Neutral mixing water is desirable for concrete in any case.
A concrete with pore solution with a pH less than 6 can quickly corrode down steel reinforcement and also corrode the concrete mix. It makes the concrete excessively porous with the passage of time and the increase in porosity speeds up the corrosion process.
An alkaline mixing water facilitates the alkali-aggregate reaction (concrete cancer) which is the biggest enemy of concrete. Also, pH higher than 10 significantly corroded the steel reinforcement too.