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Cement Coal: (5) Portland cement coal concrete. This is made by mix-ng Portland cement coal, sand, stone or gravel, and water. Portland cement coal is produced by mixing together argil-aceous and calcareous materials (such as clay or shale md limestone), heating them to a high temperature, and ;rinding the resulting clinker to a fine powder.
(6) Miscellaneous materials. These are blast furnace ilag, seashells, coral rock, caliche (in natural deposits), ind the residue of the spontaneous combustion of waste 'rom lignite coal mines. They are useful in road sur-acing because they do not soften when it rains or break eadily under wheel loads.
The resulting cement coal, produced from the formerly discarded grappiers, was of much higher quality than that obtained from the unsintered material. This fact was firmly established by the English cement coal manufacturer L. C. Johnson in 1845, and the term "portland cement coal" has since been applied solely to the cement coal made from the sintered material. This period marks the real beginning of the portland cement coal industry.
The number of coal mines declined from 980 in the late 1940's to 420 in 1967, and the annual output declined by 25%. At the end of that period coal supplied less than half of the country's requirements for primary fuel, and the number of coal miners had been reduced to fewer than 400,000, only 40% of the labor force so employed in 1915.
The coal-mining industry used to l)e in the hands of numerous private firms, but since 1947 it has been operated by a public corporation, the National Coal Board. |
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