How Does Concrete Harden?

Concrete is a mixture of cement (usually Portland cement), water, sand, and gravel. When water is added, it triggers hydration — a complex exothermic reaction that produces calcium silicate hydrate (C-S-H) gel and calcium hydroxide crystals. The C-S-H gel is the primary binding agent, forming an interlocking matrix that glues aggregates together. Concrete reaches about 70% of its design strength within 7 days and is typically considered fully cured at 28 days, though it continues to strengthen for years. Temperature and moisture conditions significantly affect curing: too-fast drying leads to cracks, while moist curing yields stronger concrete.