Fly ash, the most widely used supplementary cementitious material
in concrete, is a byproduct of the combustion of pulverized coal in
electric power generating plants. It produces spherical glassy particles
that are finer than Portland cement. Two types of fly ash are available,
depending on the type of coal that was burned to make the ash. Class
F fly ash has pozzolonic properties, and Class C fly ash has both
pozzolonic and cementitious properties. Class F fly ash is typically used
at 15–25 percent by mass of the cementitious material, and Class C at
15–40 percent by mass.
Requirements for class C and F fly ash are provided in ASTM C618
Kohn and Tayabji (2003: 61) note that “Class C fly ash may be detrimental
to the performance of concrete, causing premature stiffening of the
fresh concrete, thermal cracking, and/or reduced sulfate resistance (it) is
generally not effective in controlling expansions due to ASR.” Early stiffening
problems are more likely in hot weather with certain water reducing
admixtures. The potential for early stiffening may be assessed by making
trial batches at the highest temperatures anticipated on the project, and
measuring slump loss and setting times.
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