Summary of the participant's procedures for sample preparation

In the area of sample preparation, the following key information was requested where appropriate:

• Dispersion medium
• Concentration used and how dilution was achieved
• Surfactant
• Type and duration of ultrasonic treatment

Each of these issues should be clearly defined when a standard test is proposed to ASTM.

Table 12 shows a comparison of the reported medium used for dispersion of the cement powder in a liquid prior to and during analysis. Over 50 % of the participants used isopropyl alcohol (IPA). There were only two participants who used a non-alcoholic medium (i.e., water-based). One of the participants (#206) used water with no added surfactant and no ultrasonication. Participant #450 used water, with no added surfactant, but ultrasonication of 20 W for 60 s.

Previously (see Section 3.1.2), results #206 and #450 were both identified as outliers by the statistical analysis. Given the propensity for reaction of cement powder with water, this is not a unexpected result. It is possible that the PSD could change during the course of the measurement due to the hydration of cement, or that the reactive cement particles cling to the optical cell walls and thereby influence the precision and accuracy of the results.

The second issue concerns the concentration of cement in the measuring cell and the dispersion method. This information is paramount because it can affect the capability to correctly disperse the cement and, therefore, could lead to a bias or increased variability in the measurements. It is clear from Table 13 that the vast majority of participants prepared their cement powder suspensions in one step (i.e., without dilution from a stock concentrate). 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 optimal percentage obscuration range was predetermined by the manufacturer of the device. As can be seen in Figure 5, the concentration varies widely and is reported explicitly by only 12 participants out of 26 who used liquid dispersion. It could be concluded from these results that the most common practice is to adjust concentration based on obscuration. It might therefore be difficult to prescribe a fixed solids concentration for a standard test method, since different instruments may require different obscuration levels. An alternative route would be to specify the solids content for a stock concentrate, which would be used to control sample dispersion properties. The stock could then be diluted as needed to obtain the optimal obscuration level for a particular instrument. Existing standards outside the U.S. should also be closely examined to determine the best method.

Only one participant (#736) used a surfactant during sample preparation. The medium used in this case was ethanol and the surfactant was SrCl₂ at a dose of 0.06g/L. No conclusions can be drawn from this lone test.

Figure 5: Concentration of the dispersion in the cell for each participant. The values on top of the bars represent values that are off the scale selected.

Another factor is how the dispersion was further prepared by using ultrasonication. Analysis results show that 69 % employed ultrasonic treatment to disperse cement suspensions prior to measurement. Of these, 63 % used the on-line ultrasonication provided by the instrument, while the remainder used an externally applied ultrasonic treatment prior to the introduction of the sample to the device. One participant (V2) reported using both external and in-line ultrasonication.

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 clearly 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 ultrasonic geometry and sample volume. In some cases the frequency is reported and in others it is not. Table 14 summarizes the available information. On the other hand, the duration is always reported and this is shown in Figure 6. The values range from 10 s to 300 s, with a median value of 60 s. It does not seem that there is a correlation between the duration and whether the distribution contains outliers. Further studies to determine the impact of ultrasonic treatment duration and power output on dispersion of cement in alcoholic media are ongoing at NIST. Preliminary results indicate that after an initial treatment duration (<120 s), further ultrasonic treatment provides no additional particle dispersion in alcoholic media.

Figure 6: Distribution of the ultrasonication duration for all applicable participants.