Reactive silica in fly ash is a key component that determines its usefulness as a supplementary cementitious material. It refers to the form of silica within the fly ash that is capable of participating in a chemical reaction with calcium hydroxide (lime) in the presence of water.
Understanding Reactive Silica
Fly ash is widely utilized as a pozzolana in construction materials like cement, mortar, and concrete. The term 'pozzolana' refers to a siliceous or silico-aluminous material that, in finely divided form and in the presence of water, reacts chemically with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.
According to the provided information: Reactive silica is one of the parameters that determines the pozzolanic potential of a fly ash, which is its tendency to react with available calcium hydroxide to form cementitious hydration products.
This highlights the critical role of reactive silica. It's not just the total amount of silica in the fly ash, but specifically the reactive portion that contributes to the desired cementitious reactions.
Key Aspects of Reactive Silica
- Pozzolanic Reaction: Reactive silica reacts with calcium hydroxide (a byproduct of cement hydration) to form calcium silicate hydrate (C-S-H) gel. This C-S-H gel is the primary binder responsible for the strength and durability of concrete.
- Determining Pozzolanic Potential: The quantity and reactivity of the silica content in fly ash are direct indicators of how effectively it can improve the properties of concrete by participating in this pozzolanic reaction. Higher reactive silica content generally means higher pozzolanic potential.
- Source: Reactive silica in fly ash is typically present in the amorphous (non-crystalline) glass phases formed during the combustion of pulverized coal. This amorphous structure makes it much more reactive than crystalline forms of silica.
Why is Reactive Silica Important?
The pozzolanic reaction involving reactive silica offers several benefits when fly ash is used in concrete:
- Increased Strength: The additional C-S-H gel formed contributes to higher long-term strength and reduced permeability.
- Improved Durability: It refines the pore structure, making the concrete less susceptible to sulfate attack, alkali-silica reaction (ASR), and chloride ingress.
- Reduced Calcium Hydroxide: It consumes the free calcium hydroxide, which is a less stable hydration product, improving overall stability.
In essence, reactive silica is the active ingredient in fly ash that allows it to act as a binder alongside cement, enhancing the performance and sustainability of concrete.
