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Bulletin 88—Sources for lightweight shale aggregate in New Mexico

By R. W. Foster, 1966, 86 pp, 4 tables, 22 figs., 1 index.

This report is the result of studies on the shales of NM for their possible use as a lightweight aggregate in concrete. Of the 42 shales sampled and tested, four are considered suitable for this purpose and six others may be suitable but require additional testing. Shales that might be used are restricted to the Devonian, Pennsylvanian, Triassic, Cretaceous, and Tertiary Systems.

Limited testing of Devonian shales in central NM indicates that the carbonate content of these rocks is too high for proper expansion. In most sections examined, Pennsylvanian shales are intimately interbedded with other rock types and low-cost surface mining methods cannot be used. In addition, most of the Pennsylvanian shales tested have very short expansion ranges and would be difficult to fire in a rotary kiln. However, two possibly useful deposits were discovered in rock of this age, one near Abo Pass, the other in the Mud Springs Mountains. Red shales of Triassic are were not investigated.

The Cretaceous System contains numerous thick shales that crop out in many parts of the state. Large deposits with good expansion properties occur in the Pierre Shale of northeastern NM. Other thick intervals of shale present in the northeast melt at relatively low temperatures and either have short expansion ranges or are too refractory. Possible sources in the Mancos Shale of northwestern and central NM include the Mount Powell and Carthage areas. Additional Cretaceous shales that look promising occur in the Fruitland Formation at the Navajo Coal mine, in the Menefee Formation near Chaco Canyon National Monument, and in the Lewis Shale near Dulce. Considerable sampling would have to be done to evaluate fully the potential of the Cretaceous section. Tertiary shales possibly suitable for expanded aggregate are restricted for the most part to the San Juan and Raton Basins. Limited sampling in the Raton area failed to reveal any suitable deposits.

All samples were tested initially by flask firing at 2,100ºF for 15 minutes. After examination to determine the degree of expansion, favorable shales were tested further by firing at temperatures between 1,800–2,200ºF at increments of 100ºF and retention times of 5, 10, and 15 minutes. Each locality sampled is described in the report, and bulk specific gravity, weight in pounds per ft3, absorption %, and internal structure are discussed for the more favorable deposits.

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