Hydration

Hydration is the chemical reaction between cement and water that binds aggregate into a solid mass and drives concrete's set and strength gain. Hydration generates heat, releases calcium hydroxide (part of why washout water runs highly alkaline), and continues for weeks after a pour — though most of a mix's design strength is reached within the first 28 days.

Hot weather speeds hydration and can cause a mix to set too quickly; cold weather slows it and can stall strength gain entirely if the concrete freezes before curing.