LIGHTWEIGHT AGGREGATES 



339 



the rock. Meanwhile, Gervida (1957) admitted that 

 carbonates, suliides, sulfates, and oxides could all 

 participate in the gas-forming process but asserted 

 that the gas was formed at temperatures (550°- 

 825°C) well below the rock-softening level (1,050°- 

 1,150°C). He suggested that only the dehydroxyla- 

 tion of micas and amphiboles could account for the 

 bloating, inasmuch as these reactions occurred at 

 rock-softening temperatures. Hill and Crook (1960, 

 p. 382) specifically refuted this contention, stating 

 that it was intrinsically "improbable because the 

 dehydroxylation temperature is considerably lower 

 than the bloating temperature * * *," and they 

 very strongly emphasized the role of the reduction 

 of ferric to ferrous oxide, with the oxygen evolved 

 as the cause of the bloating. Chopra, Lai, and Rama- 

 chandran (1964) decided that both organic matter 

 and calcium carbonate produced the gas that does 

 the bloating, and that hydroxyl water, entrapped 

 pore air, and ferric oxide were not significant con- 

 tributors. Donnelly, Brennan, and Peacock (1968) 

 concluded that iron oxide and water reacted with 

 carbon (as little as 0.1 percent) to produce carbon 

 monoxide, carbon dioxide, and a little hydrogen and 

 that the decomposition of carbonates does not con- 

 tribute to the bloating. This brief history suggests 

 (1) that the divergence of opinion as to what causes 

 bloating is considerable, (2) that no single factor 

 is most important, and (3) that insufficient consid- 

 eration has been given to the possible effects of the 

 suggested reactions operating simultaneously, at 

 least as far as the production of gas is concerned. 



Riley (1951) investigated the combinations of 

 constituents that apparently acted as fluxes and 

 affected pyroplastic conditions at the appropriate 

 temperatures for gas formation and retention. In 

 plotting the distribution of some 85 chemical 

 analyses of bloating and nonbloating clays and 

 shales in terms of their SiOa, AI2O3, and combined 

 Fe^Os, FeO, CaO, MgO, and (K, Na)20 (flux) con- 

 tent, he was able to define a range of compositions 

 that included most of the bloatable materials and 

 excluded most of the nonbloatable materials. That 

 range was about SiOs, 52-80 percent; AI2O3, 11-25 

 percent; and combined fluxes, 10-25 percent (fig. 

 41) . Additional investigations since 1951 have shown 

 that this bloating range is less reliable than it first 

 seemed to be, but it is a useful guideline. As exam- 

 ples. White (1960) showed that the range had to be 

 extended in the direction of higher AI2O3 and lower 

 flux contents, whereas Sweeney and Hamlin (1965a) 

 had to extend the range to a higher flux content 

 (fig. 41). At the same time, more materials pre- 

 viously considered to be nonbloatable were included 



in the bloating range. 



Laboratory methods for evaluating raw materials 

 have been described by Conley, Wilson, Klinefelter, 

 and others (1948), Klinefelter and Hamlin (1957), 

 and Hamlin and Templin (1962). The ultimate evalu- 

 ation is how the material expands in a commercial 

 operation; for this determination to be made, sev- 

 eral tons of material may need to be run. 



To distinguish between favorable and unfavorable 

 rocks in the field is difllicult because so many differ- 

 ent argillaceous rocks have been found to be suit- 

 able. Burnett (1964) suggested that field estimates 

 be made of the content of iron, alkali, carbonate, 

 organic carbon, and silt. Because too much of any 

 of these constituents can cause the bloating range 

 to be too narrow, the firing temperature to be too 

 high, or the expanded product to be too heavy, an 

 overabundance can serve as a negative criterion. 

 There are no really positive field criteria. Illitic and 

 montmorillonitic sediments seem to be more con- 

 sistently favorable than kaolinitic ones. The darker 

 colored clays, shales, and slates (green, gray, black) 

 are more suitable than the light colored (particularly 

 red) ones, especially if they have a moderate con- 

 tent of dispersed organic carbon. In general, un- 

 weathered materials are more suitable than weath- 

 ered ones. Common or brick clay is more suitable 

 both physically and economically than flint or ball 

 clay. Thinly flssile shales and slates tend to expand 

 unidimensionally, which is undesirable. Most of 

 these features tend to vary more greatly in short 

 vertical intervals across the sedimentary section 

 than laterally along the sedimentary units, but 

 lateral variation is enough that units unsuitable at 

 one point may be eminently suitable a few hundreds 

 of yards to a few miles away. 



In addition to the materials that can be made to 

 expand, argillaceous rocks can be ground, mixed 

 wth combustible materials — such as ground coal — 

 and sintered to produce lightweight aggregates. 

 The mass becomes pyroplastic, or cohesive, and voids 

 are left as the combustibles are burned out; some 

 bloating may occur, but it is incidental to the sin- 

 tering. Inasmuch as units that may not bloat of 

 themselves can thus be used for lightweight aggre- 

 gate, classifications of argillaceous rocks as unfit is 

 unwise unless the rocks have been tested and found 

 unsuitable for both processes. 



Marine, littoral, lacustrine, and fluviatile clays 

 and shales, and slates derived from them, are all 

 possible raw materials. They occur in formations 

 that range in thickness from a few feet to more 

 than 1 thousand feet and in age from Precambrian 

 to Holocene, and they are distributed throughout 



