Hydration Kinetics

Figure 5 shows the model predictions and experimental results for degree of hydration under saturated and sealed conditions for cement 135. The VCCTL does an excellent job of fitting the experimental data and in matching the observed differences between saturated and sealed curing observed at longer hydration times. Early in the hydration process, excess water is readily available, so that the water lost due to self-desiccation in the sealed specimens does not have a major influence on hydration kinetics. As the specimen ages, the amount of self-desiccated (empty) porosity becomes significant, and the hydration rate of the sealed specimens trails behind those of the corresponding saturated specimens. While not a large difference is observed for the w/c=0.4 samples investigated in this study, for lower w/c ratios (now often being employed in high-performance concretes) sealed conditions can lead to a substantial reduction in the degree of hydration (and the strength) achieved at long hydration times (e.g., > 7 d).

  

Figure 5: Computer model (lines) and experimental results (data points) for degree of hydration of CCRL cement 135, w/c=0.40, hydrated under saturated and sealed curing conditions at 25 ºC. Crosses indicate ± one standard deviation in experimental measurements, and generally fall within the boundaries of the data point symbol itself. Model cycle to actual hydration time conversion factor was 0.0003.

Figure 6 shows the equivalent plot for hydration under saturated conditions only for cement 141. While this hydration was performed at a lower temperature of 20 ºC (assuming an activation energy of 40 kJ/mole for the cement hydration reactions), reasonable agreement between model and experiment is still observed, as noted previously [9]. For these two cements, the conversion factor between hydration cycles and real time is nearly a constant (0.00030 for cement 135 and 0.00035 for cement 141). These values are in good agreement with other conversion factors determined previously for CCRL cements 133 (0.00030) [10] and 136 (0.00036) [12].

  

Figure 6: Computer model (lines) and experimental results (data points) for degree of hydration of CCRL cement 141, w/c=0.40, hydrated under saturated conditions at 20 ºC. Crosses indicate ± one standard deviation in experimental measurements, and generally fall within the boundaries of the data point symbol itself. Model cycle to actual hydration time conversion factor was 0.00035 (at 25 ºC).