There are problems that can incur when the weather is too hot. One of the biggest problems that happens is you are more likely to get crackings in the concrete, regardless of if it is walls, floors, steps or walkways. You can also get reduced durability in the concrete. And in both walls and floors it is a reduced strength that can happen when you pour concrete in hot weather.
A fourth thing that can happen is variance in the appearance of concrete, you can get concrete or the concrete will look difference. Last, you can get reduced bonding of the concrete to re-bar.
The hot weather helps concrete to cure faster but the problem is that it cures too fast. Concrete has an ideal curing temperature, so if the temps are too far below or too far above, you could have problems.
Despite this, concrete can be poured all summer long. How? First, lets look at the concrete. The material you choose to mix the concrete needs to be kept cool. That means that the aggregate, the stones, that you mix into it should be shaded from the sun. Also, the water that you use should be cold. Also, when you are pouring concrete it should be when the temperatures are dropping, not when they are rising. You could also add cold water to the concrete just before you pour it, but you have to be very careful when doing that so that it meets the standards required.
Also, an additive can be added to the concrete to slow the curing process down in hot weather. But you really need to know what you are doing, when adding additives. Also, you never want to keep the concrete truck waiting, because the concrete inside will just keep heating up.