As I understand it, your water heater is supplying hot water for heating the slab in an open loop configuration. Because that same hot water is used for bathing, laundry, dishwashing etc, fresh water will be regularly entering the water heater to replace the hot water as it is consumed. When the water feeding the WH is softened, the WH will be filled with soft water, including the water circulating through your heating system loop.
A water softener removes calcium and magnesium minerals which were dissolved into the water when the water was in the ground in contact with rock. Once those hardness minerals have been removed, they will continue to be removed so there will be no benefit for the water to reenter the softener. Although there will be some initial mineral scale within the WH and piping, the softened water will slowly dissolve that scale back into the water, for it to be eliminated through regular hot water use.
With no new scale being formed and with the existing scale being dissolved and eliminated, your hot water system will eventually become scale free. How long that will take will be dependant on the amount of scale now present.
There are some industrial applications where hot water must be softened. In those situations, specialty tanks, valves etc will be utilized as even a relatively small amount of pressurzed hot water can damage a fiberglass media tank as evidenced in post #5 & #14 at the link below.
https://terrylove.com/forums/index.php?threads/funny-and-interesting-pictures.88495/