Answer by Sumanth Reddy:
Cement is inherently alkaline and this results in an alkaline pore solution in concrete. It is quite alkaline if you think about it. It could reach as high a pH as 13.5 – 13.7.
The reason for this particular difference in ready mixed and hand mixed concrete could be due to the quantity of cement in the mix. I do not think any of the admixtures would have a significant effect on the alkalinity of concrete.
The temperature due to mixing will not play a dominant role since the rate of mixing is not high enough to generate additional heat and as for the heat of hydration, it relates to the quantity of cement again.
It would be interesting to know things like exposure time, temperature during operation, concrete strength and purpose of concrete (application) to consider other factors that could have influenced the observation.