Thermal Analysis of Hydrated Calcium Aluminate Cements

2 Thermal Analysis of Hydrated Calcium Aluminate Cements 

Differential thermal analysis (DTA), differential scanning calorimetry (DSC), and differential thermogravimetric analysis (DTG) have been used to provide estimates of the degree of conversion in CAC concretes. 

Carboaluminate phases (e.g., C4ACH1 j) can form when hydration takes place in normal atmospheres. The presence of this phase is indicated by a DTA peak in the region 180-200°C, which can overlap with the peak due to the presence of C2AHg. A shoulder may be present for the phase present in the lesser amount. 

The presence of AH3 can be detected by DTA showing a strong endothermic peak at about 270 to 300°C and (when large amounts form) an additional endothermal peak at temperatures up to 240°C. 

Triple DTA peaks have been observed in the region 27Q-340°C with Secar 71 hydrated at 40°C and 50°C (and other Secar systems). Thermograms are shown in Fig. 5. The peak at the lower temperature (290-300°C)can be attributed to AH3, andthatat31 l-325°CtoC3AH. An extra phase (attributed to the third peak) was not detected by XRD suggesting that this phase was amorphous or poorly crystalline. There is doubt, however, with this explanation given that the additional peak occurs at a higher temperature than that observed for crystalline AH3, contrary to expectation. 

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