Lightweight aggregates are usually made from
Correct Answer: D. volcanic source
📚 Detailed Explanation: Volcanic Origin of Lightweight Aggregates
Lightweight aggregates (LWA) have a much lower bulk density than conventional aggregates (typically 300–1200 kg/m³ vs. 1400–1700 kg/m³). Their lightness comes from the high porosity (void content) in the aggregate particles themselves, not just between them.
Why D (volcanic source) is correct: Natural LWA include pumice, scoria, and volcanic cinders — all products of volcanic activity. When magma erupts, dissolved gases form bubbles in the molten rock; when the rock solidifies rapidly, these bubbles are trapped as voids, creating the characteristic vesicular texture and low density. Quartzite (A), limestone (B), and granite (C) are dense rocks with virtually no internal porosity — they are used to make normal-weight or heavyweight aggregate, not lightweight aggregate.
Types of Lightweight Aggregates
| Type | Source | Bulk Density | Application |
|---|---|---|---|
| Pumice | Volcanic (silicic lava) | 500–900 kg/m³ | LW concrete, insulation |
| Scoria / Cinders | Volcanic (mafic lava) | 700–1200 kg/m³ | LW concrete blocks |
| Expanded clay (Leca) | Kiln-fired clay (artificial) | 300–700 kg/m³ | Structural LW concrete |
| Expanded shale | Kiln-fired shale | 400–900 kg/m³ | Structural LW concrete |
Key Concepts for Students
- Lightweight aggregate concrete has a density of 1600–1900 kg/m³ vs. 2400 kg/m³ for normal concrete — significant dead-load savings in high-rise structures.
- Natural LWA (volcanic origin) must be tested for soundness as pumice can disintegrate when saturated.
- Artificial LWA (expanded clay, shale, fly ash beads) are manufactured to specific grades for structural use.