The hydraulicity of the hydraulic lime is mainly due to____.
Try it first: pick an option, then read the explanation.
Understanding Hydraulicity
Hydraulicity is the property of a binder (like lime or cement) that allows it to set and harden by reacting with water, even in the absence of air (specifically, carbon dioxide). This is in contrast to non-hydraulic materials, like fat lime, which require exposure to air to harden through a process called carbonation.
Detailed Analysis of the Options
B. Clay
This is the correct answer. Hydraulic lime is produced from limestone that naturally contains clay impurities (typically 5% to 30%). During burning, the calcium oxide (CaO) from the limestone reacts with the silica (SiO₂) and alumina (Al₂O₃) from the clay to form calcium silicates and aluminates. These compounds are responsible for the hydraulic set; they react chemically with water to form a strong, stable, water-insoluble binder.
A. Calcium oxide
This is incorrect. While Calcium Oxide (CaO) is the main component of all limes, it is not responsible for the hydraulic property. Pure CaO hardens by reacting with atmospheric carbon dioxide (CO₂) to re-form calcium carbonate (CaCO₃), a process that requires air.
C. Sulphur
This is incorrect. Sulphur is an undesirable impurity in lime and does not contribute to hydraulicity.
D. Water
This is incorrect. Water is necessary for the hydraulic reaction to occur (it's a reactant), but it is not the ingredient within the lime that *causes* the hydraulic property. The clay is the essential ingredient that gives the lime its hydraulic potential.
Summary: Cause of Setting in Limes
| Lime Type | Key Ingredient for Setting | Setting Mechanism | Can it set under water? |
|---|---|---|---|
| Fat Lime | High Purity Calcium Oxide (CaO) | Carbonation (Reacts with CO₂ from air) | No |
| Hydraulic Lime | Clay (Silica & Alumina) | Hydration (Reacts with H₂O) | Yes |
Study Tips
- Hydraulic = Clay: This is the most important association to remember. The clay impurities give hydraulic lime its special power.
- Think "Cement": The setting action of hydraulic lime is similar to that of cement, which also relies on the hydration of calcium silicates and aluminates.
- Fat Lime vs. Hydraulic Lime: Fat lime is pure and needs air (CO₂) to set. Hydraulic lime is impure (with clay) and needs water (H₂O) to set.