When evaluating coal fly ash for use in mortar and concrete, there are many properties to assess. These include:
Important aspects of durability include air entrainment, permeability, and prevention of detrimental aggregate reactions (e.g. alkali-silica reaction, or ASR).
ASTM specification C 311 provides test methods for sampling and testing fly ash for use as an admixture in Portland cement concrete and mortar. ASTM C 618 describes requirements for fly ash composition and performance.
A thorough chemical analysis is often the first step in characterizing a fly ash. The CAER typically analyzes for:
Carbon in fly ash will not only affect the color of the concrete, but can affect the final water and entrained air content. The major oxide content is the basis for the determination of the ASTM fly ash class, F versus C ash, and can reveal information on the coal burned.
For example, a high iron oxide (Fe2O3) content in the ash typically indicates that a high-sulfur coal was burned, whereas a high calcium oxide content suggests that a low-rank coal, such as western U.S. subbituminous coal, was burned. Available alkali is important because it can exacerbate detrimental ASR reactions. Thus, ASTM C 618 specifies maximum alkali content for fly ash and cement.
The inorganic portion of fly ash has essentially two components:
The majority of the fly ash is composed of spherical glass particles, with lesser amounts of crystalline material. X-ray diffraction (XRD) is used to determine the crystalline compounds present in the ash, and can also be used to estimate the amount of glassy material. The glass component is important because it is this material that gives fly ash its pozzolanic reactivity in mortar and concrete.