What causes the deterioration of gypsum board when exposed to heat?

The deterioration of gypsum board when exposed to heat is primarily caused by the evaporation of calcium sulfate. Gypsum board, or drywall, is composed mainly of calcium sulfate dihydrate (CaSO4·2H2O). When subjected to elevated temperatures, particularly in a fire, the water molecules in the gypsum begin to evaporate, leading to the loss of its structural integrity. As the temperature rises, the hydration begins to break down, resulting in the transformation of gypsum into anhydrite (CaSO4), which is a less stable form.

This process directly undermines the board's fire-resistive properties, as the loss of moisture weakens the material and can lead to crumbling or disintegration. Understanding this mechanism is critical for fire safety and building material assessment, particularly in fire scenarios where the integrity of structural materials is vital for maintaining safety and stability.