What minerals are primarily responsible for scale formation in boilers?

Scale formation in boilers is primarily caused by minerals that are present in the water being used in the system. Calcium and magnesium are the key minerals involved in this process. When water is heated in a boiler, the solubility of these minerals decreases, leading to precipitation and the formation of scale on the boiler surfaces.

Calcium, in the form of calcium carbonate (CaCO3), and magnesium compounds can crystallize out of the water as temperature and pressure increase. This scale can significantly reduce the efficiency of heat transfer, increase fuel consumption, and even lead to damage if not managed properly.

Other minerals listed, such as sodium and potassium, while they can affect water quality, do not typically contribute to scale formation in the same manner as calcium and magnesium. Similarly, iron and aluminum don’t commonly lead to scale formation in the boiler context, and silica's role is more associated with the formation of deposits rather than traditional scale in the same way as the carbonates of calcium and magnesium. Thus, calcium and magnesium are the correct answer when considering the specific minerals responsible for scale build-up in boiler systems.