Heating principle of iron powder used for heat pack materials

The mechanism for generating heat is that the iron reacts with the oxygen in the air to generate heat in the salt solution.
Among them salt and water belong to medium, carrier;
Activated carbon reaction bed, because the inside of activated carbon evenly distributed large pore, can make the iron powder in the mixture reaction speed even and the role of the catalyst;
Vermiculite is an insulating material designed to slow the heat of the insulation.
"You can generate heat just by tearing it apart, but why not react when you need it, and not react when you need it?"
The reaction that generates heat requires oxygen in the air, which insulates the iron powder from the thermal insulation to prevent it from reacting.