Thought I would toss this out for you to look at. There are lots of pages on specific heat of liquids and solids so this should just be a starter. Specific heat is expressed as (BTU/lb per degree F)
-OR- (kJ/kg K). The higher the specific heat of a substance, the more effective that substance is at storing/holding heat.
Keep in mind the specific heat of water is very high compared to most building materials. When one looks at a traditional cast iron woodstove the specific heat of cast iron is .46 kJ/kg K. Many high end woodstove manufacturers will add soapstone into their stoves. Soapstone has a specific heat of .88 kJ/kg K which is twice a good as cast iron alone. What is interesting is that coal has a specific heat of 1.32 kJ/kg K. Water has a specific heat of 4.185 kJ/kg K.
I would think that using coal rather than salt would hold more heat, but if one can figure out how to use water it is even better. The trick with water is it must stay below the boiling point, or it will change phases and go to vapor. If that phase change occurs in a closed system you could have a steam explosion.