Tag Archives: Particle size

How to define particle size?

Particle size is a very important property in the research field of cementitious materials, e.g. modeling, reaction kinetics.

Particles are 3-dimensional objects, and unless they are perfect spheres, e.g. emulsions or bubbles, they cannot be fully described by a single dimension such as a radius or diameter.

In order to simplify the measurement process, it is often convenient to define the particle size using the concept of equivalent spheres. In this case the particle size is defined by the diameter of an equivalent sphere having the same property as the actual particle such as volume or mass for example.

It is important to realize that different measurement techniques use different equivalent sphere models and therefore will not necessarily give exactly the same result for the particle diameter.

Below is a schematic figure illustrating the definition of particle size based on different equivalent spheres.

particle-size
Definition of particle size

Sample preparation by McCrone Micronizing mill before XRD/Rietveld test

In the cement science field, XRD analysis combined with Rietveld method can give quantitative results of crystalline phase of hydration products, so I try to use XRD/Rietveld method to quantify the hydration products of cementitous material. To achieve good performance of XRD test, fine powder is required and of great importance.

The question is how fine the particle sizes should be? It is showed by research that the particle size of powder has to be reduced to 10 μm or less, which is especially important for quantitative analysis (Smith, 2001).

Here comes the problem, it is very difficult to obtain these grain sizes by hand grinding, which means that mechanical grinding is required to reduce the grain size sufficiently.

There are several points should be stated. For soft minerals such as zeolites (hardness 3–4) grinding can introduce problems of size broadening and amorphous layers can be formed around grains. Prolonged grinding can even make the minerals completely X-ray amorphous (Okada et al., 1993; Kosanovic et al., 1993). Wet grinding generally lowers these effects, but in ball mills even wet grinding can still lead to problems of amorphization and line broadening (Okada et al, 1993).

Sample preparation by McCrone Micronizing mill before XRD/Rietveld test

By the help of Dr. Lieven MACHIELS from KuLeuven, it is reported in his doctor thesis that several authors have shown that wet grinding using a McCrone Micronizing mill is the most efficient method of reducing the particle size of most materials, while avoiding many of the deleterious effects that can be associated with mechanical grinding (O’Connor and Chang, 1986; Buhrke et al., 1998; Eberl, 2003).

Therefore, we applied the method to grind hydrated pastes to get particles smaller than 10 μm for XRD analysis. In the future, we will analyze and assess whether this method is good enough to obtain quantitative results.