LIGHTWEIGHT AGGREGATES 



335 



share of the market for aggregates. Pumice-pumicite 

 and diatomite claim the remaining third in about 

 a 10:1 ratio. The production data for pumice, pumi- 

 cite, scoria, and volcanic cinders are so commingled 

 and the terms are applied so unspecifically in regard 

 to uses that only the broadest guess can be made; 

 the ratio for pumice-pumicite may be low by a factor 

 of 3. 



All in all, about 13.5 million short tons of rock 

 was used in 1970 for structural lightweight aggre- 

 gates, and perhaps as much as one-half million short 

 tons was used for ultralightweight aggregates for 

 non-load-bearing concretes, plaster insulation board, 

 and loose fill insulation. Their values after processing 

 for final use were about $71 million and $14 million, 

 respectively. As these materials also have other 

 applications, their annual production value for all 

 uses was somewhat greater for the structural light- 

 weights, about $74 million, and much greater for 

 the ultralightweights, about $78 million; we have 

 an overall value, then, of more than $150 million 

 per year. 



The natural subdivision of the lightweight aggre- 

 gates into those used for structural purposes and 

 those used in non-load-bearing applications will be 

 followed in this chapter. 



STRUCTURAL UGHTWEIGHT AGGREGATES 

 INTRODUCTION 



Despite their varied origin, the aggregates suit- 

 able for structural purposes have several character- 

 istics in common: they are porous, ideally highly 

 so; they are permeable, ideally poorly so; and the 

 pore walls are relatively thick, as compared with 

 the diameter of the voids. This last characteristic 

 gives the aggregates fairly high compressive 

 strengths. The porosity of all aggregates results 

 from heat treatment — natural (volcanic) heat for 

 the scoria; intentional rotary kilning or horizontal 

 furnace sintering for the expanded clays, shales, 

 and slates; and unavoidable heat in blast furnaces 

 for the slags and in coal-fired steam power plants 

 for the fly ash. 



Because of their porus but fairly impermeable 

 texture, the lightweight aggregates are both ther- 

 mal and acoustic insulators. These properties make 

 the aggregates desirable for bulk use in floor and 

 roof fill, and they are an added advantage in con- 

 cretes where the primary reason for the use of these 

 aggregates is weight reduction. 



Compressive strength that can be obtained for 

 concrete made with these aggregates ranges from 

 perhaps 1,200 psi (pounds per square inch) to more 



than 6,000 psi, depending upon the proportions of 

 sand, cement, aggregate, and water. Higher strength 

 and increased weight go hand in hand ; for example, 

 a scoria aggregate concrete at 1,200 psi weighs 

 about 60 lb per cu ft, whereas a sintered expanded 

 shale concrete at about 6,000 psi weighs about 120 

 lb per cu ft. In this broad range of strengths, the 

 lightweight aggregate concretes can be used for fill, 

 bridge decks, shell walls, concrete fioors, concrete 

 frames using precast, prestressed, or posttensioned 

 girders, precast tilt-up units, poured barge and ship 

 hulls, and lightweight masonry blocks. In bulk form 

 the aggregates are used as railroad ballast, as high- 

 way and embankment fill, and in the surface course 

 of bituminous paving mixtures. 



Structural lightweight aggregates and concretes 

 play an important though not indispensable role in 

 urbanized, industrialized nations. They enhance effi- 

 ciency — they allow buildings to be made higher, 

 their steel framework to be more fire resistant, and 

 their floors and ceilings to be better insulants. They 

 contribute to more economical use of funds, ma- 

 terials, and time, inasmuch as concrete can be 

 pumped higher and farther from a single location; 

 plates, girders, and shell walls can be mass produced 

 offsite; and additional thermal and acoustic insula- 

 tion can be reduced. On the esthetic level, the light- 

 weight aggregates and concretes allow innovative, 

 experimental construction and design. 



The United States is by far the largest producer 

 and consumer of structural lightweight aggregates. 

 Inasmuch as domestic supplies are abundant and 

 the materials have low unit value but high place 

 value — that is, transportation costs largely govern 

 whether the materials can be used — there are vir- 

 tually no import or export markets. To all intents, 

 the country is self-sufficient in these commodities. 

 Production data for pumice reveal some imports 

 used for lightweight block, but in 1970 less than 

 400,000 tons was imported for all purposes, as com- 

 pared with more than 8 million tons of domestic 

 lightweight aggregates used in lightweight block. 

 Overall, imports make up less than 2 percent of the 

 structural lightweight aggregates market. Almost 

 all this usage was along the eastern seaboard, where 

 marine freight rates allowed competition with pum- 

 ice from domestic western sources. 



EXPLOITATION 



Pumice first was used as a lightweight construc- 

 tion material by the Romans about the beginning 

 of the Christian era, but because production data 

 are somewhat sketchy between then and about 1940, 

 only the production trends since 1940 are really of 



