True, but nobody I know makes a cake out of concrete
If you mix the ingredients and leave them in a cake tin on the work surface, will you end up with a cake?
No. The reaction only occurs because you heat the mixture to temperatures in excess of the boiling point of water. If all the ingredients reacted and made a cake before heating, there would be no need to invest energy in the reaction, would there?
[edit] Actually it occurs that concrete and water have an exothermic reaction, whereas baking a cake is endothermic. They aren't really comparable. [/edit]
Not necessarily. You're proposing there that all the water turns to steam, when in fact that has not been claimed. What has been claimed is that the water boils, which it most certainly does. Some of it does indeed turn into steam, and escapes into the oven.
I don't need to experiment with this; I can simply recall all the occasions where I have checked on the progress of a cake, loaf or similar in the oven.