The "stone" walls are actually a mix of cement powder and crushed beer bottle or windshield glass.

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That information revealed the two February tests by Halliburton of a slightly different cement slurry mix that showed it was unstable, the letter said.

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Between its 14 cement plants, hundreds of ready-mix concrete plants and quarries, and distribution stations in the U.S., Cemex employs about 15, 000 American workers.

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As part of the investigation, the letter said, Chevron conducted independent tests of similar cement slurry materials supplied by Halliburton and found the mix to be unstable.

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