Which material is NOT acceptable for use with moisture content from the nuclear density gauge?

The use of a nuclear density gauge for measuring moisture content relies on the interaction of gamma radiation with the materials being tested. Certain materials can interfere with the gauge's readings due to their composition or density. Concrete, while commonly used in construction, is dense and can absorb radiation in a way that reduces the accuracy of moisture measurements from a nuclear density gauge. This means that using concrete could lead to misleading results regarding moisture content, making it an unsuitable choice for such testing.

In contrast, limerock, cemented coquina, and shellrock have qualities that allow for more reliable readings when using the gauge. Therefore, the acceptance of those materials is due to their physical properties that do not adversely affect the gauge's performance.