Why is water glass called an "air-hardening cementitious material"?

Water glass is referred to as an "air-hardening cementitious material" because:

1. It meets the definition of a cementitious material: It can harden from a liquid paste into a solid state and bind other materials together.

2. Its hardening process is essentially a physico-chemical process, the core of which is the precipitation and coagulation of silica gel.

3. This hardening process strongly relies on carbon dioxide (CO₂) in the air to trigger and complete the key chemical reaction.

4. It can only harden and gain strength in air. It cannot harden effectively or achieves very low strength in water or environments isolated from air.

Therefore, the term "air-hardening" precisely summarizes the fundamental characteristic of water glass: its dependence on air (specifically the CO₂ within it) for hardening. This property also determines its applications (primarily for acid-resistant engineering, soil stabilization, fireproof coatings, etc., in above-ground or dry environments) and its limitations (it cannot be used underwater or in constantly damp environments).