Calcium leaching in cement and concrete refers to the process where calcium ions are dissolved and removed from the cement paste, typically when exposed to water or other fluids. This phenomenon is particularly critical in structures exposed to aggressive environments, such as underground facilities, marine structures, or those in acidic or highly alkaline soils. The leaching of calcium can affect the durability and integrity of concrete, as it leads to a gradual loss of material cohesion and strength, potentially compromising the structural performance over time.