As was mentioned earlier, there is evidence that the initial precipitation of ettringite is as a gel which transforms into needle shaped crystals due to the nucleation process modelled by Billingham and Coveney (1993). Since random aggregation generally forms isotropic structures, we have developed special rules to ensure that ettringite grows as needles (Kuzel and Pöllmann 1991). Two approaches have been applied to date.
One approach is to label the head of a growing ettringite 'crystal' as a special state and only allow diffusing ettringite species to aggregate onto the head of the crystal, or to make the probability of aggregating on the head higher than on other sites. Thus, three states are used to represent ettringite: diffusing, solid, and solid at the head of a crystal. When further aggregation occurs at the head of the crystal, the new solid ettringite pixel is labelled as the head of the ettringite crystal and the former head is simply labelled as solid ettringite. Figure 6 shows a 3D visualization of the single-pixel-thickness needles that can be generated.
A second approach is to utilize a surface curvature measurement algorithm originally developed for simulating the sintering of ceramic powders (Bentz et al 1992, Pimentia et al 1992). By counting the number of pore space pixel neighbours, a measure of local curvature can be obtained (Bentz et al 1992). By evaluating the local curvature at prospective ettringite growth sites, ettringite crystal growth can be biased towards pixels with a high local curvature, causing needlelike structures to form. The amount of bias can be used to control the variability allowed in the morphology of the ettringite crystals, with no bias corresponding to the totally random growth used for other 'crystalline' hydration products such as CH and FH3. Planar ettringite crystals could be produced in a similar fashion by specifying an orientation and appropriately biasing the aggregation process.