462 rhoades. CONCRETE AGGREGATE [Ch. 24 



may augment alkalies in cement; other salts may alter the course of cement 

 hydration. In some cases, coatings may be essentially inert and strongly 

 bonded and may actually increase strength of aggregate-cement bond. 



Deleterious Impurities 



Natural aggregate is commonly contaminated by dirt, silt, clay, mica, coal, 

 humus and other organic matter, and various salts. These substances act in 

 a variety of ways to decrease strength and durability, cause unsightly appear- 

 ance, or complicate processing and mixing operations. They may increase water 

 requirements of the concrete; they may be physically weak or susceptible to 

 breakdown by weathering; they may be so active chemically as to inhibit 

 normal hydration of cement or react with cement constituents. Fortunately, 

 excesses of deleterious impurities may frequently be removed by simple treat- 

 ment. Silt, clay, soluble salts, and some lightweight substances are usually 

 removable by washing. Special processes may be necessary for less amenable 

 substances, or their removal may not be practicable. 



Mineral and Chemical Properties 



The mineralogic and chemical composition and the internal texture and 

 structure of particles control the physical and chemical properties of aggre- 

 gates. Few, if any, rocks or minerals are inert when enclosed in concrete. 

 Some may contain water-soluble constituents which can be leached, with 

 attendant loss of strength and increase in porosity. Leached material may 

 form unsightly efflorescence or modify normal hydration of cement. Un- 

 stable minerals are susceptible to oxidation, hydration, or carbonation and may 

 cause concrete to become distressed. Certain minerals are capable of base- 

 exchange action in which alkalies in the minerals are exchanged for calcium 

 in the cement solutions ; released alkalies may then attack susceptible aggregate 

 particles. Some minerals — opal, chalcedony, tridymite, cristobalite, and heu- 

 landite, and rocks such as glassy or cryptocrystalline rhyolites, dacites, and 

 andesites — and opaline and chalcedonic cherts and phyllites are reactive with 

 high-alkali cements (cement containing more than 0.6 percent Na20 + K2O 

 expressed in equivalents of Na 2 0) and can cause deterioration of concrete 

 through production of alkalic silica gels which subsequently absorb water 

 osmotically from the cement paste, developing hydrostatic pressures in excess 

 of the tensile strength of concrete. 



Thermal compatibility between mineral particles and cement paste signifi- 

 cantly influences quality of cement when the linear thermal coefficient of ex- 

 pansion of the mineral is decidedly lower than that of the cement paste. Some 

 difficulty is experienced with minerals that have different thermal coefficients 

 of expansions in different crystal directions. For example, with increasing 

 temperature calcite expands greatly in one crystallographic direction and 

 contracts in another direction. 



