The solution to this is to stop thinking about how things would work according to mundane physics and think how they would work according to Supernal magic. For something like the spell to change how concrete holds/transmits heat, I'd compare what a spell of that level would be capable of to the existing spell 'Alter Conductivity'. It would increase/decrease the amount of heat said section of concrete could hold/transmit by 2 per point of potency, limited by whatever direct form of heat you, the ST, decide it is directly connected to, advanced math not needed. If you decide that from a magic standpoint...