To illustrate the predictive capabilities of the VCCTL, two cements distributed by the Cement and Concrete Reference Laboratory (CCRL) were utilized [4,5]. Twice each year, the CCRL sends out proficiency samples of two cements for evaluation by laboratories throughout North America and 22 other countries. Proficiency samples of CCRL cements 135 and 141 were obtained in January of 2000 and June of 2001, respectively. The samples were provided double-sealed in plastic bags in cardboard boxes. Samples of the two cement powders were subsequently analyzed with respect to particle size distribution (PSD) and by scanning electron microscopy (SEM) and X-ray microanalysis. This characterization provides the inputs needed by the VCCTL for the detailed simulation of hydration and microstructure development.