There is no direct correlation between the compressive strength of concrete and its corresponding shrinkage. In fact, high strength concrete tends to have higher shrinkage due to the higher content of cement used in those types of concrete. Fly ash is able to reduce the shrinkage in concrete as a result of its dilution effect on cement and the slow pozzolanic reaction.

The decrease in compressive strength you mentioned after 10% replacement of cement with fly ash is due to the slow pozzolanic reaction of the fly ash. Also, these decrease in compressive strength after 10% are mostly observed at early ages (i.e. 7 days and 28 days). When long term investigations (i.e. after months or years) are carried out, the compressive strength of concrete incorporating higher content of fly ash should be the same as the control due to the formation of hydration and pozzolanic reaction with time.

Adeyemi Adesina thank you