Because the round robin results suggest that a large majority within the cement industry (over 90% of participants) use some form of LAS, it would seem logical to focus initial efforts on the development of a standard method (or best practice) for cement powder sizing based on this technique.
To establish such a standard, the procedures used by industry to measure PSD were examined by analyzing the information that was provided by the round robin participants. Although there is a wide range of methods used to prepare and analyze test samples, some trends could be identified. Additional experiments at NIST were used to examine the impact of key parameters related to the use of LAS techniques (ultrasonication and the index of refraction have been previously discussed). A combination of industry practice and supplemental experiments could be used as the basis for a draft standard method.
LAS-W is subject to the greatest amount of variation due to sample preparation. In LAS-W, the following parameters could be established based on the results of these studies: suspending medium-IPA; solids concentration-fixed stock suspension; obscuration level-modulated at dilution.
Based on measurements performed at NIST using several common alcohols (data not shown), IPA appears to be the best choice. Although the measured PSD varied very little between methanol, ethanol, and isopropanol in these experiments, the latter medium offers a viscosity advantage that could reduce variability arising from the sedimentation of large particles during sampling or measurement. For example, at 20 ºC the viscosity of IPA (η = 2.41) is roughly twice that of ethanol (1.20) and nearly four times higher than methanol (0.597). A number of surface-active agents were also tested to determine if an improvement in dispersion could be achieved in alcoholic media. No consistent or clear advantage could be observed for the addition of common commercial surfactants.
In regards to solids concentration, this parameter could be controlled in two ways. First, the solids content in the test suspension could be fixed, and therefore a single solids loading would apply to all cement formulations, users, and instruments. Second the optimal obscuration value as registered by the instrument would determine the final solids concentration after dilution from a stock concentrate. In the first case, the sample would be introduced to the instrument as prepared. In the second case, a concentrated stock suspension would be prepared at a fixed solids loading by all users and followed up by obscuration-based dilution into the pure suspending liquid preloaded into the instrument circulation system. The former method offers complete control over the sample preparation procedure, which is a large potential source of error, but optimal measurement concentrations may vary between cement formulations and instruments. The latter method offers less control over sample preparation, but is more flexible, broadly applicable, and may be a good compromise (prescriptive versus performance based methodology).
On the other hand, a better understanding of the influence of the following parameters on the results should be investigated further: 1) Refractive index used in Mie model and 2) Duration and intensity of ultrasonication.
Because there were fewer parameters to select for the LAS-D method, the standardization should be less complicated. The use of compressed air versus vacuum for powder dispersion is a factor determined by the manufacturer and not the user, and thus cannot be standardized. The pressure used in compressed air systems can be varied somewhat, but the possible influence of this factor on the measured PSD of cement requires further study. As the size of the finest fraction of particles cannot be accurately determined using Fraunhofer ([ISO 13320-1:1999(E)]), and the refractive indices of cement are not accurately known for cement, the authors suggest that both models be included in a standard or that a lower size limit be set for reporting PSDs. Further research to determine appropriate refractive index values for cement, in the case of Mie analysis, should be conducted or a consensus complex refractive index should be set.
Therefore, there is a strong argument for ASTM committee C01.25.01 and NIST to make an investment in time and effort to develop a standard test method to measure the PSD of cement powder using LAS (both wet and dry). From this report, several parameters could already be narrowed or fixed. A small task group could conceivably help define the next set of specifications to be recommended for a future round robin as part of the standards development process.