silicates are not sand. silica is sand. silicates are mineral salts.
After flushing my cooling system i had LOTS of sand/silicate/salt, or whatever you want to call it at the bottom of my drain pan. It covered the entire bottom of the pan in a pretty thick layer.
I agree that clean metal will conduct better, that is why i only use distiled water, water wetter and a bit of the orange stuff.
Incedentally, coolant is a horrible heat conducter compared to pure water. Not only is its thermal conductivity bad but it also has a much lower specific heat capacity, meaning it absorbs and holds much less heat energy as it circulates. This means that it can't possibly transfer as much heat into the radiater.
The bottom line is that the only thing "coolant" is good for is raising the boiling point and lowering the freezing point of the real coolant- water.