Hydration
The chemical reaction between cement and water that causes concrete to harden and gain strength
Hydration is the chemical reaction between cement and water that causes concrete to harden and gain strength. Unlike drying (which weakens concrete), hydration is an exothermic chemical process that permanently transforms cement paste into a strong, stone-like material.
Why It Matters
Understanding hydration explains why concrete needs water to cure, not to dry. Many DIY failures stem from letting concrete dry out too quickly, stopping hydration and leaving weak, dusty concrete. Proper curing maintains water availability for hydration to continue, developing maximum strength.
The first 7 days of hydration are critical—concrete gains approximately 70% of its 28-day strength during this period. Inadequate moisture during this time permanently reduces final strength.
Technical Details
The chemistry: Cement compounds (primarily tricalcium silicate and dicalcium silicate) react with water to form calcium silicate hydrate (C-S-H) gel—the primary strength-giving component of concrete—plus calcium hydroxide. This isn't drying; it's crystal formation creating an interlocking network.
Hydration stages:
- Initial reaction (0-45 min): Rapid reaction on cement particle surfaces. Heat generation begins.
- Dormant period (45 min-3 hr): Reaction slows. Concrete remains workable.
- Acceleration (3-8 hr): Major heat generation. Concrete stiffens and gains early strength.
- Deceleration (8-24 hr): Reaction continues but slows. Most setting occurs.
- Diffusion (days-months): Slow, ongoing strength gain as water diffuses into unreacted cement.
Heat of hydration: The reaction is exothermic (generates heat). Mass concrete (large pours like dams) can reach internal temperatures of 160°F+, requiring special cooling measures to prevent thermal cracking. Residential work rarely has this problem.
Water requirements: Complete hydration requires about 0.25 w/c ratio by weight. Concrete typically uses 0.40-0.60 w/c; the "extra" water creates workability but also capillary pores (reducing strength). Some unreacted cement always remains in properly proportioned mixes.
Temperature effects:
- Cold weather (below 50°F): Slows hydration, extends set time, requires longer curing
- Hot weather (above 85°F): Accelerates hydration, reduces working time, increases cracking risk
- Freezing: Stops hydration; ice formation disrupts developing structure
Related Terms
- Curing - Maintaining conditions for hydration to continue
- Cement - The material that undergoes hydration
- Heat of Hydration - Temperature rise from the reaction
Learn More
- How Long Does Concrete Take to Cure? - Hydration timeline and strength development
- Concrete Basics - Understanding concrete chemistry
- Concrete Calculator - Plan your project