Measuring the Particle Size Distribution of Cement

Measuring and controlling the particle size distribution of cement is important both in order to achieve the desired product performance and to control manufacturing costs. Historic techniques of sieve and air permeability are still in use, but laser diffraction is becoming a more popular method to determine the particle size distribution of cement. The laser diffraction technique is quick, easy, reproducible, and provides a complete picture of the full size distribution. Introduction

How Cement is Made

Cement is made by heating limestone with small quantities of other materials such as clay or sand to 1450°C in a kiln. The resulting ‘clinker’ is then ground with a small amount of gypsum into a powder to make Portland cement. The most common use for Portland cement is in the production of concrete – a composite material consisting of cement, aggregate (gravel and sand), and water.

Particle Size Distribution and Hydration Rate for Portland Cement

The relationship between the particle size distribution of Portland cement and hydration rate and strength has been studied and published. As a general rule, reducing the particle size increases rate of hydration and strength. On the other hand, grinding cement to smaller sizes requires additional energy costs, creating a critical need for measurement and control of final product size.

Measurement Techniques

Historic techniques to determine the particle size and surface area of cement include sieves and air permeability, or Blaine, tests. More recently, modern cement laboratories deploy laser diffraction to perform particle size analysis. Laser diffraction trumps historic techniques with advantages in speed, ease of use, and reproducibility. In addition, laser diffraction provides a more complete description of the particle size distribution. Although it is possible to correlate laser diffraction results to sieve and Blaine values, the correlation typically falls apart if the size distribution changes - a likely occurrence in a plant upset condition. Vendors of laser diffraction analyzers therefore promote the use of directly-calculated volume distribution results in the instrument’s software package

Case Study

Experimental

Accurate measurement of cement in laser diffraction particle size analyzers proceeds either as a dry powder dispersed in air, or as a suspension dispersed in alcohol (typically isopropanol). Method development for wet or dry cement measurements follows the basic guidelines described in other HORIBA Application Notes.

Choosing the Refractive Index for Cement

Choosing the refractive index to use for cement requires consideration since cement consists of so many different chemical species. The chemistry of cement is complex, but the basic ingredients include tricalcium silicate, dicalcium silicate, tricalcium aluminate, tetracalcium aluminoferrite, and calcium sulfate (gypsum). The NIST SRM 114q certificate suggests using 1.70 and 1.0i, thus all results presented here incorporate those values.

Measuring Particle Size Using Laser Diffraction

Dry results for cement shown in this document were collected on the Partica LA-950 laser diffraction analyzer (seen below). Owing to the hard, robust nature of the material, all measurements used the highest air pressure setting for the PowderJet Dry Feeder. The PowderJet maintains a constant mass flow rate through the measurement zone via clever use of an automatic feedback loop: control of vibration rate based on light source transmittance readings.

The system settings used for the dry measurements are presented below:

  • PowderJet: Small nozzle, max pressure
  • Iteration number: 15
  • Refractive index: 1.70-1.0i in 1.000 (air)

Measurements Using Two Laser Diffraction Particle Size Analyzers

The wet results presented here were collected on both the LA-950 and LA-300 laser diffraction particle size analyzers. The LA-950 has a broader dynamic range (0.01-3000 µm) compared to the LA-300 (0.1-600 µm), but since these and most cement samples contain little to no powder below 0.1 µm, both systems accurately and precisely measure cement. The LA-950 does provide the additional benefits of higher levels of automation and a sophisticated software package users and labs find valuable.

Note that all measurements were made in isopropanol (IPA). Monitoring the optical background is important when using IPA as thermal fluctuations may cause additional scattering leading to inconsistent results. When encountering this phenomenon, recirculating the IPA to obtain a stable background is typically required.

The system settings used for the wet LA-950 and LA-300 measurements are presented below:

  • Iteration number: 15
  • Refractive index: 1.70-1.0i in 1.390 (IPA)
  • Ultrasound: 60s @ level 7 (full power)
  • Circulation speed: 6
  • Measurement time: 5000 acquisitions/second

Results

The results from three measurements of Portland cement measured on the LA-950 are shown below in Figure1 and Table1. Note the extreme reproducibility of the results, with the coefficient of variation well below those suggested in ISO 13320-1. The coefficient of variation calculations are performed directly in the LA-950 software. The HORIBA applications support team recommends all customers utilize this approach to vetting measurement methods before accepting results.

LA-950 Dry Results.

Figure 1. LA-950 Dry Results.

Table1. LA-950 Dry Results

File Name

Material

10.00%

50.00%

90.00%

200710232103113.NGB

Portland Cement

3.256

11.152

24.586

200710232104114.NGB

Portland Cement

3.116

11.183

24.671

200710232105115.NGB

Portland Cement

3.112

11.128

24.92

         

Average

 

3.161

11.154

24.726

Std. Dev

 

0.082

0.027

0.173

CV (%)

 

2.589

0.245

0.701

Wet Portland cement results on the LA-950 appear below in Figure 2 and Table 2. These results also exhibit excellent reproducibility and meet ISO 13320-1 guidelines.

Wet LA-950 Results

Figure 2. Wet LA-950 Results

Table2. Wet LA-950 Results

File Name

Material

10.00%

50.00%

90.00%

200710232235124.NGB

Portland Cement

2.122

11.81

27.047

200710232245128.NGB

Portland Cement

2.058

11.696

26.743

200710240103131.NGB

Portland Cement

1.999

11.614

27.001

         

Average

 

2.06

11.707

26.93

Std. Dev

 

0.062

0.098

0.164

CV (%)

 

2.996

0.838

0.607

Portland cement was also measured wet on the LA-300 laser diffraction system (seen below). This instrument measures suspensions, and is well suited for routine QC measurements where a smaller, more portable, and economically priced solution is beneficial.

The LA-300 System

Figure 3. The LA-300 System

Wet LA-300 Results

Figure 4. Wet LA-300 Results

Table 3. Wet LA-300 results

File Name

Material

10.00%

50.00%

90.00%

3 – 2 min EUS – 2 - A

Portland Cement

1.034

12.05

31.573

3 – 2 min EUS – 3 - A

Portland Cement

1.015

12.112

33.649

3 – 2 min EUS – 4 - A

Portland Cement

1.052

12.533

32.235

         

Average

 

1.034

12.232

32.485

Std. Dev

 

0.019

0.263

1.060

CV (%)

 

1.790

2.148

3.264

NIST SRM 114q Standard for Cement

The cement industry uses the NIST Standard Reference Material 114q in order to calibrate fineness testing equipment according to ASTM Standard Methods. This material has also been used as a source material for round robin studies using laser diffraction to measure particle size distribution. Analysis of NIST SRM 114q on the LA-950 shows the instrument’s response compared to the NIST documentation. Graph 4 (below) displays the LA-950 result along with the certified 114q values, and the lower and upper limits of accepted results. The LA-950 results fall within the accepted range.

Dry LA-940 Results for NIST 114q

Figure 5. Dry LA-940 Results for NIST 114q

Conclusions

Every passing day, laser diffraction becomes the preferred method for particle size analysis. Many customers prefer measuring cement as a dry powder thus eliminating the need to use and recycle alcohol. Other customers still prefer to use IPA as the dispersant. Both the LA-950 and LA-300 successfully meet the needs for research and routine quality control through the provision of speed, ease of use, repeatability, reproducibility, and reliability.

This information has been sourced, reviewed and adapted from materials provided by HORIBA Particle Characterization.

For more information on this source, please visit HORIBA Particle Characterization.

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Comments

  1. ifan mzaenudin ifan mzaenudin Indonesia says:

    how to obtain particle size values ​​from intensity weakening information using the lambert-beer equation?

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