Concrete sidewalks are always laid in sections, with gaps between each section. For example, a drawing show three identical 2.4-m sections, the outer two of which are against immovable walls. The two identical gaps between the sections are provided so that thermal expansion will not create the thermal stress that could lead to cracks.
What is the minimum gap width necessary to account for an increase in temperature of 21 C degrees?
for 21 degrees they won't have to be very large. The source in "sources" below states that the average expansion is about 10 millionths per degree, for your 7.2m total, the expansion would be 0.000072m * 21 = 0.001512m divided by two, for the two joints (the question states the outer panels are "against" walls, if there are four joints, then each joint is proportionally less) , it is less than one millmeter.
Depending on how much water is used in the mix, the gap needed for temperature may well be less than the drying shrinkage, especially if the pour is done at the higher temperature.
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