The Daily Grind At Lehigh

How Cement -- Not Concrete -- Is Made

October 20, 1991|By Kerry O'Rourke | Kerry O'Rourke,Staff writer

Concrete and cement: Same thing, right?

No, no, no, no, says David H. Roush, plant manager at the Lehigh Portland Cement Co., shaking his head emphatically.

"It's sort of a mission I have to proselytize the world" about the difference between concrete and cement, he said.

It may not seemimportant to you, but Roush, who has spent his career making cement,cringes when people misuse the words.

Cement is what they make atLehigh. It's an ingredient -- a dry, powdery one -- in concrete. Cement is the glue, or bonding agent, in concrete, which is used to makesidewalks, highways, roads and bridges.

Remember that if you evermeet Roush at a cocktail party.

Making cement is a grueling, hot,dirty and -- most of all -- dusty process that requires the right mix of ingredients and the proper chemical reaction.

Lehigh makes Portland cement, developed in 1824 and so named because it resembled the color of stone quarried in Portland on the English coast.

The first step in making cement at Lehigh is digging limestone and shale from a quarry adjacent to the plant.

The quarry, 250 feet deep, is 1,000 feet wide and about a mile long, or 85 acres. Fifty-ton trucks carry 18 to 20 loads of boulders up from the quarry every hour on weekdays.

The trucks dump the stone onto a conveyor belt to crushers that break the stone into pieces as small as one inch.

The limestone, shale and other raw materials -- sand and iron ore -- then are ground into a fine powder in mills that can grind 240 tons an hour.

The raw materials and fuel then are blown into four rotary kilns, each400 feet long and 11 1/2 feet in diameter. The kilns -- long, steel tubes lined with bricks -- are tilted slightly. The raw materials areblown in at the higher end and the fuel -- ground coal and waste oil-- at the lower.

The raw materials travel down the length of the kiln over a two-hour period. The temperature reaches 2,700 degrees Fahrenheit and the raw materials are transformed into clinker -- round,rough, dark gray lumps that when ground with gypsum make cement. Gypsum, a white and gray rock, acts as a retardant so the concrete won'tset too quickly, Roush said.

It takes 1 2/3 tons of raw material to make 1 ton of cement, he said.

Throughout the process, laboratory workers test the quality of the rock, ground raw materials, clinker and finished cement, Roush said.

Cement at Lehigh and plants throughout the country must conform with standards set by the American Society for Testing and Materials.

In Union Bridge, Lehigh produces1 million tons of cement a year, Roush said; 50,000 tons are sold inbags, and the rest is shipped out by truck or rail.

The bags are sold to retail and wholesale outlets and to large masonry contractors, and the bulk cement is sent to companies that make concrete and concrete products, Roush said.

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