process. It requires lower energy input during grinding and contributes to efficient kiln operations due to its predictable decomposition into lime (CaO) and CO2. High calcite content leads to strong and durable cement. However, variations in calcite concentration can cause inconsistencies in the chemical composition of the raw mix, affecting clinker formation and final product quality.
Dolomite (CaMg(CO3)2): Dolomite introduces both calcium and magnesium into the raw mix. While calcium is necessary, magnesium can cause problems. High MgO levels can lead to the formation of periclase (MgO) in clinker, which can cause delayed expansion and cracking in concrete. Dolomite requires higher kiln temperatures for decomposition, leading to increased energy consumption. Additionally, it is more abrasive, leading to increased wear on grinding equipment. The presence of dolomite in limestone can be detrimental to cement quality if MgO levels exceed the acceptable limits. This requires careful management during quarrying and processing to ensure that dolomite-rich zones are either avoided or blended properly with high-calcite limestone.
Clay Minerals in Limestone (Kaolinite, Illite, Montmorillonite)
These clay minerals can introduce silica (SiO2), alumina (Al2O3), and iron oxide (Fe2O3) into the limestone. While these elements are necessary for clinker formation, their excessive presence can lead to an imbalance in the raw mix, requiring additional corrective materials. Limestone with significant clay content tends to be more difficult to grind and may result in increased kiln coating and ring formation, leading to operational inefficiencies. Excessive clay content in limestone can result in lower clinker reactivity and strength, affecting the quality of the final cement.
Kaolinite (Al2Si2O5(OH)4): Kaolinite is a key source of alumina and silica. High kaolinite content is desirable as it provides the necessary Al2O3 and SiO2 for clinker formation without introducing excessive impurities. It is relatively easy to process and decompose in the kiln. It contributes positively to the formation of tricalcium aluminate (C3A) and dicalcium silicate (C2S), essential for cement strength. High-quality clay rich in kaolinite supports the production of strong, durable cement with consistent setting times.
Illite (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]: Illite introduces both alumina and silica but also potassium, which is an alkali metal. Alkalis can be problematic as they may cause alkali-silica reaction (ASR) in concrete. The presence of illite can increase the alkali content in the raw mix, requiring adjustments in the raw material blending and process control to avoid quality issues. While illite contributes necessary alumina and silica, its alkali content must be managed carefully to prevent ASR in the final concrete.
Montmorillonite (Na,Ca)0.3(Al,Mg)2Si4O10(OH)2·nH2O):
Montmorillonite is a swelling clay mineral that can introduce variability in the alumina, silica, and water content of the raw mix. Its sodium content adds to the alkali levels, potentially leading to ASR (Alkali Silika Reaction). Montmorillonite’s water content and swelling nature can complicate the grinding process and increase the risk of material handling issues like stickiness and clogging. It can also contribute to Portuneven kiln feed, impacting clinker formation. Montmorillonite must be carefully managed within the raw mix to prevent quality issues in cement, such as increased porosity and reduced strength in concrete.
Gypsum
Gypsum is a common mineral, with thick and extensive evaporite beds in association with sedimentary rocks. Gypsum is deposited from lake and sea water, as well as in hot springs, from volcanic vapours, and sulphate solutions in veins. It is often associated with the minerals halite and sulphur. Gypsum (CaSO4·2H2O) used in cement making acts as a retarder, controlling the setting time of cement. This allows for better workability and placement of the cement before it hardens. The amount of gypsum added to cement is carefully controlled to achieve the desired setting time. Too much gypsum can result in excessive retardation, while too little can lead to rapid setting and poor workability. It also provides sulphur, which can improve clinker formation. The quality of gypsum depends on its purity and its ability to dissolve readily in water.
Iron Ore
Iron ore is often used as a minor component in cement production. While not a primary ingredient like limestone or clay, it can provide several benefits: The addition of iron oxide from iron ore (hematite (Fe2O3) or magnetite (Fe3O4)) can improve the strength and durability of the cement. Iron oxide can also be used to adjust the colour of the cement, which is sometimes desired for aesthetic purposes. It can help in the formation of certain clinker phases, such as tricalcium aluminate, which can improve the cement’s early strength. However, it’s important to note that excessive iron oxide can have negative effects on cement properties, such as increased shrinkage and susceptibility to sulphate attack. Therefore, the use of iron ore is typically limited to small quantities to achieve the desired benefits without compromising the overall quality of the cement.
Other Materials
Supplementary cementitious materials (SCMs) can also be used in cement making. Typical SCMs include blast furnace slag, fly ash from coal-fired power plants, silica fume (a byproduct of silicon metal production). SCMs can enhance cement’s properties, reduce its carbon footprint, and improve its sustainability. The use of SCMs varies depending on local regulations and the desired properties of the cement. The specific properties and proportions of these raw materials can significantly influence the quality and performance of the final cement product. Careful selection and blending of raw materials are essential for producing cement that meets the desired specifications.
Impact on Product Quality, Material Handling and Processing
The composition of the raw materials can a great influence on the final product as well as the stages to create it. The effect can be seen in several areas ranging from the clinker formation to material handling to processing. The effects can be summarised below:
Clinker Formation
The mineralogical composition of the raw materials directly impacts the formation of clinker phases like alite and belite. For instance, high calcite (CaCO3) content leads to optimal alite formation, contributing to the strength and durability of cement. Alite reacts relatively rapidly with water and is responsible for most of the early strength development of concretes. Belite is less reactive at early ages but can contribute appreciably to strength at later ages.
Setting Time and Strength Variations in the alumina and silica content, due to different clay minerals, affect the formation of compounds like tricalcium aluminate (Ca3Al2O6 or C3A) and tricalcium silicate (C3S). Tricalcium aluminate reacts most strongly with water of all the calcium aluminates, and it is also the most reactive of the Portland clinker phases. These phases govern the setting time and early strength development of cement.
Durability
The presence of minerals introducing excessive MgO, alkalis (Na2O, K2O), or sulphur can lead to durability issues like expansion, cracking, or reduced sulphate resistance in the final concrete.
Grinding Efficiency
Minerals like dolomite and clay minerals (e.g., montmorillonite) can be harder or more abrasive than pure calcite, requiring more energy for grinding and leading to increased wear on equipment.
Kiln Feed Consistency
Variability in mineralogy can lead to inconsistent kiln feed, causing operational challenges such as fluctuating temperatures and clinker quality. Proper blending of materials with different mineralogical characteristics is crucial to maintaining consistent feed.
Storage and Transport
Some clay minerals, particularly those that swell or retain water (e.g., montmorillonite), can cause stickiness and clogging in storage bins, hoppers, and conveyors, leading to handling difficulties.
Kiln Operations
The mineralogical composition of the raw mix affects the thermal behaviour during clinker production. For example, dolomite requires higher temperatures to decompose, increasing energy consumption. The presence of swelling clays can lead to ring formation in the kiln, disrupting operations.
Energy Consumption
Harder minerals or those with higher impurity levels require more energy for both grinding and clinker formation. Efficient quarrying and blending based on mineralogical data can help minimize these energy demands.
Emissions Control
Certain minerals, especially those containing sulphur or alkalis, can increase emissions during the cement manufacturing process. Proper management and blending can help control these emissions and comply with environmental regulations.
Chemical Approach vs Mineralogical Approach in Cement Making
The traditional approach to cement production has mainly relied on chemical analysis to understand and control the properties of the final product. However, there are more benefits to using a mineralogical and geological approach, as it can be linked to the original depositional conditions. This approach can help achieve better resource management by establishing a robust geological model through the incorporation of existing geological and mineralogical information to control the quality of the final product. Additionally, this can lead to better reporting practices using any of the CRIRSCO templates for resources and reserves (Kahraman 2024), which is a critical factor for the industry’s future success. Given the increasing scrutiny from regulators, investors, and financial institutions, accurate and transparent reporting will be essential for maintaining competitiveness, securing financing, and achieving sustainability goals.
Chemical Approach
The chemical approach focuses on the elemental composition of a material. Chemical analysis involves determining the proportions of different elements using techniques such as X-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), and atomic absorption spectroscopy (AAS). However, while this provides valuable information about elemental composition, it may not fully capture the structural or physical properties of a material.
Chemical analysis aims to identify and quantify impurities that may affect the quality of cement, such as excessive amounts of magnesium, sulphur, or alkalis. Subsequent process control is based on blend optimisation and kiln operation based on the chemical recipe. Blend optimisation aims to achieve the desired chemical composition for the clinker, the intermediate product in cement production, while kiln operation ensures that the desired reactions are occurring and that the clinker is being formed correctly.
Mineralogical Approach
Conversely, the mineralogical approach focuses on identifying and characterizing specific minerals within a material. This involves examining the crystal structure, chemical composition, and physical properties of individual minerals using techniques like X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), mineral liberation analysis, and electron microprobe analysis (EMPA).
This approach provides a comprehensive understanding of a material’s properties, including its microstructure, texture, and behaviour, by identifying the specific minerals present, such as calcite, dolomite, clay minerals, and iron oxides. This information is crucial for understanding their potential contributions to the clinker formation process, particularly in relation to particle size distribution, which influences their reactivity and mixing behaviour during clinker production.
The mineralogical approach can also be used to study the phase composition of clinker, identifying key minerals formed during the firing process, such as alite, belite, tricalcium aluminate, and tricalcium silicate. Techniques like scanning electron microscopy (SEM) can be used to examine the microstructure of clinker, revealing the distribution and morphology of different mineral phases.
Furthermore, this approach can be used to identify the hydration products formed when cement is mixed with water, such as calcium silicate hydrates (C-S-H), calcium hydroxide (CH), and calcium aluminate hydrates [AFm phase which is an “alumina, ferric oxide, monosubstituted” phase, or aluminate ferrite monosubstituted, or Al2O3, Fe2O3 mono, in cement chemist notation (CCN)]. This information also contributes to understanding hydration kinetics, providing insights into the rate at which different minerals react and contribute to the development of cement strength and durability.
Mineralogical approach may also evaluate the durability of cement, by examining the formation of deleterious phases (e.g., sulphate attack products) and assessing the stability of the hydration products over time. This can also be extended to develop new cement formulations with tailored properties, such as enhanced durability, reduced environmental impact, or specialised applications.
Concept of Geo-metallurgy in Cement Making
Geo-metallurgy is an interdisciplinary field that was developed particularly for metalliferous mines, which integrates geological, mining, metallurgical, and environmental information to optimise resource extraction and processing. It involves understanding the variability of ore bodies and how this variability affects processing performance. It bridges the gap between geological knowledge of ore deposits and metallurgical practices for extracting valuable metals from them.
Key aspects of geo-metallurgy include ore characterisation, metallurgical process selection, process optimisation, and predictive modelling.
Ore characterisation is implemented by conducting mineralogy (identification of specific minerals present in the ore body), texture (spatial distribution and relationships between different minerals), liberation (determination of the degree to which valuable minerals are physically separated from gangue (waste material)), and particle size distribution (analysis of size distribution of ore particles affecting processing efficiency).
Metallurgical process selection involves choosing the appropriate methods for crushing, grinding, and concentration of the ore whilst selecting the most suitable extraction techniques (e.g., smelting, leaching, electrolysis) based on the ore’s characteristics. Refining at later stage involves determining the necessary refining processes to obtain pure metals or alloys.
Process optimisation includes energy efficiency by minimising energy consumption during processing and environmental Impact by reducing the environmental footprint of mining and metallurgical operations. This ideally leads to economic viability by maximising the profitability of mineral extraction.
Predictive modelling employs process simulation by using computer models to predict the behaviour of ore during processing. Once an acceptable model is established, the optimisation process can identify optimal operating conditions for maximum efficiency and profitability.
The authors of this paper suggest that similar approaches can be employed for cement industry by adopting the term “Geo-cementing” and its approaches, details of which will be given in a subsequent paper later.
Conclusion
This article highlights the importance of geological and mineralogical aspects of the raw materials in cement making where the primary component is mainly limestone and other associated materials. The focus of the paper is to establish the foundations for the “geo-cementing” concept, which will be the topic of a subsequent paper, as the geology and mineralogy of these materials directly determines the chemical composition, which in turn affects the quality of the cement produced.
The traditional approach to cement production has predominantly relied on chemical analysis to understand and control the properties of the final product. However, utilising a mineralogical approach could offer more benefits, as it can be linked to the original depositional conditions. This approach can facilitate better resource management by establishing a strong geological model that incorporates existing geological and mineralogical information to control the quality of the final product.
Therefore, the intention here is to collate and remind the basics of the raw material in cement making since geological and mineralogical variations in raw materials like limestone, clay and others affect chemical characteristics, and the subsequent impact on product quality, material handling, and processing in cement production.
The “Geo-cementing” approach, as proposed by the authors in this paper, is similar to geo-metallurgy developed particularly for metalliferous mines. It integrates geological, mining, comminution, processing and environmental information to optimise resource extraction and processing. This approach aims to bridge the gap between geological knowledge of limestone deposits and processing practices for the final product. Detailed information about the geo-cementing approach will be provided in a follow-up article later.
Kahraman Arden H. 2024. Çimento Endüstrisinin Kaynak ve Rezerv Tahminine Hazırlığı: Birleşmeler, Satın Almalar, Finansman ve Paris Anlaşması için Çıkarımlar [The Cement Industry’s Preparedness for Resource and Reserve Estimation: Implications for Mergers, Acquisitions, Financing, and the Paris Agreement]. CemenTürk Magazine. Eylül-Ekim Sayısı. pp.66-70.
