In a water-tube boiler, waterside scale formation is caused by __________.

Correct Answer: Option C — calcium sulfate

Calcium sulfate exhibits an inverse solubility curve, decreasing rapidly as temperature increases. In high-pressure water-tube boilers complying with Title 46 CFR Subchapter F chemistry controls, this property causes calcium sulfate to precipitate directly onto heated tube walls, forming hard, highly insulating waterside scale.