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Facts of the Matter Richard Brill |
Concrete solidifies
reputation as
useful materialPlain, ordinary, everyday concrete is not so plain, and it is definitely not ordinary.
Though it is common, this synthetic rock is a leading candidate for the title of the most amazing and useful manmade chemical in history. If not the most useful, it is certainly the most used.
Reciting a detailed list of things that are made of concrete would be a daunting task. Buildings, roads, bridges and dams are the most obvious, but there are also concrete pipes, canals, swimming pools, ornaments, light poles, missile silos, and nuclear waste containment. There are even concrete boats and concrete Frisbees.
Concrete and cement are not the same thing, although the two terms are often used interchangeably.
Cement is glue, any kind of glue. Concrete is synthetic rock, a calcium-silicate fruitcake with sand and gravel nuts, held together by a cake called portland cement.
Concrete is as near to the perfect building material as there is. It is made from a virtually inexhaustible, self-renewing resource. Various aggregates such as lightweight vermiculite or dense scrap iron can be substituted for sand and gravel to alter its properties, and it naturally recycles itself.
Concrete is economical, and not only because it is inexpensive. It requires little maintenance, has a long lifetime, resists rot, corrosion and decay, and withstands high temperatures, wind, water, rodents and insects.
It is easy to work with, can be cast into literally any shape, and the forms can be built on site. It can be placed anywhere, even underwater, and can be easily transported, and can be mixed on site or delivered in quantity. It cures itself during normal usage as its strength increases for a decade or more.
As near to the ideal building material as concrete is, there are environmental costs. Its extraction and processing leave ugly scars, add carbon dioxide and dust to the atmosphere, and are energy intensive.
Concrete has minor disadvantages as a building material as well. It has a low tensile strength, which limits the span of concrete beams and arches, and a low shear strength that limits the height of concrete structures. It cannot be shaped easily once it has set, is susceptible to cracking and is not very strong for its weight.
Today, concrete rules. But its primacy as a building material did not mature in the modern world until the 20th century, with the innovation of steel reinforcement. Cement made from lime (anhydrous calcium oxide) has been around for a long time.
As early as 3000 B.C. Egyptians used limey mud mixed with straw to make bricks, and used mortars of lime and gypsum in the pyramids. In China around the same time, limey mud concoctions cemented bamboo boats and rock in the Great Wall.
Much later, in the first century B.C., builders in Greece and Mesopotamia used lime mortars.
Influenced by early builders, Roman engineers used a cement called pozzolana, made from the volcanic rocks and sand of Mount Vesuvius. With pozzolanic cement, they built the Appian Way and the streets of Rome, the Roman baths and the aqueducts that brought water to them, the Coliseum and the domed Pantheon.
The art and science of cement was lost after the Roman Empire fell apart in the fifth century and lay hidden until manuscripts of Pollio Vitruvius, a famous Roman engineer, were discovered in a Swiss monastery 900 years later.
The first known use of concrete in modern times was at the end of the 15th century, in a pier of the Pont de Notre Dame, a picturesque bridge in Paris.
Three hundred years later, during the American Revolution, an English engineer rediscovered that baking lime with certain other earth materials at high temperature makes a cement that is activated by water. A few years later, he baked limestone that also contained clay and produced a "dirty" lime cement that hardened under water, which he called hydraulic lime. It was the beginning of the era of concrete.
Meanwhile, experimentation increased in all areas as the first wave of science swept over the land. The 19th century began just at the time when the ancient and practiced arts of alchemy were giving way to the systematic and quantitative science of chemistry.
By the time portland cement was patented in 1824 by British bricklayer John Aspdin, several patents had already been issued for various lime-clay cements and one was issued for steel-reinforced concrete.
Portland cement is not a trade name. Aspdin named it after the high-quality building stones from the quarries at Portland, England, and the name stuck.
The portland cement made today is basically the same as Asprin's original recipe: bake finely pulverized and purified lime and clay at 1,400 degrees Fahrenheit until nearly melted, then cool it.
Near melting temperature, the lime and clay react chemically. Calcium ions from the lime abandon their monogamous union with oxygen ions and join with them in an electronic dance with silicon, aluminum, oxygen, and iron atoms from the clay.
When the mixture cools, the party is over and what remains are solid chunks of ceramic, consisting of three unique but related crystalline substances -- two distinct calcium silicates and calcium aluminate that contains trace amounts of iron. The chunks are mixed with small amounts of gypsum and ground into a fine powder. The resulting portland cement harbors a latent chemical magic that will not manifest until water is added.
When water is added to the powder, a chemical reaction called hydration begins. It is not, as is incorrectly assumed, merely the cement powder dissolving in water, nor is it just the powder absorbing water to make a slurry. It is cold stone chemical soup that jells and then solidifies.
The atoms of crystalline solids such as those in cement are in a highly ordered state, held together by an electrical field that is created by the atoms, molecules, or ions that comprise the crystal. The arrangement of the crystal lattice depends on the size, shape and the distribution of electric charges on the respective atomic particles.
Water molecules fit into some crystal lattices because they are like electrically "bumpy" oxygen atoms, but they are different enough from oxygen atoms to disrupt the electrical fields within crystals. This is why water is a good solvent.
In this case, water and rock are dissolving in each other as their atoms rearrange and redistribute themselves, creating new crystals with different lattices, releasing bond energy as heat in the process.
We observe all of this and simply call it a hydration reaction.
Hydrated crystals are common on water-laden earth, and much of the planet's water is locked up in rocks that contain hydrated minerals. The variety of the mineral hematite that NASA scientists seek on Mars is a hydrated iron oxide, and its presence would be almost certain evidence that there was once liquid water on the surface of the Red Planet.
There are actually two phases of hydration in the curing process, as there are two different unhydrated, or anhydrous calcium silicates in the portland cement that react at different rates, one very quickly and the other much more slowly.
Hydration begins when anhydrous crystals release calcium ions and water releases hydroxide ions. The mixture becomes extremely alkaline immediately and emits a large amount of heat. At some critical concentration of calcium hydroxide, crystals of hydrated calcium silicate begin to form.
As fast as anhydrous crystals release ions, hydrated crystals form and remove them, locking up water molecules in the crystal. By the time hydrated crystals have grown to colloidal size the reaction virtually stops, having run out of the faster reacting anhydrous crystals, leaving the mixture viscous and gelatinous.
There is a lag time between the two hydration phases that gives a two-hour window, during which the rock jelly can be worked and shaped before it sets. The cement hardens as the second phase begins and eventually gets rolling.
The second phase peaks about 12 hours after mixing but will continue as it cures and hardens at an ever-declining rate for another 24 hours.
The curing and hardening process slows for two reasons: The now-solid mass of hydrated crystal forms an interlocking web with few pore spaces for water to move through, and the aluminum in the few remaining anhydrous crystals retards the hydration process.
The ideal mix is exactly 30 percent water and 70 percent portland cement by weight, which provides just the right balance of water molecules, calcium, silicon, aluminum and iron atoms. This produces the strongest concrete.
If the mix is too wet, liquid water will remain in microscopic pore spaces and will weaken the concrete. On the other hand, if the mix is too dry there will not be sufficient water to complete the hydration process, and anhydrous crystals will fill the pore spaces, likewise weakening the concrete.
A mix that is best for strength is too viscous to work, so strength is sacrificed to workability by adding a small excess of water.
Various additives can alter the mix to control speed, strength, density, permeability and water requirements or to serve other special needs.
Today, steel-reinforced concrete buildings tower a quarter of a mile high. Steel-reinforced concrete dams provide flood control, create irrigation and recreational waterways and generate a significant proportion of electrical power worldwide. Pre-stressed reinforced concrete spans wider chasms and supports greater loads, giving rise to a topologist's delight of bridges and overpasses. A network of concrete and steel ribbon lines the landscape, while million-pound metal birds arrive and depart on thick slabs of concrete.
We take a mixture of rock jelly, rock chips and water and mold it into earthen fruitcake structures, creating colossal architectural and engineering sculptures that characterize 21st century landscapes and will no doubt do so for centuries to come.
Richard Brill picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be contacted by e-mail at rickb@hcc.hawaii.edu
