Particle size distribution by LASER diffraction spectrometry: application to cementitious powders

Particle size distribution (PSD) measurements are now routinely employed to characterize cement powders. A variety of techniques are available for this purpose. However, at present, an industry standard does not exist for PSD analysis, nor do uniform methods exist for sample preparation. Two international round-robin tests sponsored by ASTM committee C01.25.01 revealed high variability in reported PSDs, even among participants using similar instruments. The round-robin studies also identified LASER diffraction spectrometry (LDS) as the most common technique used by the cement community for routine determination of PSD. Therefore, studies were conducted to identify and examine the factors that significantly influence the determination of the PSD in cement powder by LDS. Potentially, the most significant influence on variability of PSD measurement is the state of dispersion of the powder prior to analysis by LDS; dispersion efficiency will depend on factors such as solids concentration, choice of dispersion medium, and the application of chemical and/or mechanical de-agglomeration methods. Another potentially significant source of error originates from the conversion of the measured optical spectrum to a PSD, a process that requires input of optical constants (real and imaginary components of the refractive index) of the solid phase which in cement is a multi-phase particle. The current work was undertaken with the objective of improving the precision, and therefore the degree of confidence, associated with the LDS technique in its application to cement characterization, and to assess the overall measurement precision of LDS under controlled conditions. We report relevant experimental data gathered in the course of these studies, and briefly summarize each set of results pertaining to a specific influence or parameter. Cement is composed of particles of finely interground clinker and gypsum. Knowledge of the separate PSD of the two components is critical for the prediction of cement properties. A novel technique to extract the PSD of gypsum from the total cement PSD by matching optical constants will be presented.