For high-strength concrete and to mitigate alkali-silica reaction, which class of fly ash is preferred?
- AClass C (high calcium)
- BClass F (high silica/alumina)Correct
- CClass N (natural pozzolan)
- DClass S (high sulfur)
Solution
Concept
Class F fly ash for ASR mitigation
Formula
SiO2 + Al2O3 + Fe2O3 ≥ 70%
Step-by-step solution
1. Fly ash is designated as Class F or Class C based on source coal and chemical composition. 2. Class F fly ash has high siliceous and aluminous content (SiO2 + Al2O3 + Fe2O3 ≥ 70%) and low calcium content (CaO < 10%). 3. The pozzolanic reaction of Class F fly ash consumes free calcium hydroxide and traps alkali ions, suppressing alkali-silica reaction (ASR) expansions. 4. Hence, Class F fly ash is preferred for high-strength concrete and ASR mitigation. Answer: B
Explanation
Class F fly ash is derived from anthracite or bituminous coal and features a high content of pozzolanic silica and alumina with low calcium oxide content (less than 10%). Its reaction with free calcium hydroxide consumes alkalis and densifies the cement matrix, thereby arresting destructive alkali-silica reaction (ASR) expansions. It is therefore preferred for high-durability and high-strength concrete.
Why the other options are wrong
A) Class C fly ash contains high lime (>20% CaO), making it less effective at controlling alkali-silica expansion. C) Class N designates raw or calcined natural pozzolans, not coal combustion by-products. D) Class S is not a recognized ASTM C618 fly ash category.
Shortcut
Class F = Low calcium + High silica -> Best for ASR control and long-term strength.
Common mistake
Assuming Class C fly ash is superior for ASR control, whereas its high calcium content reduces ASR mitigation efficacy.
Correct answer: B) Class F (high silica/alumina)