How do additives in ice change Latent Heat of Fusion?

When using ice to keep a room (or in my case, a cooler) cold, one would want it to have a high specific heat capacity and a high latent heat of fusion, so that it would absorb a lot of energy as it melts. Basically, so that it melts as slowly as possible.



I know that adding salt to road ice will lower the melting point of the ice, making it melt faster. But what if you were to mix salt with water and then freeze it? To me it would make sense that the ice melts sooner than ice made from pure water, because of the lower melting point. However, after doing some research I've found that the opposite is true, and that salt-ice takes longer to melt than water ice.



Why does this happen? If KCl was used instead of NaCl, would that make the ice melt more slowly, or less slowly? What substances would you add to ice in order to make it melt as slowly as possible?



Thanks for the help.