Setting Behavior and Ettringite Nucleation

Compressive strength results for three pastes (OPC/CAC ratios 92.5/7.5 80/20 20/80) prepared at w/c ratio = 0.40 are presented in Table 1 [43] jjjg early strength development of the 92.5/7.5 and 20/80 pastes is very similar to that for pure OPC and CAC pastes. The 80/20 paste develops strength earlier than the other two pastes, and it appears to have unique strength development behavior. 

Many factors affect the strength development of OPC/CAC systems. In the OPC/CAC binaiy cement pastes, ettringite can form in the first few minutes of the hydration mainly through the following reactions  

The early strength of the binary paste systems is attributed to the above reactions. The OPC normally provides the sulfate and the CAC is 

In the 20/80 paste, appreciable strength is developed at 4 hours hydration (Table 1). SEM examination at a half hour hydration failed to identify ettringite crystals. X-ray diffraction also failed to detect early ettringite formation. There is apparently not enough sulfate available and the hydration and strength development behavior of this system are similar to those for pure CAC paste. 

The observation of a Ca(OH)2 peak at 48 hours by x-ray diffraction analysis suggests that the slow strength development in the 80/20 paste may be due to a delay in the hydration of C3S. The observation that C3S remains unhydrated until the conversion is completed suggests an incompatibility between the OPC and CAC hydration products. 

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