When fly ash is used as a partial replacement for ordinary portland cement in concrete, it chemically reacts with Calcium Hydroxide (Ca(OH)2). What is the primary benefit of this specific "pozzolanic reaction"?
- AIt rapidly accelerates the setting time of the concrete in cold weather.
- BIt produces excess heat to cure the concrete faster.
- CIt consumes the soluble, weak Ca(OH)2 and converts it into additional, strong Calcium Silicate Hydrate (C-S-H) gel.Correct
- DIt acts as an air-entraining agent to improve freeze-thaw resistance.
Solution
Concept
Pozzolanic reaction in concrete
Formula
Ca(OH)2 + SiO2 + H2O -> C-S-H
Step-by-step solution
1. Ordinary Portland Cement hydration produces calcium hydroxide Ca(OH)2 as a byproduct. 2. Ca(OH)2 is soluble and mechanically weak, contributing little strength. 3. Pozzolanic silica in fly ash reacts with Ca(OH)2 to produce secondary C-S-H gel. 4. Therefore, fly ash converts weak Ca(OH)2 into strong C-S-H gel. Answer: C
Explanation
During cement hydration, calcium silicate reaction produces calcium hydroxide Ca(OH)2, which contributes little to strength and is prone to chemical attack. Pozzolanic fly ash contains reactive amorphous silica and alumina that consume this soluble Ca(OH)2 in the presence of water, converting it into additional, insoluble calcium silicate hydrate (C-S-H) gel that significantly enhances concrete durability and long-term compressive strength.
Why the other options are wrong
A) Fly ash retards rather than accelerates initial setting. B) Pozzolanic reactions generate less heat of hydration, which prevents thermal cracking. D) Fly ash does not introduce air voids like air-entraining agents.
Shortcut
Pozzolana + Ca(OH)2 = Extra C-S-H gel (strength & durability).
Common mistake
Assuming fly ash acts as an accelerator or increases early heat of hydration.
Correct answer: C) It consumes the soluble, weak Ca(OH)2 and converts it into additional, strong Calcium Silicate Hydrate (C-S-H) gel.