Particle size distributions (PSDs) of cements and mineral admixtures are critical to the properties and performance of cement-based materials, as illustrated in the case studies given below. However, a universally recognized standard method for characterizing the size of cement particles does not currently exist (Jillavenkatesa and others, 2001; Hackley and Ferraris, 2001). Standard sieve analysis works fine for most aggregates, but particle size measurement of ultrafine aggregates (diameter <75 µm) suffers from the same lack of a standard method as does cement. Consequently, implementation of different measurement methods for fine particles varies widely. In general, round-robin tests conducted to date have demonstrated a high variability among results, even between participants using instruments based on the same underlying principles. Potential sources of variability in sizing methods include adjustable instrument parameters or material property data required as input parameters 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 slightly different weighting. Thus, the "mean" particle diameter values found by different methods are expected to differ.
ASTM Subcommittee C01.25 is currently discussing the development of a standard test for cement PSD. Its approach consists of two steps: 1) define a consensus PSD curve using a standard cement such as NIST Standard Reference Material 114P, and 2) develop standard PSD test procedures.
The first step to achieve these goals was to organize round-robin tests. Two were organized within ASTM (Ferraris, 2002). The results were analyzed and the following conclusions drawn.
First, by far the most commonly used technique for characterization of cement PSD is laser diffraction, with two dispersion options: wet = the specimen is dispersed in liquid (suspension) ,and dry = the specimen is dispersed in air (aerosol). The wet technique is currently somewhat more popular than the dry technique and uses various organic solvents in which to suspend the cement particles.
Second, machine parameters used by the various laboratories are not the same and the influence of these parameters is not always clearly understood. These parameters range from the properties of the cement, such as its complex diffraction indices, to the model used to interpret the data measured (Fraunhofer or Mie) and the dispersion medium used (e.g., isopropanol,methanol, or ethanol).
Finally, the dispersion of the PSD obtained with the various methods on the same cement can be very wide, even if the reproducibility of the results by the same operator is very good. For example, Figures 10.3.4 and 10.3.5 show three different results obtained with the laser diffraction dry and wettechniques during the round-robin conducted by the VCCTL members for Cement and Concrete Reference Laboratory (CCRL) (Pielert, 2002) cements 141 and 142 (Ferraris, 2002 - Part II).
NIST is working within the VCCTL consortium and within ASTM to develop a standard test for cement PSD determination by laser diffraction. The collaboration with ASTM will yield at first a "consensus" PSD using NIST cement 114P, a cement that is normally sold to laboratories to calibrate their Blaine fineness device. The consensus PSD of this cement is being determined by around-robin test in collaboration with CCRL and ASTM. More than 200 laboratories have been asked to measure the PSD of this cement and a CCRL cement in the context of the existing CCRLproficiency sample program.
NIST will also examine, both experimentally and theoretically, the influence of various parameters on the results obtained from the round-robin tests. This fundamental study, along with the round robin results, will yield the methodology needed to standardize the measurements of cement PSDwith this technique. This will allow the VCCTL to more routinely bring cements into its database, since the cement PSD is needed as an input.
Figure 10.3.4. Laser diffraction, dry PSD.
Figure 10.3.5. Laser diffraction, wet PSD.