ACI 232.2R-18 Report on the Use of Fly Ash in Concrete.
3.2—Chemical composition Bulk chemical composition (Table 3.2) has been used by ASTM C618 to classify fy ash into two types: Classes C and F. The chemical composition data used to determine compli- ance with ASTM C618 do not directly address the reactivity of the particles, but are used as a quality control or quality assurance tool. Minor variations in the chemical composi- tion of a specifc fy ash do not relate directly to the long- term performance of concrete containing that fy ash. Fly ash composition is reported as percent oxides by mass, although the elements analyzed may not always be present in a pure oxide form, and may be incorporated within glassy or other mineral phases. The crystalline and glassy constituents that remain after the combustion of the pulverized coal are a result of materials with high melting points and incombus- tibility. The amounts of the four principal constituents vary widely. Typical values are SiO 2 (35 to 60 percent), Al 2 O 3 (10 to 30 percent), Fe 2 O 3 (4 to 20 percent), and CaO (1 to 35 percent). The sum of the frst three constituents—SiO 2 , Al 2 O 3 , and Fe 2 O 3 —need to be equal to or exceed 70 percent for the material to be classifed as an ASTM C618 Class F fy ash, whereas their sum need only exceed 50 percent for the material to be classifed as an ASTM C618 Class C fy ash. Class C fy ashes typically have a higher CaO content than a Class F fy ash.
Fly ash color and the amount used can infuence the color of the resulting hardened concrete in the same way as changes in cement or fne aggregate color. Fly ash color is not an engineering concern unless aesthetic considerations relating to the concrete require maintaining a uniform color in exposed concrete. A change in the color of a fy ash from a particular source can indicate changes in coal source, carbon content, iron content, or burning conditions and, therefore, changed properties. Refer to Fig. 3.5 for examples of the range in color seen with diferent fy ash sources. 3.5.1 Particle shape—Particle size and shape charac- teristics of fy ash depend on the source and uniformity of the coal, the degree of pulverization before burning, the combustion environment (temperature level and oxygen supply), uniformity of combustion, and the type of collec- tion system used (mechanical separators, baghouse flters, or electrostatic precipitators). Lane and Best (1982) reported the shape of fy ash particles is also a function of particle size. The majority of fy ash particles are glassy, solid, or hollow, and spherical in shape as singles or clusters. Examples of fy ash particle shapes are shown in Fig. 3.5.1a and 3.5.1b. Fly ash particles that are hollow are translucent to opaque, slightly to highly porous, and vary in shape from rounded to elongated. The intergrinding of fy ash with portland cement clinker in the production of blended cement has improved its contribution to strength (Diamond 1982). ACI 232.2R pdf download.