The inert material used in concrete is
Correct Answer: C. aggregate
📚 Detailed Explanation: Aggregate as the Inert Material in Concrete
The term “inert material” in the context of concrete refers to a component that does not chemically react with the other ingredients (particularly cement and water) during the hydration and hardening process. Aggregates satisfy this definition in normal concrete.
Why C (aggregate) is correct: Aggregate — both fine (sand) and coarse (gravel or crushed stone) — is chemically inert in standard concrete. Its role is physical: it provides bulk, reduces the amount of cement needed, and contributes to the load-carrying skeleton. Cement (A) reacts chemically with water through hydration reactions to form C-S-H gel — it is not inert. Water (B) acts as the reactant that triggers cement hydration — it is also not inert. Therefore, aggregate is the only truly inert component in normal concrete.
Role of Each Ingredient in Concrete
| Ingredient | Chemical Role | Classification |
|---|---|---|
| Cement | Reacts with water (hydration) | Reactive binder |
| Water | Reactant in hydration | Reactive liquid |
| Aggregate | Chemically inert (no reaction) | Inert filler |
Key Concepts for Students
- Some aggregates are not truly inert — reactive silica (chert, flint, opaline silica) can cause Alkali-Silica Reaction (ASR), expanding and cracking concrete over time.
- For aggregate to be classified as inert, it must pass IS 2386 Part VII soundness and chemical reactivity tests.
- Even chemically inert aggregate must be clean (no silt, clay, organic matter) to bond properly with cement paste.