Fly ash is a commonly used pozzolan, and this relationship is important to the modern construction industry. It is a fine, powdery byproduct recovered from the gases of coal-fired power plants. Its value lies in its chemical composition, allowing it to serve as a supplementary cementitious material when mixed into concrete. By reacting with other components, fly ash enhances the properties of the final material, providing both technical and environmental benefits.
Understanding the Pozzolanic Reaction
A pozzolan is defined as a finely ground siliceous or aluminous material that possesses little or no cementitious value on its own. In the presence of water, it chemically reacts with calcium hydroxide (\(\text{Ca}(\text{OH})_2\)) at ordinary temperatures to form compounds with cementing properties. This process, known as the pozzolanic reaction, is responsible for the long-term strength gain in concrete.
The reaction consumes calcium hydroxide, a byproduct released during the hydration of Portland cement, and converts it into a stable binder. The primary product formed is Calcium Silicate Hydrate (C-S-H) gel, the same compound responsible for the strength and durability of the cement paste itself. Pozzolans enhance the long-term strength and durability of concrete by utilizing a component that would otherwise be a weak link in the matrix.
This chemical transformation allows the material to act as an effective binder, improving the overall integrity of the concrete structure. The reaction contributes to a denser material structure, which reduces the concrete’s permeability and its vulnerability to chemical attacks, such as sulfate exposure. A high pH environment, typically around 12, is necessary to facilitate the reaction by increasing the solubility of the silica and alumina.
Origin and Basic Composition of Fly Ash
Fly ash is generated as a residue during the combustion of pulverized coal in thermal power plants. The fine particles solidify while suspended in the exhaust gases and are collected by electrostatic precipitators or filter bags. The physical characteristics of fly ash include tiny, spherical, glassy particles, which contribute to improved workability when mixed into concrete.
The chemical makeup of fly ash is predominantly composed of oxides of silicon, aluminum, and iron, with smaller amounts of calcium oxide. Silicon dioxide (\(\text{SiO}_2\)) often makes up 40 to 60 percent of the material, while aluminum oxide (\(\text{Al}_2\text{O}_3\)) is typically 20 to 40 percent. The presence of these active siliceous and aluminous components gives fly ash its ability to undergo the pozzolanic reaction.
The specific chemical composition is not fixed and varies depending on the source of the coal, the furnace temperature, and the operating conditions of the power plant. This variability leads to differences in the final ash product, particularly in the amount of calcium oxide. These differences necessitate a classification system to ensure the material is used appropriately in construction applications.
Classification and Practical Application in Cement
Due to the variation in coal sources, fly ash is classified into two main types for use in concrete, as defined by standards like ASTM C618: Class F and Class C. Class F fly ash is derived from burning anthracite or bituminous coal and contains a low percentage of calcium oxide, usually less than 10 percent. This type is primarily pozzolanic, meaning it has little inherent cementing value and requires the calcium hydroxide released from Portland cement to react and form the C-S-H binding gel.
Class C fly ash originates from lignite or sub-bituminous coal and is characterized by a higher calcium oxide content, sometimes exceeding 20 percent. Because of this higher calcium content, Class C fly ash exhibits both pozzolanic and self-cementing properties, allowing it to achieve strength gain without a large external source of calcium hydroxide. The choice between the two classes depends on the desired properties of the concrete mix and the required rate of strength development.
Fly ash is widely used as a supplementary cementitious material (SCM), either by blending it with Portland cement during manufacturing or by adding it directly to the concrete mix. This practice offers improved workability of the fresh concrete due to the spherical shape of the particles. The fine particles act like tiny ball bearings, making the concrete easier to place and finish.
The long-term impact of fly ash is significant, as the pozzolanic reaction leads to increased late compressive strength and enhanced durability after the first 28 days of curing. By consuming calcium hydroxide and producing additional C-S-H gel, fly ash reduces the concrete’s permeability. This increases its resistance to sulfate attacks and the alkali-silica reaction. Utilizing fly ash also offers an environmental advantage by diverting a waste product from landfills and partially replacing Portland cement production, a major source of carbon dioxide emissions.