In the area of sample preparation, the following key information was requested where appropriate: dispersion medium, solids concentration, dispersion procedure, surfactant (if used), and type and duration of ultrasonic treatment (if used).
Each of these items should be clearly defined if a standard test is proposed to ASTM. Only a summary will be given here as the details are described in reference [Ferraris et al., 2002b].
Most LAS-W participants used alcohol as a dispersion medium: 54% used isopropanol (IPA), 31% used ethanol and 8% used methanol (percentage were rounded). However, the use of alcohol was not universal. Two participants (8%) used an aqueous medium, despite the obvious problem with hydration.
The second issue concerns the concentration of cement in the measuring cell, and the dispersion and/or dilution method used to achieve that concentration. This information is paramount because it can affect the ability to fully disperse the cement, and could lead to a user bias or increased variability in the measurements. Furthermore, solids concentration is a key parameter for any light scattering technique, due to the effects of multiple scattering when concentrations are too high andminimum signal requirements when they are low.
The majority of round robin participants prepared their cement powder suspensions in a single step (i.e., they analyzed the samples as prepared, without further dilution). In some cases, a known amount of cement was added, while in other cases the addition amount was varied to achieve a certain optical obscuration level in the cell. The optimum percentage obscuration range is predetermined by the measurement device requirements. As a result, the concentration used in the round robin varied widely and was reported explicitly by only 12 of 26 participants who used liquid dispersion. It could be concluded from these results that the most common practice is to adjust concentration in situ (i.e., with the suspending liquid in the measurement cell) based on obscuration levels. It might be difficult, therefore, to prescribe a fixed-solids concentration in a standard test method, since different instruments may require different obscuration levels. An alternative approach would be to specify the solids content for a stock concentrate, allowing some control over sample preparation, but the stock would then be diluted in situ (using the chosen suspending medium) to obtain the optimal obscuration level for a particular instrument.
Another factor influencing dispersion relates to the use of ultrasonication to break up agglomerated particles, a common practice in many industries. Round robin results show that 69% of LAS-W users employed ultrasonic treatment of cement dispersions before measurement. Of these, 63% used in-line ultrasonication provided by the instrument, while the remainder used an externally applied ultrasonic device prior to sample introduction to the measuring cell. One participant reported using both external and in-line ultrasonication, in series. The power and the duration of the ultrasonication should be compared to determine the best procedure for an ASTM standard. Unfortunately, the power cannot be explicitly compared because values are not always reported in fundamental units (i.e., Watts versus a relative % scale) nor is output power always clearly defined with respect to the device geometry and sample volume. Furthermore, the sound frequency is rarely known or reported. On the other hand, the duration of treatment was reported by all participants using ultrasonics, and it ranged from 10−300 s, with a median value of 60 s. A comparison of reported ultrasonic treatment durations and the measured D10 values5 is given in Fig. 4. There is no clear dependence of the fine fraction size on duration, nor is there any apparent correlation between duration and the occurrence of outliers. Further studies to determine the impact of ultrasonic treatment duration on dispersion of cement in alcoholic media were performed at NIST (Hackley et al., 2004). In these studies, an external ultrasonic immersion device was employed. Suspensions were prepared at a solids volume fraction of 5%in IPA. Results, summarized in Fig. 5, indicate that after an initial treatment duration of 60 s at an output power of 90 W, further treatment provided no additional dispersive benefit. From the resulting PSDs, it is clear that the initial treatment improved dispersion of the finer fraction (below 20 µm), while having no appreciable impact on the coarse fraction.
FIG. 4−Correlation of reported ultrasonic treatment duration and the dispersion of the smaller size fraction represented here by D10.
FIG. 5−Cumulative PSD of cement powder (CCRL 135) in IPA as a function of ultrasonic treatment duration.