IPA: /ˈhɑːrdənɪŋ/
KK: /ˈhɑrdənɪŋ/
The process of becoming firm or solid, often used in reference to materials or substances that change from a soft to a hard state.
The hardening of the concrete took several hours before it was strong enough to support the structure.
Hardening → It is formed from "hard" (from Old English "heard", meaning solid or firm) and "-en" (a suffix used to form verbs indicating to make or become). The word "hardening" means the process of making something solid or firm.
Think of something becoming more 'solid' ('hard') — that's why hardening means to make something hard.