Quantitative Phase Analysis of Blast Furnace Slag Cements

Granulated ground blast furnace slag (GGBFS) is a major constituent of CEM II/A-S, CEM II/B-S and CEM III cements. Blast furnace slag is a predominantly amorphous product, containing small amounts of crystalline phases like Akermanite (CaMgSiO), Merwinite (CaMgSiO), or Quartz (SiOCO2). It offers hydraulic properties similar to those of Portland Cement and hence may partly substitute the clinker component in cement blends.

Benefits of Blast Furnace Slag Cements

Obviously clinker burning is a main source of CO2 emissions. Therefore blast furnace slag cements are increasingly produced in order to reduce cement making specific CO2 emissions.

Quality Control of Blast Furnace Slag Cements

Quality assurance and surveillance requires the determination of the main constituents of the finalized cement. Current regulations allow several methods to estimate the slag content in cements, such as gravity separation (ENV 196-4:1993), selective dissolution (ENV 196-4:1993), microscopic analysis (DIN 1164-1:1990) or the determination based on the chemical composition. These methods are either time consuming, require special laboratory work. or pre-knowledge about the sample.

This Lab Report demonstrates how the analysis of amorphous phases can be seamlessly integrated into the TOPAS Rietveld calculation without further need for calibration or adding a standard.

Rietveld Quantification of Amorphous Phases

By definition, traditional Rietveld analysis takes only crystalline phases into account. The relative weight fractions are normalized to 100 wt. %. Amorphous amounts can only be determined indirectly by adding a known weight fraction of an internal standard to the sample (also called the "spiking method").

In an automated process laboratory this method can hardly be realized. Furthermore, systematic errors can arise from microabsorption effects due to differences in mass absorption of sample and standard. Accurate analysis using the spiking method requires similar mass absorption and grain size distribution of both sample and standard.

A Comparison with the Traditional Rietveld Quantification Method

In contrast to traditional Rietveld TOPAS allows the consideration of phases with partially or no known crystal structure in the calculation (PONKCS [1]) by using hkl_Phases (Pawley or Le Bail fitting).

Quantitative phase analysis applying such models requires an empirical "calibration" step, because of lacking structure information. The mass of such phase is not known. To do a proper calibration, samples of known composition are mandatory to define the mass accordingly.

PONKCS - Quantitative Rietveld Analysis of Phases with Partially or No Known Crystal Structure

The weight fraction wi of the i-th phase in Rietveld analysis is defined by: the scaling parameter si, the volume Vi of the unit cell. the weight of the atoms Mi Zi (M = Mass of one formula unit, Z = number of formula units in cell) inside the unit cell and

Rietveld quantification using hkl_Phases instead of structures: Using hkl_Phases the intensity values are derived from a measurement of the peak intensities. If an hkl_Phase is used in quantitative Rietveld analysis, only the volume V of the unit cell is known. Rietveld quantification requires the "calibration" of the Mass (MZ) of the hkl_Phase, because of the lack of structural information.

X-Ray Diffractometer for Quantitative Rietveld Analysis

The measurements were executed using a D4 ENDEAVOR diffractometer in Bragg-Brentano Geometry equipped with the 1-dimensional LynxEye™ compound silicon strip detector (fig. 1). The settings are given in Table 1. The quantitative phase analysis was done using the DIFFRACplus TOPAS (Version 4) software.

Table 1. D4 ENDEAVOR configuration with the LynxEye Detector

Goniometer

D4 ENDEAVOR Theta/2Theta

Measurement circle

401 mm

Tube

2.2 kW Cu long fine focus

Tube power

35 kV / 50 mA

Primary optics

Divergence slit fixed to 0.5° 4° Soller slit

Sample stage

Rotating sample holder

Secondary optics

Nickel Kß Filter 4°Soller slit

Detector

LynxEye (opening 3.9°)

Step size

0.02°

Time per step

0.2 s

Angular range (2Theta)

10° to 65°

Total Measuring time

9 min 50 sec

Preparation of Blast Furnace Slag Cements

In 2006, the VDZ (German Cement Works Association) organized a round robin on quantitative phase analysis of blast furnace slag cements using XRD methods. A set of samples was distributed to the participants to be analyzed. This set comprised three Slag Cements of different compositions. 10 grams of each sample were ground in an automatic preparation unit POLAB®APM, using Polysius tablets as binder. The samples were pressed in steel rings. The preliminary published outcomes [2] provided the reference values of the slag amounts in each cement sample (table 2).

Table 2. Composition of the Slag Cement Samples used in the VDZ Round Robin

Sample No.

Sample Description

Slag Content in wt.%

1

CEM II/B-S

25

2

CEM III/B 32,5 N-NW/HS/NA

67

3

CEM III/B 42,5 N-NW/HS/NA

72

The 1-dimensional LynxEye compound silicon strip detector.

Figure 1. The 1-dimensional LynxEye compound silicon strip detector.

Phase Modeling of Blast Furnace Slag

The modeling of the amorphous diffraction data is realized by the following steps:

  • Measurement of the pure blast furnace slag
  • Whole Powder Pattern Decomposition of the amorphous intensities by Pawley fitting using an arbitrary start model
  • Empirical „calibration" of the mass (MZ) of this model to meet the results of the reference samples

This approach results in a perfect description of the amorphous diffraction characteristics as shown in figure 2.

Structurless modelling of amorphous blast furnace slag. The blue curve represents the measured data. The calculated model is represented by the red curve. The difference is plotted in grey.

Figure 2. Structurless modelling of amorphous blast furnace slag. The blue curve represents the measured data. The calculated model is represented by the red curve. The difference is plotted in grey.

Analysis of hkl Phase Model Calculations

All three round robin samples were quantified using the same hkl_Phase model for calculating the blast furnace slag. The repeatability of measurement was investigated by analyzing each sample five times. For each of the runs the sample was unloaded and reloaded to the diffractometer. Figure 3 shows measurement data of round robin sample 1 and the TOPAS quantification result.

Measurement result (blue) and TOPAS calculation (red) of Slag Cement sample 1. The difference of both is given in grey. The marks indicate the peak positions of each phase with a known structure. The blue curve above the difference curve indicates the intensity contribution of the amorphous blast furnace slag.

Figure 3. Measurement result (blue) and TOPAS calculation (red) of Slag Cement sample 1. The difference of both is given in grey. The marks indicate the peak positions of each phase with a known structure. The blue curve above the difference curve indicates the intensity contribution of the amorphous blast furnace slag.

Table 3. TOPAS quantitative phase analysis of the VDZ round robin slag cement samples (values given in wt.%)

Measurement Number

Sample 1

Sample 2

Sample 3

Measurement 1

25.0

67.2

71.7

Measurement 2

25.1

67.3

71.9

Measurement 3

24.7

67.0

71.6

Measurement 4

25.1

67.3

71.9

Measurement 5

25.3

67.0

71.5

Mean

25.1

67.2

71.7

Std. Dev.

0.2

0.2

0.2

Summary

The TOPAS PONKCS method provided accurate results (Table 2 and 3) covering a broad range of slag concentration. The repeatability was significantly better than known from existing methods currently established. The absolute standard deviation of the calculated slag concentrations was 0.2 wt.%.

Note: In this study, predominantly amorphous blast furnace slags were analyzed. The analysis can be easily extended to slag qualities showing a larger degree of crystallinity, by simply adding additional crystalline phases to the TOPAS calculation. No further work is required.

This information has been sourced, reviewed and adapted from materials provided by Bruker X-Ray Analysis.

For more information on this source, please visit Bruker X-Ray Analysis.

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