The hydration of cement generates heat. If the rate of heat produced in hydration is greater than the loss of heat from the concrete surfaces, the concrete will expand. On the other hand, if the rate is lower there will be an overall contraction. From the generation of the difference in temperatures within the slab, tensile stresses can form between the cooler, exterior part of the slab and the interior part of the slab, in which shrinkage occurs slower, causing it to crack.
Some ways to reduce the temperature gradient in very thick slabs:
This process can also be referred to as early age thermal cracking.