In the LAS technique, the angular distribution of light scattered from a dilute particle dispersion is measured. To be precise, light can be scattered, diffracted, or absorbed by the dispersed particles (Bohren et al., 1983). Scattered light consists of reflected and refracted waves and depends on the form, size, and composition of the particles. Diffracted light arises from edge phenomena and is dependent only on the geometric shadow created by each particle: diffraction is independent of the composition of the particles. Absorption occurs when light is converted to heat or electrical energy by interaction with the particles, and is influenced by both size and composition. The so-called laser diffraction technique incorporates all three of these effects, but is generally limited to the more forward scattering angles. The key material parameter for LAS is the complex refractive index, m = n − ik, where n is the real component and k is the imaginary (absorptive) component. Scattering arises due to differences in the refractive index of the particle and the surrounding medium (and internal variations in the case of heterogeneous particles). Values of n have been published for many bulk materials (see for example Handbook of Optical Constants of Solids, 1985, 1991), but in the case of cement, n is routinely estimated based on a mass average of the refractive indices for the individual material components (Cyr et al., 2000). Absorption becomes important primarily in the fine fraction, especially below 1 µm. Cement is generally gray to off-white in color, and therefore a finite, but relatively low value for the imaginary component is expected. The value k = 0.1 is often reported for cement, although the origin of this value is unclear and its appropriateness for general use has not been established.
There are two principal methods of data analysis for LAS: Mie and Fraunhofer. Mie theory describes scattering by homogeneous spheres of arbitrary size and is the most rigorous optical scattering model available. For non-spherical particles, Mie provides a volume-weighted equivalent spherical diameter. Mie theory has been applied with mixed success to the analysis of powders with diameters from several hundreds of micrometers down to about 100 nm. An accurate representation of the "true" size distribution by Mie scattering is dependent on the input of an accurate value for the complex refractive index. For particles much larger than the wavelength of light, the Fraunhofer method can be used without knowledge of the refractive index, because it is based on the diffraction effect only. The range of validity for Fraunhofer is limited at the fine end to diameters a few times greater than the wavelength of light, denoted by λ, for particles that are opaque or have a large refractive index contrast with the medium (ISO 13320- 1:1999(E)). For more transparent particles, or particles with a moderate refraction contrast, the lower limit is raised to about 40 x λ. For λ = 633 nm (red light), this corresponds to about 25 µm. The benefit of using Fraunhofer diffraction is that the interpretation is not dependent on the absorptive or refractive properties of the material. On the other hand, use of the Fraunhofer approximation beyond the valid range can lead to large systematic errors in the calculated PSD (ISO 13320-1:1999(E)).
The LAS method requires that the particles be in a dispersed state, either in liquid (suspension) or in air (aerosol). The former is presently referred to as the "wet" method (LAS-W) whereas the latter is termed the "dry" method (LAS-D). Differences between LAS-D and LAS-W methods arise primarily from the different ways in which the particles are dispersed in each case.
In liquid, it is possible to modify solution conditions by changing pH or adding chemical dispersing agents, and one can disrupt aggregates using mechanical or ultrasonic energy. Thus, for the very fine fraction, a better state of dispersion can be achieved in an appropriately selected liquid medium. Generally, water is an excellent dispersing medium. However, due to the reactive nature of cement in water, alcohols, such as isopropanol, methanol, and ethanol, are commonly used in its place.
In the LAS-D method, a stream of compressed air (or a vacuum) is used to both disperse the particles and to transport them to the sensing zone. This method of dispersion works best for the coarse size fraction, where the interparticle contacts are weak. For particles smaller than a micrometer, or highly asymmetric particles, air dispersion is generally not appropriate for sizing.