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Web phones, rock-hard concrete

Q: Can Internet phones call non-Internet phones? If so, how do they do it? (Shirley, Panama City, Florida)

Information from ZDnet, How Internet calling works [Corel Corporation icons]

A: Yes. With an Internet phone, we can call anyone in the world that has a phone — extremely cheaply or even free.

The basic procedure is simple. See figure. Plug your current phone into a gateway box (a phone adapter and router) that the Internet phone company provides. Plug the gateway box into your broadband cable, satellite, or DSL modem to connect to the Internet. Pick up the phone, hear a dial tone, press the phone number into the phone’s keypad, let it ring, and then talk when the other person comes on the "line." It seems like a normal phone call.

Sometimes, the connection is outstandingly clear with beautiful high fidelity — much better than we can ever get on a regular phone. Other times, when the Internet is crowded, we endure annoying lags in our conversation — something like the "buffering" lags when trying to watch video over the Internet.

How it works. An Internet phone service uses the Internet (instead of phone lines) to send voice, much like an Internet Service Provider uses the web to send our emails. Here’s the event sequence:

1. We pick up the phone and talk.
2. The gateway box converts our voice audio waves to digital data (0s and 1s).  The gateway formats the digital data into data packets.
3. The cable or DSL modem hands the packets to the
4. Internet for routing over the web.
5. The packets arrive at the Internet phone service provider, who then converts the data signal to a phone (radio) signal and passes the signal to the public phone network.
6. The public phone network makes the final connection, as usual. The other guy’s phone rings.

That’s it. Internet phones can talk to any phone and we send the voice across the Internet like we send email.

Further Reading:

Vonage Internet phone service, product reviews

ZDNet: Internet phones by Rafe Needleman

Q: My kids and I just poured some steps with concrete. They would like to know how concrete gets so hard and how cement is made. (George, Greeley, Colorado)

Concrete-hard steps [Brickform Rafco Products]

A: Concrete is a mixture of cement, water, sand, and gravel. The cement and water combine to make a paste. We mix the paste with the sand and gravel to coat the individual gravel pieces and sand grains and to fill the gaps in between.

Then, the cement and water combine chemically in such a way that the whole thing sticks together and the concrete hardens. We understand the process empirically — after all, we’ve been using Portland cement for over 175 years and more primitive cement for 5000 years. But, the exact chemistry that takes place is complex and not fully understood.

Basically, the cement reacts with water in a process called hydration. "During this reaction, the cement and water combine to form fibrous crystals which grow and expand until they interlink with crystals from other cement particles or attach to nearby sand grains or gravel chunks," says Terry Collins, a concrete construction engineer. As the crystals form connections, the concrete stiffens, hardens, and gains strength.

The crystallization process is somewhat similar to how water freezes and ice crystals form.

Before the mixture gets too stiff, we pour the concrete into forms or slabs. Then we smooth it and let it cure for a few days. Curing involves keeping the concrete moist enough and at the right temperature for, typically, 3 to 7 days. This allows hydration to continue and, therefore, the concrete to get harder. The stiffening occurs in a couple of "sets." The initial set (when it begins to stiffen) takes about ½ to 3 hours and the final set (too stiff to change with finishing tools) about 1 to 6 hours.

But that’s not the end of it. Concrete gains most of its strength in 28 days, with typical strength gain of about 45% in 3 days, 75% in 7 days, and 100% of design strength in 28 days. "Then, with adequate moisture and temperature, it gains strength slowly for decades," says Collins. "Temperature and moisture are big players in the actual strength — at any time." High temperatures accelerate and low temperatures retard strength gain. Too dry conditions stop strength gain.

That’s how concrete gets hard. Now, your second question — how we make cement.

We mix calcium, silica, alumina, and iron together. The calcium comes from limestone or chalk. The silica, alumina, and iron come from sand, clay, and iron ore. We stick the mixture in a kiln and heat it to — 2550 to 2800̊F (1400̊ to 1550̊C) — hot enough so the calcium and the silica interact chemically to form calcium silicates that emerge from the kiln as ball-shaped pellets about 0.75 inches (2 cm) in diameter.

These small grayish black pellets are called clinker. We cool the clinker, add a bit of gypsum to control setting time, and crush the mixture into a fine powder. Ta, ta! Portland cement.

By the way, in 3000 B.C., ancient Egyptians made a concrete somewhat like our modern cement. They used lime and gypsum to make this primitive cement and built the pyramids with the concoction.

Further Reading:

Portland Cement Association: Cement & concrete basics

WonderQuest: Concrete in the pyramids and the coliseum

Washington State Department of Transportation: Portland Cement

(Answered Dec. 3, 2004)