For construction of structures under water, the type of lime used is ..........
Try it first: pick an option, then read the explanation.
Understanding Lime for Underwater Use
The primary challenge of building underwater is that the construction material must be able to set and harden in the complete absence of air. Most common binders, like pure lime, require carbon dioxide from the air to harden. For underwater work, a special type of binder is needed that can undergo a chemical reaction with water itself to gain strength. This property is known as hydraulicity.
Detailed Analysis of the Options
A. hydraulic lime
This is the correct answer. Hydraulic lime is specifically designed for this purpose. It contains clay impurities (silica and alumina) which, upon heating, form compounds that react directly with water to set and harden. This allows it to be used in foundations, bridge piers, and other structures submerged in water.
B. fat lime & D. pure lime
These are incorrect. Fat lime and pure lime are essentially the same thing—high-purity calcium oxide. They harden by reacting with atmospheric carbon dioxide (carbonation). Underwater, there is no air supply, so this reaction cannot occur. The lime would simply wash away without ever setting.
C. quick lime
This is incorrect. Quicklime is the raw, unslaked form of lime (CaO). It is highly reactive with water but does not possess hydraulic properties on its own. It needs to be slaked before use, and unless it contains hydraulic impurities, it will behave like fat lime.
Summary: Lime Types and Their Setting Action
| Lime Type | Setting Mechanism | Requires Air (CO₂)? | Suitable for Underwater Use? |
|---|---|---|---|
| Hydraulic Lime | Sets by reacting with water (Hydration) | No | Yes |
| Fat / Pure Lime | Sets by reacting with air (Carbonation) | Yes | No |
Study Tips
- Hydraulic = Hydro = Water: Connect the word "hydraulic" with "hydro," which means water. Hydraulic lime is the one that works with water.
- Fat Lime Needs to Breathe: Think of fat/pure lime as needing to "breathe" air (CO₂) to harden. If it can't breathe (i.e., it's underwater), it can't set.
- Key Application: The most important use case that distinguishes hydraulic lime is for underwater construction or for very thick walls where air cannot penetrate easily.