Calibration/Prediction of Degree of Hydration and Heat Release

  

Figure 15: Experimental (data points) and model predicted (lines) degrees of hydration for cement 133, w/c=0.3 and 0.45, hydrated under both saturated (solid line) and sealed (dashed line) conditions.

To model the hydration behavior of a specific cement, a calibration is required to determine the input parameter $\beta$ governing the relationship between time and cycles, as described above. For cement 133, this calibration was performed by conducting studies of the degree of hydration vs. time achieved for two different w/c ratios (0.3 and 0.45) and two different curing conditions (saturated [28] and sealed). The degree of hydration was assessed experimentally based on the loss on ignition (non-evaporable water content) [1,2] of samples hydrated for various ages (typically 8 h and (1, 3, 7, 14, 28, and 90) days). A value of 0.225 g H₂O/g cement was used to represent the non-evaporable water content at complete hydration, to convert from loss on ignition measurements to degrees of hydration. Based on these results and execution of the hydration model, it was determined that the most appropriate value for $\beta$ in the equation: time =  x  cycles² is 0.0003. This can be compared with previously determined values of 0.0011 [1,2], 0.0017 [24], and 0.001 [10]. The new value is lower than the previous values due to the incorporation of the induction period directly into the model, which required that the dissolution rates be slowed down (parameter DISBIAS was increased from 20.0 to 30.0 in disrealnew.c) to improve the early age hydration predictions. This means that each cycle of the hydration will represent a smaller value of real time, hence the reduction in $\beta$ from about 0.001 to about 0.0003. As shown in Fig. 15 and Fig. 16, using this value for $\beta$ results in very good fits to the experimental data for degree of hydration under both saturated and sealed conditions and the heat release under sealed conditions. Experimentally, the heat release was measured using isothermal calorimetry for small sealed samples (typically containing about 125 mg of cement paste). For heat release, the model predictions are excellent between 6 h and 30 h, but do overestimate the very early age (< 6 h) heat release. The model appears to slightly underpredict the longterm hydration occuring under saturated conditions for the w/c=0.45 specimens, while providing an excellent prediction for the sealed specimens. However, results in section 6.2 will show that the model does an excellent job of predicting the strength development of mortar cubes prepared with w/c=0.485 and hydrated under saturated conditions.

  

Figure 16: Experimental (solid lines) and model-predicted (dotted lines) cumulative heat release curves for hydration of cement 133 under sealed conditions, with w/c=0.3 (left) and 0.45 (right).