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Heat of Hydration

The heats of hydration of the two cements were assessed using a multichambered microcalorimeter constructed at NIST. 28 A known mass of cement and several small stainless-steel balls to facilitate mixing were placed in a seated calorimeter cell that was then equilibrated in the main calorimeter chamber. After a steady heat flux signal was obtained, the cell was removed, the appropriate mass of water (also thermally equilibrated to the calorimeter temperature) quickly added using a syringe, and hand mixing performed (by shaking the cell) before restoring the cell to the calorimeter chamber. The voltage signals produced (proportional to heat flux) by the calorimeter cells were digitized using a PC-based high-resolution analog-to-digital (A/D) data acquisition system. Thus, during the initial hydration, data could be taken at 30 s intervals. Once the reactions slowed, data were typically acquired every 5 or 10 min over a period of at least 7 d. At longer times, the signal of the calorimeter was very close to its background level, so that detection of the slow but ongoing hydration became unreliable. In analyzing the heat release data, the initial exothermic "mixing" peak was ignored because of the necessity of removing the sample cell from the calorimeter chamber to assure adequate mixing. This was considered to be the major contribution to the uncertainty in the heat of hydration measurements and could result in a difference in the cumulative heat released over a period of 7 d on the order of 10 kJ/kg or ~4%, as estimated from samples mixed in situ in the calorimeter. Because of the mixing difficulties at low w/c ratios, calorimetric measurements were performed only at the two higher w/c ratios of 0.40 and 0.45.

Figure 3 provides a sample plot of the obtained signal for Cement 116 at 25ºC for w/c = 0.40 for the first 24 h of data acquisition. This signal versus time was then numerically integrated to obtain the cumulative heat release (kJ/(kg of cement)) versus time curves, which are presented in the results.

Fig. 3. Experimental heat release signal versus time.


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