Lime Classification as per IS 712 - 1984

IS 712 - 1984 provides a standard classification for building limes used in construction. This standard categorizes lime based on its properties, such as hydraulic nature, setting time, and suitability for different applications.

Classes of Lime under IS 712 - 1984

The standard classifies building lime into different categories based on their characteristics and uses:

• Class A: Eminently hydraulic lime.
• Class B: Semi-hydraulic lime.
• Class C: Fat lime (also includes white lime).
• Class D: Magnesium/Dolomitic lime.
• Class E: Kankar lime.
• Class F: Silicious Dolomitic lime.

Class A – Eminently Hydraulic Lime Properties

Class A lime, specified as Eminently hydraulic lime, is particularly suitable for structural purposes. This is because it possesses significant hydraulic properties, meaning it can set and harden in the presence of water, including under water or in damp conditions. This makes it ideal for applications like mortar, concrete, construction works, and foundations where strength and durability are required, and exposure to moisture is expected.

According to IS 712 - 1984, Class A lime has specific requirements for its setting time:

• Initial setting time: Not less than 2 hours (minimum).
• Final setting time: Not more than 48 hours (maximum).

These setting times ensure that the lime mortar or concrete allows for sufficient working time after mixing (initial setting time) but also gains strength relatively quickly (final setting time) to support structural elements.

Setting Time and Structural Use Analysis

The question asks for the class of lime used for structural purposes with a minimum initial setting time of 2 hours and a maximum final setting time of 48 hours, as per IS 712 - 1984. Based on the standard, Class A – Eminently hydraulic lime fits this description precisely due to its hydraulic nature suitable for structural applications and its specified setting time requirements.

Let's consider why other options are not suitable:

• Class C – Fat lime: Fat lime is non-hydraulic. It hardens slowly by absorbing carbon dioxide from the air. It is primarily used for plastering and whitewashing, not structural concrete or mortar requiring strength in damp conditions.
• Class D – Magnesium/Dolomitic lime & Class F – Silicious Dolomitic lime: While these are hydraulic limes, their specific properties, uses, and setting times might differ from the requirements specified for Class A and the structural applications mentioned in the question. Class A is specifically designated for such eminent structural use based on its hydraulic strength and controlled setting times.

Therefore, the lime class meeting the criteria of structural use (mortar, concrete for construction and foundation) and the specified setting times (initial $\ge$ 2 hours, final $\le$ 48 hours) according to IS 712 - 1984 is Class A – Eminently hydraulic lime.