The term 'Harden, as Concrete' can be solved by the three letter word 'SET'. When concrete is exposed to air and water, it begins to harden as a result of a chemical reaction called hydration. This process causes the cement particles to bind together and form a solid mass. The rate of hardening depends on the amount of water used in the mix, the temperature, and the humidity. The process of hardening can be accelerated by adding accelerators to the mix or by increasing the temperature.