I. What is Cement Clinker

Cement clinker composition: A semi-finished product obtained by proportioning limestone and clay as the main raw materials into raw meal, burning it until partially or fully molten, and then cooling. In the cement industry, the most commonly used Portland cement clinker has main chemical components of calcium oxide (CaO), silicon dioxide (SiO₂), and small amounts of aluminum oxide (Al₂O₃) and iron oxide (Fe₂O₃). Its principal mineral constituents are tricalcium silicate (C₃S), dicalcium silicate (C₂S), tricalcium aluminate (C₃A), and tetracalcium aluminoferrite (C₄AF). When Portland cement clinker is ground together with an appropriate amount of gypsum, it becomes Portland cement.

II. The Complete Cement Production Process

1. Mining

The process of extracting and crushing limestone ore and transporting it to the limestone silo by various means.

2. Raw Meal Preparation

The process of crushing limestone raw material together with a small amount of argillaceous raw material and corrective materials, then proportioning, grinding, and blending them into raw meal of suitable composition and uniform quality.

3. Clinker Calcination

The process in which raw meal undergoes a series of physical and chemical changes through high-temperature calcination, ultimately forming Portland cement clinker with calcium silicate as the principal component.

The properties of clinker determine the principal properties of cement; therefore, the clinker formation process is the most critical stage in cement production.

4. Cement Manufacturing and Dispatch

The process of grinding clinker together with appropriate amounts of gypsum and supplementary cementitious materials into powdered cement, which is then packaged or dispatched in bulk.

III. Applications of Cement Clinker

Suitable for rapid-hardening, early-strength engineering and high-strength-grade concrete. Not suitable for mass concrete engineering, or for projects subject to chemical attack, pressure-water action, or seawater erosion.

IV. Clinker Quality Assessment

(1) Clinker Mineral Composition Analysis

In traditional cement production, X-ray fluorescence (XRF) is used to analyze the chemical composition of cement clinker, and the Bogue method is employed to calculate the mineral composition from the chemical composition. However, this method is based on ideal crystal equilibrium conditions, i.e., it assumes that all chemical components undergo complete chemical reactions during the firing process. In actual production, due to factors such as reaction temperature, heating rate, calcination atmosphere, particle-size distribution, and homogeneity of the feed, the firing reaction cannot proceed under ideal conditions. Moreover, clinker must be cooled, during which various minerals exhibit solid-solution phenomena to varying degrees; phases such as periclase (free MgO) or free quartz may even be present. Consequently, the Bogue method often fails to reflect the true characteristics of the clinker composition, and its results frequently differ substantially from the actual phase contents.

(2) Crystal-Form Analysis of Clinker Minerals

Different process conditions (e.g., firing and cooling rates) and the solid solution of trace elements (MgO, R₂O, SO₃, P₂O₅, and heavy metals) affect the mineral composition of clinker as well as the crystal forms or lattice parameters of C₃S, C₂S, C₃A, and C₄AF, which in turn lead to changes in clinker performance. XRD technology can analyze changes in clinker mineral composition, crystal form, and lattice parameters, helping to identify the root cause of variations in clinker performance and providing a basis for production control.

V. Experimental Principle of Quantitative Phase Analysis of Cement Clinker

From diffraction-intensity theory, the diffraction intensity of a given mineral phase in a multiphase mixture increases with its relative content. The X-ray diffraction method uses the raw XRD pattern of the test sample and the crystal information files (CIF files) of each mineral phase to qualitatively identify the phase composition, and then obtains the relative content of each phase through quantitative analysis by fitting and calculating the peak intensities of the pattern. XRD patterns are well suited for the phase analysis of Portland cement clinker. Rietveld quantitative analysis is a full-pattern fitting method that directly employs the least-squares method to refine crystallographic and instrumental parameters against the measured XRD pattern. This data-analysis approach minimizes the difference between the calculated and measured patterns and can be used to determine the relative content of mineral phases in cement clinker.

Advantages of the Rietveld analysis model over traditional standard-based methods: it enables full-pattern fitting of XRD polycrystalline-structure data. Rietveld analysis accounts for the reproducibility of intensity measurements, sample-position offsets in the pattern, characteristic-peak crystal parameters, and characteristic-peak intensities affected by solid-solution doping.

Using the scale factor, phase density, and unit-cell volume of each mineral, the relative phase content is calculated according to the following formula:

Where:

W_p —  mass fraction of mineral phase (p);

S_P —  Rietveld scale factor;

Z  —  number of formula units contained in a single unit cell;

M  —  relative molecular mass;

V  —  unit-cell volume.

VI. Test Case

In this experiment, LANScientific FRINGE series desktop XRD was used to test and analyze Portland cement (GBW 03201c) and sulphoaluminate cement clinker (GSB 08-2048-2006), obtaining sample diffraction patterns. The Rietveld analysis model was applied for full-pattern fitting of the XRD polycrystalline-structure data to perform standard-free quantitative calculation of the content of each phase in the cement samples (reference: GB/T 40407—2021, "X-ray diffraction analysis method for mineral phases of Portland cement clinker"). During quantitative phase analysis, the XRD quantitative-analysis program combines these calculated patterns to fit the original measured pattern, yielding the relative content of each mineral phase at the best fit [when the weighted-profile R-factor (Rwp) converges and Rwp < 8%].

Sample Display

Figure 1. Reference cement samples: Portland cement (GBW 03201c, left) and sulphoaluminate cement clinker (GSB 08-2048-2006, right)

Analysis Results

The calculated pattern was fitted to the original measured pattern, achieving the best fit with a weighted-profile R-factor of 7.33% (< 8%), indicating that the analysis results are highly reliable and satisfy the requirements of quantitative analysis.

The mineral phases in Portland cement (GBW 03201c) are: 65.5% alite (tricalcium silicate), 8.6% belite (dicalcium silicate), 11.9% aluminate (tricalcium aluminate), 9.2% aluminoferrite (tetracalcium aluminoferrite), 3.6% anhydrite (calcium sulfate), and 1.2% periclase (magnesium oxide). Converting these mineral phases into their corresponding oxides and comparing with the national-standard certified values provides useful reference information; the absolute errors for the oxides of silicon, iron, magnesium, aluminum, and sulfur are below 2%. As shown in the table below:

Table 1. Comparison of XRD phase-converted oxide contents with national-standard certified values for Portland cement (GBW 03201c)

The calculated pattern was fitted to the original measured pattern, achieving the best fit with a weighted-profile R-factor of 7.65% (< 8%), indicating that the analysis results are highly reliable and satisfy the requirements of quantitative analysis. The mineral phases in sulphoaluminate cement clinker (GSB 08-2048-2006) are: 26.0% belite (dicalcium silicate), 8.8% aluminate (tricalcium aluminate), 4.1% aluminoferrite (tetracalcium aluminoferrite), and 61.1% calcium sulfoaluminate. Converting these mineral phases into their corresponding oxides and comparing with the national-standard certified values provides useful reference information; the absolute errors for the oxides of calcium, silicon, iron, aluminum, and sulfur are basically below 3%. As shown in the table below:

Table 2. Comparison of XRD phase-converted oxide contents with national-standard certified values for sulphoaluminate cement clinker (GSB 08-2048-2006)

VII. Conclusion

The analysis results demonstrate that using LANScientific FRINGE series desktop XRD to collect diffraction patterns of cement samples, followed by Rietveld standard-free quantitative analysis of the phase composition and content of each component in the cement, helps to ascertain information such as the proportion of each phase component in the cement. This provides a robust data basis for practical cement production, research and development, and quality control.