In basic civil engineering materials, lime mortar used in traditional masonry construction is generally prepared using which type of lime?

Introduction / Context:
Lime mortar is one of the oldest binding materials used in masonry construction, especially before Portland cement became common. It is made by mixing lime, sand, and water to form a workable paste that hardens over time. Civil engineering students need to know which type of lime is generally used for preparing this mortar in standard building practice. The answer reflects how different kinds of lime behave in the presence of water and air.

Given Data / Assumptions:

• The question is about lime mortar, not cement mortar or other binders.
• Options mention hydraulic lime, quick lime, fat lime, and plain lime with only water.
• Lime types differ in calcium content, impurities, and how they set (hydraulically or in air).
• We assume typical masonry work above ground level, in ordinary building construction.

Concept / Approach:
Lime can be broadly categorised into fat (high-calcium, non-hydraulic) lime and hydraulic lime. Fat lime contains high calcium content with few impurities and sets slowly by reacting with carbon dioxide in the air (carbonation). It is used for making good quality lime mortar for plastering and masonry in dry conditions. Hydraulic lime contains clay and other impurities and can set even under water, making it more suitable for damp conditions and foundations. Quick lime is the unslaked form that reacts vigorously with water to form slaked lime; it is not used directly in mortar without proper slaking and mixing. Plain lime mixed only with water without sand does not form a proper structural mortar. Therefore, traditional lime mortar is generally prepared using fat lime that has been slaked and mixed with sand in the correct proportions.

Step-by-Step Solution:
Step 1: Recall that lime mortar is a mixture of lime, sand, and water, used as a binding material between bricks or stones. Step 2: Identify that fat lime (high-calcium non-hydraulic lime) is commonly used for making good quality mortar for masonry and plastering. Step 3: Understand that hydraulic lime is more specialised, used where setting is needed under water or in very damp conditions, not as the general choice for ordinary mortar. Step 4: Note that quick lime is highly reactive and must first be slaked with water; it is not used directly as quick lime in mortar. Step 5: Recognise that plain lime mixed only with water lacks sand and does not produce a structurally sound mortar for masonry joints. Step 6: Conclude that fat lime slaked and mixed with sand is the standard lime used for preparing lime mortar.

Verification / Alternative check:
Building materials textbooks often state that fat lime is suitable for plastering and masonry work because it has high plasticity and produces a smooth, workable mortar when mixed with sand. They also explain that hydraulic lime is preferred for structures exposed to moisture or for underwater works, where non-hydraulic lime would set too slowly or be washed away. Practical construction guidelines further emphasise that quick lime must be converted into slaked lime (powder or putty) before use. These consistent descriptions confirm that fat lime is generally used for making lime mortar in typical building applications.

Why Other Options Are Wrong:
Hydraulic lime that sets under water without exposure to air is used in special situations like damp foundations, not generally for all lime mortars in ordinary masonry work.
Quick lime used directly without slaking or mixing is unsafe and impractical; it must be slaked first to form slaked lime before adding sand and water.
Plain lime mixed only with water and no aggregate does not provide the necessary bulk, strength, or workability for a proper mortar in masonry joints.

Common Pitfalls:
Students sometimes assume that because hydraulic lime sounds “stronger,” it must always be used for mortar. However, the suitability of lime depends on the application. For many traditional above-ground walls and plaster, fat lime is preferred for its workability and finish, as long as the structure is not constantly wet. Another confusion is treating quick lime, slaked lime, and hydraulic lime as interchangeable, when in fact their properties and uses differ significantly. Remember that for standard lime mortar in typical masonry, the usual answer is fat lime mixed with sand.

Final Answer:
Lime mortar is generally made with Fat lime (high-calcium non-hydraulic lime) slaked and mixed with sand.