338 



UNITED STATES MINERAL RESOURCES 



raw material must be extracted before the area is 

 irretrievably lost to preclusive uses, such as indus- 

 trial plants, high-rise buildings, and residential 

 construction. 



CLAYS, SHALES, AND SLATES 

 Many varieties of clays, shales, and slates, of 

 diverse origins, expand somewhat on heating to 

 about 2,000°F (1,100°C). Basically, all clays, shales, 

 and slates are chemically and, to some extent, min- 

 eralogically similar — they differ mostly in their 

 physical characteristics. Lightweight aggregates are 

 used only for their physical properties, and it is 

 their chemical similarity that makes all of them 

 amenable to thermal expansion. Table 67 presents 

 several representative chemical analyses for the 

 argillaceous rocks that have been tested for use as 

 lightweight aggregate. The range in composition is 

 wide for each of the three materials, and all three 

 ranges largely overlap. Unfortunately, the analyses 

 of rocks that expand suitaby, that expand inade- 

 quately, or that do not expand, all fall in these same 

 ranges. 



The basic requirement for expansion of an argil- 

 laceous rock is a source of gas within the raw ma- 

 terial, but several other physical characteristics are 

 required for full suitability. Sufficient gas must form 

 to cause a full bloat, in the temperature range where 

 the material is pyroplastic (melted enough to be 

 viscous) , so that the gas can be trapped ; there must 

 be enough vitrification, at a high enough viscosity, 

 so that most of the gas can be held in small, evenly 



distributed pores; and the temperature range be- 

 tween softening and liquefaction must be large 

 enough (about 100°F) that the bloating can be con- 

 trolled in large-scale commercial production. Eco- 

 nomical production is had at kiln or sinter tempera- 

 tures between 1,800° and 2,200°F (1,000°-1,200°C), 

 although temperatures can be as low as 1,600°F 

 (870°C) or as high as 2,400°F (1,300°C). 



There is little agreement on the relative impor- 

 tance of the number of possible sources for the gas 

 evolved within the raw material. Conventional wis- 

 dom in the early days of the industry held that the 

 argillaceous rocks had to have a significant organic 

 content, although the amount varied with the prac- 

 titioner. However, laboratory investigation of the 

 evolved gases showed that many materials with very 

 little total carbon expanded better than those with 

 an appreciable amount (Austin and others, 1942; 

 Sullivan and others, 1942) — to perhaps 1.5 percent — 

 and that CO2, SO2, and H2O all played a part in the 

 bloating. Riley's (1951) research indicated that 

 thermal destruction of hematite, pyrite, and dolo- 

 mite provided both CO2 and SO2 in amount sufficient 

 for bloating without recourse to organic carbon. 

 Ehlers and Richardson (1958) emphasized the role 

 of CO2 derived by the heating of calcite, dolomite, 

 and ankerite in the argillaceous rocks, and subse- 

 quent more effective entrapment in illite than in 

 kaolinite (kaolinite fuses at a higher temperature 

 than illite). They deemphasized the role of iron in 

 gas liberation, considering it to be of possible sig- 

 nificance only where hematite exceeded 5 percent of 



Table 67. — Comparative composition of expansible clays, shales, and slates 



Clay Shale Slate 



Sample • 



1 2 8 4 5 6 7 8 



SiOj B3.00 61.80 36.7 49.00 65.01 69.00 64.6 54.00 



AlzOa 20.74 16.88 2B 8 24.67 17.07 11.60 12.9 17.30 



FeaOa 4.27 '9.68 2.66 6.44 2.15 6.37 1.72 '6.66 



FeO 2.30 .13 5.79 3.38 1.07 4.17 



MbO 1.82 .06 1.22 2.36 1.89 1.58 1 36 3.32 



CaO .66 1.27 10.7 .49 .70 .32 1.29 4.68 



NaaO .61 2.09 .26 .74 1.33 .95 1.28 .75 



K2O 3.4 .67 3.4 3.50 2.69 3.00 5.9 2 28 



TiOa 1.06 1.17 .57 .80 1.10 .73 



Loss on ignition 7.36 8.16 5.69 5 24 4.87 



Total =99.74 100.98 '102.01 100.15 99.81 '101.20 '100.00 =100.13 



CO2 .32 2.93 .98 .21 .37 3.47 4.21 



C (organic) .73 .72 87 .80 .35 .64 1.34 



H2O+ 7.15 4.68 1.18 



H2O- 2.81 4.28 .79 .64 .51 .41 



S .04 .08 .044 .08 



Other ».93 »7.89 



' Total iron. 



' Total of all constituents. 



•PjOb, 0.67; MnO, 0.11; 01, 0.16; F, 0.10. 



•PsOs, 7.3; F, 0.59. 



SAMPLE LOCATIONS AND REFERENCES 



1. Pennsylvania (Roen and Hosterman, 1969, sample Ro-R-lb). 



2. New South Wales, Australia (Hill and Crook, 1960, sample 4). 



3. Florida (Conley and others, 1948, sample 10). 



4. Illinois (White, 1960, sample 1415). 



5. niinois (White, 1960, sample 1334A). 



6. Pennsylvania (Root, 1968, sample 10). 



7. Florida ((Donley and others, 1948, sample 3). 



8. Pennsylvania (Conley, 1942, sample 1). 



9. New South Wales, Australia (Hill and Crook, 1960, sample 18). 



67.36 

 18.96 

 17.12 



6.98 

 99.75 

 2.96 



