LIGHTWEIGHT AGGREGATES 



337 



significance. Cinders from coal-fired furnaces and 

 boilers were used throughout the country for the 

 production of cinder block from the late 1800's 

 through World War II; conversion to gas- and oil- 

 fired installations and to the use of pulverized coal 

 has effectively removed the source of the material, 

 and it now constitutes a very small resource. Fly 

 ash and bottom ash, the residues of pulverized-coal 

 combustion, are used as aggregates and pozzolans 

 and will supply an increasing amount over the next 

 several years to the aggregate market. Expanded 

 clay and shale aggregates in lightweight concrete 

 were used as early as World War I in concrete ships, 

 and there was a small market (small, partly owing 

 to patent restrictions) for the lightweight aggre- 

 gates through the end of World War II. A rapid in- 

 crease in number of plants and production of ex- 

 panded clay and shales began in about 1947 (fig. 40) 

 with the expiration of patents on the rotary kiln 

 (Hayde) process. Data before 1953 are incomplete 

 but suggest that production tripled from 1947 to 

 1953 and tripled again between 1953 and 1956. Pro- 

 duction and consumption of "pumice" and expanded 

 iron blast-furnace slag increased very rapidly in 

 the period 1945-50 (fig. 40), as a part of the post- 

 war construction boom. Since 1956 overall production 

 of the structural lightweight aggregates has re- 

 flected construction trends fairly closely, although 

 some deviations from trend in production of pumice 

 and a long-term decline in production of expanded 

 slag have occurred. 



A fairly steady increase (with some minor fluc- 

 tuations) in the total amount of pumice produced 

 since 1957 conceals sharp periodic (1-2 years) 

 variations in the amounts classed as "volcanic 

 cinder" (fig. 40) ; inconsistency in classification 

 probably causes the variation, rather than actual 

 selective production. Deviations in trend between 

 total pumice and pumice used as concrete aggregate 

 and admixture are real, however, and reflect the 

 responsiveness of the lightweight concrete-block 

 market to construction activity in the residential 

 and light industrial fields. Since 1953 the produc- 

 tion of all pumice has more than doubled, whereas 



its use in concrete has increased more slowly and 

 has virtually leveled off since 1958-59, fluctuating 

 about 10 percent around the million-ton-per-year 

 level. During the same period use of pumice as 

 railroad ballast decreased, but its use for road con- 

 struction very markedly increased. 



Expanded slag production peaked between 1955 

 and 1958, and it has slowly declined since then, 

 hovering at about 2-2 14 million tons per year. Sig- 

 nificant short-term decreases reflect both market 

 conditions, as in 1961, 1968, and 1970, and non- 

 market influences, as in the prolonged steel strike 

 of 1959, which cut off supplies of molten slag. Over- 

 all, the decline represents a loss of markets to 

 competitive products, particularly the expanded and 

 sintered clays, shales, and slates. Sources of slag 

 predominantly are iron blast furnaces, which are in 

 only a few areas of the country. Suitable sources 

 of clay, shale, and slate are widespread ; new facili- 

 ties have been brought into production to compete 

 with slag in market areas close to the furnaces, as 

 well as in the peripheral areas where slag use a 

 marginal operation. Usage of fly ash in some of 

 the slag-production areas has also contributed to 

 the decline. 



In common with other extractive industries that 

 produce materials at the earth's surface, the light- 

 weight-aggregate industry is faced with the environ- 

 mental problems inherent in open-cast mining, in 

 the control of fine materials formed in the manu- 

 facturing process, in the destructive consumption 

 of water in some processes, and in the degradation 

 of water quality in other processes. None of these 

 problems is insuperable; all involve extra expense 

 in production, in a highly competitive market with 

 commonly slender profit margins. As low-unit-value 

 high-place-value commodities, raw-material sources 

 and production sites have to be as close together 

 as possible, and both have to be as centrally located 

 to the market area as possible. Consumption of 

 structural aggregates is mostly in urban areas; 

 environmental impacts and conflicts are increased 

 because they take place close to where many people 

 live. One further complication; in some areas the 



Clay, shale, slate. — Total production of kilned and sintered aggregate: 194&-55, J. W. Roberts, Solite Corp., oral commun., 

 Feb. 14, 1960; 1956-70, U.S. Bur. Mines Minerals Yearbooks, and A. L. Bush, U.S. Geol. Survey. 



Slag. — Expanded blast-furnace slag, does not include zinc and phosphate smelter slags; U.S. Bur. Mines Minerals Year- 

 books. 



Pumice. — Total tonnage of pumice and pumicite, 1940-70, and of scoria and volcanic cinder, 1953-70; U.S. Bur. Mines 

 Minerals Yearbooks. 



Pumice in concrete. — Tonnage used as concrete admixture and additive, excludes scoria and volcanic cinder before 1953; 

 U.S. Bur. Mines Minerals Yearbooks. 



Pumice without cinder. — Tonnage of pumice and pumicite, exclusive of scoria and volcanic cinder, 1953-70; U.S. Bur. Mines 

 Minerals Yearbooks. 



