PARTICLE SIZE DISTRIBUTION

Introduction

Particle size distribution (PSD) describes the range of particle sizes found within a substance.

Particle Size Distribution of filler is an important measure because it can strongly influence the behaviour of bitumen and asphalt in terms of the flow of the mastic and compactability and stiffness of the asphalt.

Particle size distribution is typically expressed as the percentage of particles passing a particular size. To be classified as a filler, there is a particle size requirement of a minimum of 70% passing 63 micron (BS EN 13043:2002). Test results can also be shown graphically, either by percentage mass or volume of particles within a particular range.

For many materials, sieving is a simple technique for producing PSD data. For very fine materials, such as fillers and chemical powders, this method has limitations due to electrostatic interactions between particles and blinding of the sieve apertures.

Laser diffraction is a technique commonly used to overcome the limitations of sieving and to measure the particle size distribution of fine materials. This technique represents a fast, accurate means of obtaining a particle size distribution. Laser diffraction instruments consist of a light (typically a laser), a particulate dispersing device and a detector for measuring the scattering pattern, see Figure 5.1.

Figure 5.1 Principles of particle size measurement and detection

Test Procedure

The laser diffraction technique is based on the phenomenon that particles scatter light in all directions with an intensity pattern that is dependent on particle size. The technique assumes that each particle is of spherical shape. The diffracted patterns are detected and analysed to produce a particle size distribution. The principles of laser diffraction are set out in BS ISO13320-1:1999.

Testing of the Particle Size Distributions was carried out using the `Malvern Mastersizer S`

Particle Size Analyser, see Figure 5.2. This device is capable of measuring particle sizes from

Figure 5.2 Malvern Master sizer S

To undertake testing, an optical transparent liquid of known refractive index was used. The selected test liquid was water as it was compatible with the material and has a refractive index which differs significantly from that of the particulate material. The chemical concentration was selected to allow a sufficient number of large particles to be present.

Prior to testing the instrument was allowed a period of 1 hour to stabilise. The instrument was then calibrated to ensure proper alignment of the optical part of the instrument and the background of the scattering pattern measured. Particle size measurements were then undertaken in accordance with BS ISO13320-1:1999.

Test Results

Firstly, the volumetric particle size distribution of the investigated chemicals and filler were considered, as displayed in Figure 5.3.

Figure 5.3 Volumetric particle size distribution of chemicals and fillers

Figure 5.3 shows that the limestone filler and sodium silicate are typically normally distributed around a relatively low particle size. In contrast, the sodium formate sample tested has two distinctive regions, a relatively normally distributed section with a peak at 46.

0 2 4 6 8 10 12 14 16

0 100 200 300 400 500 600 700 800 900 1000

Particle Size ( m)

Percentage of Particle by Volume (%)

Sodium Silicate Sodium Formate Limestone Filler

Volumetric particle size distribution and particle size distribution by mass can be inter-converted by making assumptions of the density and shape of the particles. Typically in such calculations the particles are taken to be spherical.

Analysing the data in this manner enables direct comparison between the components to be easier. These are demonstrated in Figure 5.4.

Figure 5.4 - Particle size distribution, represented as percentage by mass passing a particular particle size

Discussion of Test Results

The data demonstrates that the limestone filler typically used in asphalt production is significantly finer than the anti-icing chemicals being investigated as part of this project.

Considering the chemicals in conjunction with the filler requirement of a minimum of 70%

passing 63 micron, it is clear that the chemicals cannot be categorised as fillers in accordance with `BS EN 13043:2002 - Aggregate for Bituminous Mixtures and Surface Treatments for roads, airfields and other trafficked areas`. This could cause future challenges when specifying an anti-icing surface course in the future.

In order to allow more convenient comparisons between two or more sets of particle size distribution data, mathematical descriptors of PSD curves such as Fineness Modulus and Coefficients of Uniformity are sometimes used for fillers. The Coefficient of Uniformity gives an indication of the range of sizes within a particle size distribution. A low coefficient of uniformity indicates a PSD with a small range of particles. It is typically calculated as follows:

Fineness modulus is an empirical mathematical description of the fineness of a material and is typically derived by taking the sum of the percentage of material passing different sizes. The higher the value of fineness modulus, the finer the material.

This approach has been adopted for asphalt fillers (Kandhal et al., 1998; Harris and Stuart 1998) and the Fineness Modulus of asphalt fillers has been calculated using the following formula:

(Eq. 5.2) Where:

FM = Fineness Modulus

Px =

For each chemical and filler, Uniformity Coefficient and Fineness Modulus were calculated and results are presented in Table 5.1.

Sodium Silicate Sodium Formate Limestone Filler Uniformity Coefficient

(D60/D10)

3.08 2.59 26.44

Fineness Modulus 0.97 1.42 4.42

Table 5.1 - Uniformity coefficient and fineness modulus of chemicals and fillers

The Uniformity Coefficient demonstrates that sodium silicate and sodium formate are very uniform with limited particle size variation between 60 and 10 . The fineness modulus indicates that the sodium silicate and sodium formate are significantly coarser than the limestone filler.

The higher uniformity coefficient for the limestone filler indicates a wider range of particle sizes and the fineness modulus is typical for filler used in asphalt production. Such conclusions are consistent with the analysis in Figure 5.3, showing the percentage passing by volume. These parameters will strongly influence the behavior of the bituminous mastic and, consequently, the