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Model Results

Some general results of the microstructure model are presented before proceeding to the comparison of model hydration rate, heat release, and chemical shrinkage to the experimental data. Results can be summarized conveniently by plotting the phase volume fraction versus the number of elapsed dissolution cycles for each phase present in the model. Typical results are illustrated in Figs. 9, 10 and 11, which provide a series of graphs for Cement 116 at w/c = 0.40. For the anhydrous phases (C3 S, etc.), the phase fractions decrease monotonically with cycles, but at rates proportional to the assigned dissolution probabilities of the phase (i.e., C3S and C3 A react at higher rates than C2 S and C4AF). Similarly, as shown in Fig. 10, porosity decreases monotonically with cycles. C-S-H, CH, and FH3, increase monotonically with cycles.

Fig. 9. Model anhydrous cement volume fractions versus elapsed cycles for Cement 116 with w/c = 0.40.

Fig. 10. Model porosity and reaction product volume fractions versus elapsed cycles for Cement 116 with w/c = 0.40.

Fig. 11. Model aluminate reaction product volume fractions versus elapsed cycles for Cement 116 with w/c = 0.40.

The behavior of the aluminate hydration products is more complex, as shown in Fig. 11, for the first 800 cycles of hydration. Here, while gypsum remains in the system at a significant level (>10% of its initial volume), mostly ettringite (and a little C3 AH6) is formed from the reaction of the aluminate phases with gypsum. When the gypsum is nearly consumed, the formation of the monosulfoaluminate phase (Afm) begins, and the supply of ettringite is gradually depleted, while more C3 ,AH6, continues to form. The initiation of monosulfoaluminate formation prior to the complete depletion of gypsum is consistent with recent experimental results on the pure aluminate phases.54 The shapes of the curves for the ettringite buildup and decay and the monosulfoaluminate buildup are quite similar to those found in the literature, 46 as measured using XRD on pastes in which the dissolution of the ferrite phase has been specially activated. The persistence of ettringite at long times also is consistent with the recent synchrotron radiation-energy-dispersive diffraction measurements of Henderson et al., 55 who measured ettringite contents on the order of 7% after 326 d of hydration for a w/c = 0.5. This value is larger than those predicted by the model in the present study, which could be due in part to the lower sulfate content of the cements (~1.6% SO3, versus the 2.7%-2.9% in Table 1) used in the experimental study. 55

In Fig. 11, the ettringite peaks to a maximum volume fraction at ~60-70 cycles. Later results present the calibration of model cycles against experimental time; such results indicate that 60-70 cycles corresponds to ~12-15 h of real time for these cements. This is a reasonable time for the conversion of ettringite to monosulfoaluminate to begin, as indicated by a secondary peak in calorimetry measurements. 26 Such a shoulder (peak) on the heat release curve can be clearly observed for the heat release signal curve for Cement 116 in Fig. 3 (occurring at ~750 min). However, some researchers 56 have suggested that this secondary heat peak is associated with the renewed formation of ettringite and not the conversion of ettringite to monosulfoaluminate. It should be recognized that model parameters (gypsum and aluminate dissolution rates) can be adjusted to obtain this depletion of gypsum at any specific time. Here, the relative agreement with conventional experimental observations is rather fortuitous, because no specific attempt has been made to achieve this gypsum depletion at a specific time. Rather, the relative dissolution probabilities of the phases have been set a priori at reasonable values based on data in the literatures26


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