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How does incomplete hydration affect the strength of concrete?

The hydrated product of the cement compound in a grain of cement adheres firmly to the unhydrated core of the grains of cement. This means that unhydrated cement left in a grain of cement will not reduce the strength of cement mortar or concrete, as long as the products of hydration are well compacted.

Abrams obtained strength of 280 MPa using mixes with a w/c ratio of only 0.08. At such a low w/c ratio, only surface hydration of cement grains would have taken place & a considerable portion of cement grains must have remained in unhydrated state. Abrams could obtain the strength as he applied tremendous pressure to obtain proper compaction of such a mixture.