Well, first off, it's not an good idea to pour concrete in the hot sun. The concrete will hydrate faster and cause shrinkage cracks in the surface. If you used ready-mix concrete, the concrete supplier usually knows what additive to use, and what water-cement ratio to use, to suit the environmental conditions that day. So, they will normally adjust the mix to suit the weather.
The peeling you are describing is a function of curing too fast. I agree with what one other poster said - call the concrete supplier and tell them what happened. I don't care how poorly it was finished, the surface would not peel off if it didn't hydrate too fast. So, inspite of a poor placement job, the batching of the concrete might not have be right.
If you have lumps in the surface (which are not aggregate sticking up), it sounds like it was trowelled initially, and was not done a few more times as the concrete set. You need to trowel it more than just at the initial set. At the initial set, it is still quite pasty, so, you will get the "icing on the cake" look at locations because the concrete is still quite wet. If it was trowelled a few more times, you would have got a much smoother finish.
Anyway, if that is the case, I wouldn't get the same guy to grind down the surface - if he didn't do the trowelling and finishing properly, then I doubt he will be any good at grinding down the concrete.
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