Watery concrete is already a bad situation let alone adding the 105+ degree days. I have worked with ACT Asphalt Specialties
(who does concrete too) and they make sure to schedule the projects around the weather, not the other way around. Temperature can make a great impact on the finished product especially working in Minnesota. Good luck with ReadyMix, Its great they provided you with all the information. Hopefully enough to take the matter to someone who will fix it, and make it right at no additional cost to you. Thanks for sharing.