Which of the following possess more ability to resist fire?
Try it first: pick an option, then read the explanation.
Understanding Fire Resistance in Stones
Fire resistance is a crucial property for building materials. It refers to a stone's ability to withstand high temperatures without losing its structural integrity or breaking down chemically. The best fire-resistant stones are those that are dense and composed of minerals that are stable at high temperatures.
Detailed Analysis of the Options
A. Compact Sandstone
This is the correct answer. Sandstone is primarily composed of quartz (silica) grains cemented together. A compact, dense sandstone has excellent fire-resisting properties because its main component, quartz, has a very high melting point. The dense structure helps it maintain its integrity for a longer period when exposed to heat.
B. Quartz
While quartz is the fire-resistant mineral within sandstone, it is not typically used as a bulk building stone on its own. Furthermore, when pure quartz is subjected to intense and rapid heating (around 573°C), it can undergo a sudden expansion (a phenomenon called quartz inversion), which can cause it to crack or spall. Compact sandstone, being a composite material, manages this stress better. Therefore, as a building material, compact sandstone is considered the better fire-resistant option.
C. Red Marble
Marble is a metamorphic rock made of calcium carbonate (CaCO₃). It has very poor fire resistance. When heated, it undergoes a chemical process called calcination, where it breaks down into lime (calcium oxide) and carbon dioxide gas. This completely destroys the stone's strength and structure.
D. Shale
Shale is a soft, laminated (layered) sedimentary rock. It has poor structural strength and very low fire resistance. When heated, it easily splits along its layers and disintegrates, making it entirely unsuitable for fire-resistant construction.
Summary: Fire Resistance of Different Stones
| Stone | Composition | Fire Resistance | Reason |
|---|---|---|---|
| Compact Sandstone | Mainly Quartz (Silica) | Good | High melting point of quartz and dense structure. |
| Marble | Calcium Carbonate | Poor | Decomposes (calcines) under heat. |
| Shale | Clay Minerals | Very Poor | Soft, laminated structure that splits easily. |
| Quartz (as a mineral) | Silica | High | High melting point, but can spall under thermal shock. |
Study Tips
- Silica is Superior: Stones rich in silica (like sandstone) are generally good fire resistors. Stones rich in calcium (like marble and limestone) are poor fire resistors.
- Compact = Strong: The word "compact" is a key clue. A dense, compact structure is always better for resisting fire and other stresses.
- Marble Melts (Chemically): Remember that marble doesn't just get hot; it chemically changes into lime powder, losing all its strength.