What is physical and chemical drying?

Physical and chemical drying are the two mechanisms by which building materials dry or harden. In physical drying the water simply evaporates from the material, while in chemical drying the moisture is bound to other substances through a reaction.

How it works

In physical drying, the water present leaves the material through evaporation, without any chemical reaction taking place. Examples include air-drying paint, lime mortar that slowly dries out and plaster that dries on the surface. The material becomes stronger as less water remains, but its composition does not change.

In chemical drying, the moisture is instead chemically bound to other substances within the material. The material hardens through a reaction. Well-known examples are the hydration of cement and concrete, and two-component adhesives or paints that cure because their components react with one another. Here it is not evaporation that matters, but the chemical conversion.

Application

The distinction is important for estimating curing times and processing conditions. Physically drying materials need good ventilation and low humidity to dry quickly. Chemically hardening materials such as concrete, on the other hand, often need sufficient moisture for the reaction to complete; drying out too fast can reduce the final strength. Knowing which mechanism applies helps you judge when a surface is load-bearing or ready to be painted over.

Related terms

• Hydration
• Binder
• Mortar
• Primer

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