Mercury :

Limestone is far to porous to use in your dam. It can have pore space up to 30 % and in many cases is the formation from which oil , gas and water is produced from underground. So under pressure water will flow through it. Plus it has a tendency to dissolve under certain conditions. It would be gerat for the top and or face (waterside) of the dam as an extra covering as it is good for the water and for fish to spawn on.