Particle size analysis of cement using the technique of laser diffraction

Introduction

Cement has been an integral part of construction since a primitive version was developed by the Romans. Today the most commonly used form of cement is Portland cement, which was first developed in the 19th century.

Portland cement analysis

In general, the production of cement comprises three stages: preparation of a raw mix; production of the clinker; and the grinding of the clinker to form the finished cement.

The raw mix can be made up of materials such as calcium oxide, silicon oxide, aluminium oxide, ferric oxide and magnesium oxide. Most of these minerals can be obtained from local rock, but clay and limestone may also be added. These components are then ground into a raw mix, so that no more than 15% of the volume or mass of particles are larger than 90µm (Figure 1). The proportion of coarse particles in the raw mix must be controlled to ensure that the resulting mix is homogenous and that the sintering of the components within the kiln during the next stage of production is completed within a reasonable time frame.

The raw mix is heated to approximately 1400°C in order to from the clinker. Calcium sulphate may also be added, in order to provide control of the setting time, prior to grinding of the clinker to form the finished cement.

Figure 1 : Particle size distribution of a raw mix
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The properties of any cement are dependent on their composition and particle size distribution. For example, the properties of Ordinary Portland Cement can be varied by adding other components to produce different cement blends. For instance, blast-furnace slag and fly-ash can be added to Portland clinker in order to save cost, but these blends can have poorer early strength. Expansive cements can be produced in order to offset shrinkage during drying by use of expansive clinkers (such as sulpho-aluminate). The aesthetic properties of cement can also be altered by adding pigments and clays to produce coloured or white cements.

Cement particle size analysis

The critical strength and curing properties of cement are dependent on the particle size distribution resulting from the grinding phase. The compressive strength and curing qualities are increased as the particle size decreases due to the increase in surface area. For this reason cement has traditionally been specified using the Blaine number, a surface area based parameter measured by an air permeability technique.

However, additional information can be gained by measuring the particle size distribution. For example, if two cement samples have the same average size or surface area then the sample with the narrower size distribution will have a higher compressive strength. Secondly the Blaine number will be less sensitive to changes in the coarse fraction.

In a general cement, between 60 and 70% of the material should be between 3µm and 30µm in size. Excess large particles (greater than 50µm) can cause problems by reducing strength due to incomplete hydration, where as excess small particles (less than 2µm) can reduce strength and cause the cement to crack by setting exothermically.

Figure 2: Particle size distribution of CEM I and CEM II.
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Figure 3: Tromp curves showing different degrees of separation.
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Experimental

In this application note, laser diffraction has been used to measure samples of the raw mix and two cement blends. Particle size information has also been used to determine the efficiency of a separator. Additionally a cross contamination study has been carried out where alternate measurements of two different cement types have been made in order to demonstrate that there is no contamination between measurements.

Measuring the particle size of cement

The particle size distributions of the various cement samples have been measured by laser diffraction using a Mastersizer. Laser diffraction instruments operate by illuminating particles in a collimated laser beam and measuring the scattered light over a range of angles. Particle size information can be obtained as the angle to which particle scatter light is dependent on their size. Therefore by measuring the angular dependence of the scattered light, the particle size distribution can be obtained by using an appropriate scattering model.

The sample is dispersed and passed through the measurement cell using a dispersion unit. Samples can be dispersed wet or dry depending on which is more appropriate to the application. For cement, the dry dispersion unit allows for a larger volume of material to be measured easily and it also avoids the additional cost of the solvent which would be required for a wet dispersion of cement. Therefore, in this report the samples have been dispersed dry, using the Scirocco. The Scirocco operates by vibrating a tray of material that feeds into a venturi which, combined with compressed air, is used to disperse the particles. The Scirocco can be used with several trays for different volumes of sample and an additional hopper, which can take up to 25g. In addition, water-resistant, ceramic dispersing components are available, providing robust operation when handling abrasive materials.

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