Concrete and cement are not the same thing, although the two terms are 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.

When patented in 1824 by British bricklayer John Aspdin, several patents had already been issued for various lime-clay.

Aspdin named his 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 Aspdin’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 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, two distinct calcium silicates and a calcium aluminate with 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!

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.

There are actually two phases of hydration in the curing process, as there are two different 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 up. 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% water and 70% Portland cement by weight, which provides just the right balance of water molecules, calcium, silicon, aluminum and iron atoms. This produces the strongest concrete.

Various additives can alter the mix to control speed, strength, density, permeability and water requirements or to serve other special needs.

We take the mixture of rock jelly, rock chips and water and mold it into colossal, earthen fruitcake structures, creating architectural and engineering sculptures that characterize 21st-century landscapes and will no doubt do so for centuries to come.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.