Introduction

Cement can be a problematic material in the application of particle size analysis. First, the size distribution itself is extremely broad, extending in most cases over two to three orders of magnitude, from about 100 µm down to below micrometer size. In general, sizing techniques work best over a limited size range. The optimum range of particle size analysis varies according to a number of factors, including detector sensitivity and the assumptions associated with the underlying principle of measurement. Second, cement particles are highly agglomerated in the dry state, and therefore must be properly dispersed in order to determine the "true" particle size distribution (PSD). Standard protocols for dispersing cement particles prior to analysis by wet methods are nonexistent, and the degree of dispersion achieved in dry dispersion (air) methods will likely vary depending on the method used, the geometry of the dispersing device, the residence time in the sensing zone and the applied shear force employed to separate physically agglomerated particles. This introduces a potentially large source of variation at the sample preparation stage. Third, cement particles are typically irregular in shape and inhomogeneous in composition. Most commercial methods (see Table 1) are designed specifically for, or work best with, homogeneous spheres. The degree to which irregularity affects the results vary with technique, and is generally not well understood or accounted for properly n in many methods.

A universally recognized standard method for characterizing the complete PSD of cement particles does not currently exist [2]. The only standard test, ASTM C115 [1] (also known as the Wagner test), is really designed to measure the "fineness" of a cement powder and it is limited to a minimum particle size of 7.5 µm. As there is no standard procedure covering the whole range of cement PSD, the implementation of different measurement methods varies widely within the industry. Therefore, there is the necessity of having a method in which cement is used as a standard. Other potential sources of variability in sizing methods include adjustable instrument parameters or material property data required as inputs, and fundamental differences due to the nature of the technique itself. In the latter case, it must be acknowledged that different methods may "sense" a different aspect of the size distribution. For instance, a given method may be sensitive to either particle mass, particle number or projected surface area. As a result, for a polydisperse system, each method produces a distribution with a slightly different weighting. Thus the "mean" particle diameter values are expected to differ.

ASTM committee C01.25.01 sponsored a round-robin test to measure the PSD of cement. The scope of this report is to analyze the data generated during those tests and to summarize various methodologies available. The analysis of the data is conducted in two parts:

• Attempt to establish a reference distribution using a standard cement (SRM 114p)
• Discussion of the data obtained during round robin tests as an initial step toward development of a standard tests method for submission to ASTM.