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).