Fly Ash is the most used Supplementary Cementing Material (SCM) in Texas due to its availability. Fly ash is a byproduct of the coal burning electric power generating plants. After the ignition of coal in the furnace, the ash residue is carried away by the exhaust gases, and is then collected with electrostatic precipitators or in filter bag houses. As the fly ash travels to the collectors, the material cools and forms spherical glassy particles.

There are two classes of fly ash, Class F and Class C fly ash. Fly ashes are categorized into these classes based on their chemical composition. Class F fly ashes have higher amounts of silica and lower amount of calcium, while Class C fly ashes tend to have lower amounts of silica and higher amount of calcium. Both classes of fly ashes are acceptable for use in concrete pavement provided the total cementitious content of concrete is 520 lb/CY or less.

Class F fly ashes are generally pozzolanic which means they possess little to no cementing properties. However, in the presence of water and calcium hydroxide, they will react and form compounds having cementing properties. Class F fly ashes are excellent in improving the long-term durability of concrete. Class F fly ashes are effective in reducing permeability and mitigating alkali-silica reactions (ASR), delayed ettringite formation (DEF) and external sulfate attack at relatively low replacement rates.

Class C fly ashes have both pozzolanic and cementing properties. Due to the higher calcium content, Class C fly ashes will react with water and harden. Class C fly ashes are effective in reducing permeability, but not as effective in mitigating alkali-silica reactions (ASR), delayed ettringite formation (DEF) and external sulfate attack. High replacement rate of Class C fly ash are needed to mitigate ASR and DEF, and due to the chemistry of the glass phase, Class C fly ashes are susceptible to external sulfate attack and should not be used in sulfate environments. However, due to the low cementitious content of paving concrete mix designs, both ASR and DEF are not major concerns, and since concrete pavements are not in direct contact with natural subgrades, external sulfate attack is also not of concern; therefore, Class C fly ashes are allowed in paving concrete.