Area Fractions and Phase Perimeters

Table 1 shows the area and perimeter phase fractions for two different fields of view for the cement shown in Figure 3. It should be noted that the perimeter fractions will be a stronger function of image resolution than the area fractions. In both cases, the bulk area fractions are quite similar to the volume fractions computed using the Bogue equations [9 ]. Since most of the larger silicate particles are C₃ S, the C₃S particles generally have a lower surface area to volume (or perimeter to area) ratio than the C₂S particles. Thus, the C ₂S occupies a larger fraction of the phase perimeter than its area fraction. The C₂S generally reacts at a slower rate than the C₃ S suggesting that this cement might hydrate at a slower rate than a cement with the same bulk phase fractions but a proportional distribution of the two silicates on its surfaces. Since the gypsum is generally present as smaller discrete particles, it too occupies a larger perimeter fraction than its area fraction.

Scrivener has computed the area and surface fractions of silicates and interstitial phases and in general found that the interstitial phases occupied a larger fraction of the surfaces than their area fraction [2]. For this particular cement, we actually observe the opposite trend, as the interstitial phases (ferrite and aluminate) occupy a smaller perimeter fraction than their area fraction. This can be directly observed in Figure 3, where many of the C ₃A regions are part of much larger polymineralic particles and have much of their perimeter in contact with C₃S instead of porosity.