Petrographic and Mineralogic Characteristics of Aggregates

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Abstract
The petrographic and mineralogic characteristics and geologic occurrence of rock formations and sand and gravel deposits determine the composition, gradation, and quality of aggregates. Thus, because of their several modes of origin—igneous, sedimentary, and metamorphic—rock formations present different problems to the quarryman and produce aggregates of differing serviceability. The suitability of natural aggregates for any purpose also depends upon the geologic processes involved in their formation and subsequent history. Petrographic and mineralogic composition and internal texture and structure of the particles control the physical and chemical properties of the aggregate. Consequently, the suitability of aggregates for proposed uses can be evaluated by petrographic methods. The physical properties of particles important to the quality of aggregate are: porosity, permeability, and absorption; surface texture; volume change with wetting and drying; thermal characteristics; strength and elasticity; density; hardness; shape; and coatings. The important chemical properties are: solubility, susceptibility to oxidation, hydration, and carbonation; and reactivity with constituents of portland cement. The interrelation of petrographic character and serviceability of concrete aggregate is demonstrated by several case histories.