Once the phase compositions, phase surface areas, and correlation functions have been determined, the same computational techniques developed for three-dimensional Portland cement particles can be used to reconstruct three-dimensional fly ash particles following a specific particle size distribution [1,6]. In fact, using digitized spheres to represent the fly ash particles should in general be more realistic than in the case of cement particles, since fly ash particles are generally much more spherical in shape than cement particles. In addition to the computer programs for performing the reconstruction based on autocorrelation analysis, two new computer programs for distributing the fly ash phases amongst the fly ash particles have been developed. The first simply distributes the phases, in their appropriate volume fractions, totally at random amongst all of the pixels initially identified as fly ash. The second achieves this random distribution amongst the fly ash particles, so that all particles are monophase, but the user-supplied appropriate phase volume fractions are nonetheless maintained. This program has been developed because it has been observed in SEM images that the fly ash particles are often monophase. Listings for these two programs (distfarand.c and distfapart.c) are provided in Appendix B of this document.
During the assignment of phases to the fly ash particles, the tricalcium aluminate contained in the fly ash is assigned a different phase ID than that used for the tricalcium aluminate present in the Portland cement. However, within the microstructural model, the two are considered to be equivalent and participate in all of the same reactions, with the same dissolution probabilities and diffusion rates. While fly ash often replaces cement on a mass basis, for use in the model, this substitution must be converted to a volume basis. A specific gravity of 2.2 is generally used for the fly ash particles for performing this conversion. When using the program genpart3d.c (listing provided in Appendix B), the fly ash and cement can follow different particle size distributions, but the user should be sure to place all of the particles in order of largest to smallest, regardless of their phase identification.