Accurate measurement of the particle size distribution (PSD) of a cement powder is both an important practical issue for the cement industry and a key limiting factor in on-going computational efforts to simulate the microstructure and predict the performance of cement-based materials (Bentz et al., 1999a, 1999b; Garboczi et al., 2003).Hence, the PSD is essential for the complete characterization of a cement powder and is closely linked to cement performance. Presently, the only relevant standard method is ASTM C115-96, a turbidimetric method for determining fineness (Standard Test Method for Fineness of Portland Cement by the Turbidimeter ASTM C115). This method is limited in scope, however, with a lower size detection limit of 7.5 µm. Because there are no standard procedures that adequately cover the broad particle size range associated with portland cement powder, the implementation of different measurement techniques varies widely within the industry. Cement is a problematic material with respect to the application of PSD methods. First, the size distribution itself is extremely broad, typically spanning two or three decades from the submicrometer range to 100 µm. In general, sizing techniques work best over a limited size range. The optimum range for a particular technique varies according to a number of factors, including detector sensitivity, underlying principle of measurement, and assumptions inherent in the data analysis routine. Secondly, cement particles are highly agglomerated in the dry state, and therefore must be dispersed in order to differentiate between weakly bound agglomerates and primary units. Standard protocols for dispersing cement particles before analysis are nonexistent. The degree of dispersion achieved in dry aerosol methods will likely vary depending on particle size, the geometry of the dispersing device, the residence time in the sensing zone, and the applied shear force. Similarly, wet dispersion methods are subject to variations in surface chemistry of the powders, solids concentration, the nature of the medium, and the amount of mechanical energy expended to break up agglomerates. In addition, the most dispersive medium for cement, namely water, cannot be used due to the reactive nature of the solid phase. Dispersion introduces a potentially large source of variation at the sample preparation stage. Finally, cement particles are inhomogeneous in composition and often irregular (non-spherical) in morphology. Applicable commercial techniques are designed specifically for, or work best with, homogeneous spheres. Typically, an effective spherical particle diameter is reported. The degree to which irregularity affects the results will vary by technique, and is generally not well understood or properly accounted for in most methods (Bowen, 2002; Barth and Sun, 1985; Beddow and Meloy, 1980).
The issue of how to determine if a chosen method of analysis yields the true distribution must also be addressed. Within this context, defining a "true" PSD is an integral part of the method development and validation process. Currently, no material artifact (i.e., reference material) or universally accepted measurement method exists for cement powder PSD determination. Therefore, a PSD reference material for cement should be established, but also a standardized method for dispersing and analyzing test samples must be coupled to this reference.
To address these issues, two round robin tests, sponsored by ASTM Task Group C01.25.01, were conducted. The initial round robin involved 21 participants and was nonspecific with regards to methodology (i.e., participants used their in-house methods, and no protocols or parameters were specified). Four portland cements provided by the Cement and Concrete Reference Laboratory (CCRL) were included in the test: 131, 132, 135, and 136 (numbers were assigned by CCRL). The compositions and characteristics of these cements, measured as part of the CCRL proficiency program, are provided elsewhere (Ferraris et al., 2002a). The cement powder standard reference material (SRM) 114p was also included in this test. SRM 114p is routinely used to calibrate Blaine as well as other surface area measurements. The information requested from the participants included only the measurement technique and the cumulative PSD, so a detailed understanding of the various procedures used was not possible. The principal purpose of this initial round robin was to assess the overall variability in PSD measurements across the cement industry. A total of four techniques were reported by this group of participants: laser diffraction (LAS), electrical zone sensing (EZS), X-ray gravitational sedimentation (XRS), and scanning electron microscopy (SEM). Laser diffraction, using either wet or dry dispersion methods,was by far the most frequently reported technique. The high degree of variability in the data reported from the first round robin indicated the necessity for further research and testing.
The second round robin expanded to 41 participants and included both specified and nonspecified measurement methodologies. Two portland cements provided by CCRL were included in the tests: 143 and 144. The characteristics of these cements, as measured in the CCRL proficiency program, are given elsewhere [Ferraris et al., 2002b]. SRM 114p was again included, with the specific intent of establishing a consensus reference PSD. Establishing a true analytical PSD for SRM 114p was considered to be neither practical nor fundamentally sound, given our present limited understanding of cement particle dispersion and the limitations of available analytic methods. In the second test, only three techniques were reported (LAS, EZS, and SEM), with 93% of participants using LAS. The issue of how to best disperse cement powder for PSD analysis was addressed by conducting additional studies at the National Institute of Standards and Technology (NIST),which were then incorporated into the design of the second round robin.
The purpose of this paper is then to summarize findings based on the data generated during the round robins and to summarize the various approaches available to measure the PSD of portland cement. A summary of the statistical analysis of the test results is described. The analysis of reported data is conducted in two parts. In the first part, an attempt is made to establish a consensus reference PSD based on SRM 114p. This is followed by an examination of the parameters and methodology used by the participants in order to initiate discussion on developing a standard test method for cement PSD to be submitted for ASTM consideration. The complete set of raw data collected during the round robin tests, and the accompanying statistical analyses, are available in two separate reports (Ferraris et al., 2002a, 2002b).