As mentioned previously, a database of stored cement particle shapes and phase distributions may be used along with the NIST microstructure model to simulate the development of microstructure in an interfacial zone in concrete. Figure 6 shows both the original random configuration of cement particles around an aggregate (purple) for a system with a w/c or 0.45 and the same system after hydration for 150 cycles using the microstructure model. In the hydrated image, C-S-H gel is yellow, CH and iron hydroxide (FH3) are dark and bright blue respectively, and aluminate hydration products (AFt, C3 AH6, etc.) are green. Since the cement particles do not pack efficiently in the vicinity of the aggregate, the interfacial or transition zone contains less cement and more water-filled porosity, thus having a higher local w/c ratio. This local inhomogeneity will affect both the hydration process and the resultant final microstructure.
The hydrated paste microstructure in Figure 6 appears to be different near the aggregate than the bulk paste microstructure far away from the aggregate. To quantify this effect, Figure 7 shows a plot of the phase fractions as a function of distance from the aggregate surface for results averaged over five separate configurations of initial microstructure like the one shown in Figure 6. Even after 63% hydration, there is still a large increase in porosity as the aggregate surface is approached. In this region, there is also a deficiency of anhydrous cement, C-S-H, and FH 3. Conversely, the more mobile sulfate, calcium, and aluminate species lead to an increase in the CH, AFt, AFm, and C3AH6 in the area next to the aggregate. Since there is initially more porosity in this area and these mobile species tend to migrate throughout all available porosity, there is ultimately a larger volume of these hydration products formed near the aggregate than in the bulk paste. These computer model results are in agreement with numerous experimental observations [ 11-15].
Figure 6. Original and hydrated images showing interfacial zone microstructure. Global w/c is 0.45. White bars at top of images indicate extent (30 pixels) of interfacial zones.
Figure 7. Phase distribution as functions of distance from aggregate surface for hydrated interfacial zone microstructure shown in Figure 6.