Dissolution Kinetics of Cubic Tricalcium Aluminate Measured by Digital Holographic Microscopy

In situ digital holographic microscopy
is used to characterize the dissolution flux of polycrystalline cubic
tricalcium aluminate (C3A-c). The surface dissolves at
rates that vary considerably with time and spatial location. This
implies a statistical distribution of fluxes, but an approximately
steady-state median rate was obtained by using flowing solutions and
by reducing the water activity in the solution. The dissolution flux
from highly crystalline C3A-c depends on the water activity
raised to an empirically derived exponent of 5.2 and extrapolates
to a median flux of – 2.1 μmol m–2 s–1 in pure water with an interquartile range of 3.2
μmol m–2 s–1. The flux from
a less crystalline source of C3A-c has an empirical water
activity exponent of 4.6 and an extrapolated median flux of only −1.4
μmol m–2 s–1 in pure water
with an interquartile range of 1.9 μmol m–2 s–1. These data suggest that the bulk dissolution
rate of C3A-c can vary by at least 30% from one source
to another and that variability in the local rate within a single
material is even greater because of the heterogeneous spatial distribution
of structural characteristics (i.e., degree of crystallinity, chemical
impurities, and defects).