Carboaluminate phases

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. 

In cements with a sufficiently high tricalcium aluminate content the calcium carbonate of limestone may react with tlus phase, yielding calcium carboaluminate hydrate (C A CHjj). Up to about 6 wt% of calcium carbonate may be consumed in this reaction within 28 days (Ingram and Daugherty, 1992). Whether the formation of calcium carboaluminate also contributes to a strength increase of the hardened cement paste remains uncertain. 

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