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Cementing a Bond That Strengthens With Time

November 19, 1994|From Associated Press

Have you ever wondered why concrete gets so hard?

Concrete consists of portland cement mixed with sand and gravel or crushed stone. Portland cement cements the sand and gravel or crushed stone together.

The cement is made of calcium (usually finely ground limestone), silica (sand), aluminum, iron ore and small amounts of other materials (sometimes fly ash, for instance). These materials are quarried, crushed, ground and blended. Then they are fired in a kiln with an average temperature of about 2,700 degrees--hot enough to melt steel.

The material emerges from the kiln as marble-size pellets known as clinkers. These are ground into a fine powder, and this is bagged or delivered in bulk as portland cement.

The beauty of portland cement is that, to form a rocklike mass, all you do is mix it with sand, gravel or crushed stone and then add some water. The chemical properties of portland cement are such that it reacts with water in a process known as hydration.

Here's how hydration works: Water and cement particles form a solution. To the naked eye, this solution appears as a paste that coats each grain of sand and each piece of gravel or stone.

If you could see the paste at a molecular level, you would notice that water molecules arrange themselves around each particle of cement (each particle is a fraction of a millimeter in diameter).

In the process, heat is generated as the molecules rub against each other and arrange themselves. Excess water molecules make their way to the slab surface and evaporate. Other water molecules are locked out of the hydration process by the bonds formed by the hydrating cement particles.

It takes decades for this water to escape, and this is why concrete gets harder as it gets older. The water that was involved in the hydration process remains behind indefinitely as a crystal known as water of hydration.

Under ideal circumstances, concrete takes its own sweet time arranging these cement-water-of-hydration particles. By the time the hydration process is essentially complete (it takes several weeks), you have a well-organized mass of sand and stone bonded in a matrix of cement-water-of-hydration particles.

Because this bonding takes place at a molecular level, it's extremely thorough and very strong. And, as a bonus, the concrete grows stronger with age.

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