What is the amount of water cement ratio needed For complete hydration of cement?

For every kilogram of cement, about 0.42Kg of water is needed to fully complete hydration reactions.

However, a mix with a ratio of 0.42 may not mix thoroughly, and may not flow well enough to be placed. More water is therefore used than is technically necessary to react with cement. Water-cement ratios of 0.45 to 0.60 are more typically used. For higher-strength concrete, lower ratios are used, along with a plasticizer to increase flowability.