Hydro-power

General Overview

Obviously the most important material for the construction of a dam is concrete. Concrete is made by mixing cement with water, sand, and gravel. Cement is the essential ingredient of concrete and is what makes it harden to form an almost rock-like material. Cement is one of the most important building materials known to man. In essence, the use of cement consists of pulverizing and heating rocks, such as gypsum and limestone, to drive off associated water and carbon dioxide, then adding back water to form rock again, only this time in whatever shape one wishes. Cement is used not only in concrete but also in other indispensable building materials such as mortar and grout.

The two most common forms of cement are those made from heating gypsum or calcium carbonate. Gypsum is crystalline CaSO₄ 2H₂O. When heated, a portion of the water is driven off. When water is added again the crystalline material is reformed and hardens. More common is the cement obtained from heating limestone, CaCO₃. At a temperature of about 900° degrees Celsius, CO₂ is driven off, leaving a substance known as "quicklime", CaO. This process is known as "calcining". When water is added, an exothermic reaction occurs forming (Ca(OH)₂). The long hydroxide chains that form make it hard and rocklike The two reactions taking place in this process are:

Calcining: CaCO_{3 (s)} + heat --> CO_{2 (g)} + CaO_(s) (Endothermic)

And: CaO_(s) + H₂0_(l) --> (Ca(OH)₂) _(s) + heat
(Exothermic)

Fig. 3 Limestone	Fig. 4 Gypsum

History

These types of cements were invented centuries ago, and most likely accidentally. In Asia, mud was mixed with straw as an important building material as much as 6,000 years ago. The Egyptians also used this technique, but of more importance, they burnt gypsum and used it as a mortar in their constructions such as the pyramids. Gypsum is actually named after the Egyptians. The first known use of calcining occurred about 2, 450 B.C.E. The Greeks burned limestone, which they mixed with other substances to form cement that was actually capable of hardening underwater, or "hydraulic". This is largely attributed to a certain kind of earth that they mixed in, which was found on a Greek Island called "Santorini". But the Greeks never put this discovery to wide use.

The Romans were also capable of manufacturing hydraulic cement, knowledge that they most likely acquired from the Greeks, but unlike the Greeks, they used it extensively in their aqueducts and other building projects. The Romans made hydraulic cement by mixing lime with the volcanic ash. The silica present in this volcanic ash combined with the (Ca(OH)₂) to give the cement its hydraulic properties. This cement is known as "pozzolana cement". The Romans were particularly successful with cement because of their meticulous control of the ingredients, the preparation and the usage. But with the fall of the Roman Empire and the advent of the Middle Ages, the "art" of cement making was largely lost, and would not be rediscovered for centuries.

Fig. 5 Roman Aqueduct

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