454 rhoades. CONCRETE AGGREGATE [Ch. 24 



being derived from the marine and glacial sediments that mantle the 

 adjacent Pasco slope. 



Deposits of volcanic ash and pumicite may be considered a special 

 category of wind-blown sand. Glassy, volcanic tuff is useful as poz- 

 zolanic admixture with cement to impart certain desirable character- 

 istics to concrete (Lea and Desch, 1935; Lea, 1938). For this purpose 

 the materials should possess a fineness comparable to that of cement 

 (as a rough index, at least 90 percent of the material should pass a 

 325-mesh sieve). Some pumicites are prevailingly of this fine size 

 as, for instance, the extensive accumulations of pumicite in the vicinity 

 of Friant, California, from which material was obtained for use in 

 Friant Dam. If a deposit contains material prevailingly coarser than 

 this size or if it is not readily pulverulent, it must be pulverized arti- 

 ficially; the additional cost of such processing is a frequent deterrent 

 to the use of any given deposit of pumiceous material. 



Pumice and scoria in coarser sizes are coming into increasing use as 

 aggregate for the construction of special-purpose concrete, as in "cin- 

 der-block" construction or in the fabrication of acoustical or insulating 

 concrete (Price and Cordon, 1949). 



Glacial Deposits 



Glacial deposits in the form of moraines, or tills, are typically char- 

 acterized by extremely heterogeneous grading, frequently with large 

 boulders and fine rock flour intimately admixed. Individual particles 

 may be smooth or even somewhat rounded, but angular or subangular 

 particle shapes are common. Glacial deposits normally exhibit some 

 selective concentration of the harder rock types, but softer materials 

 frequently are preserved through incorporation within the ice mass it- 

 self, where they are shielded from the intense abrasion and crushing 

 that generally characterize glacial transportation. The glacial envi- 

 ronment discourages extreme chemical alteration and thus inhibits the 

 selective removal of materials which are soluble or susceptible to 

 chemical decomposition. Thus, whereas firm particles may be pre- 

 served from the weathering and decomposition to which they might 

 be susceptible in other sedimentary environments, soluble or chemically 

 unstable materials may be preserved to the detriment of the deposit 

 for use as concrete aggregate. 



Certain areas of the United States are extensively mantled with 

 glacial deposits and must rely on this source for concrete aggregate 

 (ASTM, 1948d). However, such deposits usually require expensive 

 beneficiation. Elaborate screening may be required to correct the 

 grading, and the necessity of wasting large amounts of oversized and 



