The selection of lime for building structures underwater depends on its ability to set and harden in the presence of water.

Lime Type for Underwater Structures

Hydraulic lime is the type of lime used for the construction of structures under water.

Reasoning for Hydraulic Lime

Hydraulic lime contains impurities, primarily silica and alumina. These components allow the lime to set through a Puzzolanic reaction when mixed with water, even without exposure to air. This property makes it ideal for underwater applications where air-based hardening (carbonation) is impossible.

Other Lime Types

• Fat Lime (Pure Lime): This lime is almost pure calcium oxide. It hardens solely by reacting with carbon dioxide from the air (carbonation). It does not set underwater.
• Quick Lime: This is calcium oxide (CaO). It requires slaking (adding water) to form calcium hydroxide (slaked lime). While used in mortars, it's not the primary choice for underwater setting itself.

Therefore, hydraulic lime's ability to set underwater makes it the suitable choice for such construction projects.