In addition to temperature, another material environmental condition
variable of interest in hydration studies is the moisture content of the
cement paste. Because cement hydration is generally viewed as a
dissolution-precipitation process, the availability of water in the capillary
pore space is paramount. As indicated by the chemical shrinkage measurements
presented above, in a sealed system, empty pores are created as the hydration
proceeds. This in turn would be expected to affect the kinetics of the
hydration because of changes in solution concentrations and the reduction in
available volume into which reaction products can precipitate. Modeling of
this behavior is especially important for high-performance concretes, which
often are based on mixture proportions with w/c
0.35, because of the expected reduction in
hydration (and
strength) relative to saturated curing, and because the menisci created as the
pores empty induce drying (self-desiccation) shrinkage stresses within the
microstructure
66, 67
even before the material's strength is fully developed.
To preliminarily test the capability of the NIST cement hydration model to effectively reproduce the effects of self- desiccation on hydration kinetics, the model was executed for
Cement 115 with w/c = 0.30
under conditions in which no external water was available, so that empty pores
were created as the hydration proceeded as described in the Computational
Techniques section. In addition, experimental measurements were made under
both saturated and sealed conditions. Figure 17
provides a comparison of the experimental measurements and model predictions
for degree of hydration versus time for these two systems. Model cycles were
converted to time using values of B = 0.0017 and t0, = 6.7. Once again, the model was found to
reproduce the experimentally observed difference in hydration kinetics due to
the self-desiccating conditions. Early in the hydration process, the kinetics
were not influenced strongly because sufficient water was present and few
empty pores existed. However, as hydration continued, the empty pores occupied
an ever-increasing fraction of the remaining total porosity, resulting in a
significant decrease in achieved degree of hydration. This is shown clearly in
Fig. 18, which compares two-dimensional slices from
the same z-plane after 5000 cycles of hydration under both saturated
and sealed conditions. Substituting the 90 d value of 
into Eqs. (8) and
(9), one finds a reduction in predicted
90 d compressive strength from 102 to 83 MPa, a reduction of ~20%,
emphasizing the importance of the proper curing of low w/c ratio
concretes. If an accurate model could be developed for the drying kinetics of
a hydrating cement paste, it should be possible to extend the cement
hydration model to consider hydration of cement paste exposed to different
external relative humidities, in addition to the saturated and sealed
conditions explored in this study.
Fig. 17. Predicted and measured degrees of hydration for Cement 115 with w/c = 0.30 hydrated under sealed and saturated conditions at 25ºC.
Fig. 18. Comparison of hydrated microstructures for hydration under (a-bottom) saturated and (b-top) sealed conditions (C3 S is red, C2S is aqua, C 3A is green, C4AF is orange, gypsum is pale green, C-S-H is yellow, other hydration products are magenta, water-filled porosity is blue, and empty porosity is black).