What is whitewash made of?

Lime and water. That is very nearly all of it.

Limestone is burned, then slaked with water to make a soft white paste. Thinned down, that paste becomes whitewash.

What happens next is the interesting part. As the coat dries it pulls carbon dioxide out of the air, and the lime slowly turns back into calcium carbonate. The wall ends up wearing a thin skin of limestone.

Worth knowing
This is the lime cycle, and it closes. Stone is burned, mixed with water, brushed onto a wall, and becomes stone again. Indian walls were whitewashed before the festival season every year, which kept the surface breathing and let damp escape instead of trapping it behind paint.

Source: Indian National Trust for Art and Cultural Heritage.