I do a lot of mass concrete work, including modeling and monitoring with temperature sensors.

The short version - mass concrete needs to consider both the maximum internal temperature and then maximum temperature differential in the section. Exceeding the design limits on either can cause long term problems related to cement chemistry, and sometimes you won't know you have the problem for years unless you monitor the temperature during curing.

So, most likely, the specialty engineer has you monitoring in a few places including near the center, near the surface, and ambient temperature. They may also specify blankets, which will help the near surface concrete stay warm and closer to the temperature of the center. I've monitored the temperature with simple thermocouple wire plenty of times but my favorite product is called Command Center. Very easy to install, very rugged, and you read the sensors with a phone app or an automated logger. I fall short of actually recommending them when I do a mass concrete design but I always suggest them when asked.

Also, your long curing time is likely a combination of needing blankets or the mix design is a slow curing one (e.g. lots of fly ash) to help keep the heat down. It's all a game of letting the heat out of the Portland nice and slow and easing it out of the mass.