Lime mortar – selecting the appropriate type of lime for general masonry Lime mortar for general building work is commonly prepared using which type of lime?

Introduction / Context:
Lime mortars have been used for centuries in masonry. Choosing the correct type of lime affects workability, setting mechanism, and durability. For typical, non-structural general-purpose masonry, the traditional choice is fat lime (also called high-calcium or non-hydraulic lime) after proper slaking and maturation.

Given Data / Assumptions:

• General building work (not marine or aggressive environments).
• Traditional lime-sand mortars without modern cement additions.
• Proper slaking and seasoning of lime before use are assumed.

Concept / Approach:

Fat lime provides excellent plasticity and workability for bedding and finishing mortars. It sets primarily by carbonation (reaction of calcium hydroxide with atmospheric CO2), which is slow but adequate for many traditional applications. Hydraulic lime, containing clayey impurities, sets with water and is chosen where earlier strength or damp conditions exist; however, in standard exam contexts, “lime mortar” generally implies fat-lime mortar unless specified otherwise.

Step-by-Step Solution:

Verification / Alternative check:

Historic building manuals and materials texts consistently describe fat-lime mortar as the standard for general-purpose work, with hydraulic lime specified where water exposure or early strength is needed.

Why Other Options Are Wrong:

Quicklime directly: Unsafe and unsuitable; must be slaked first.

Hydraulic lime: Useful, but not the default “general” choice in classic exam framing.

Plain lime (unspecified) / None of these: Ambiguous or incorrect given the traditional definition.

Common Pitfalls:

Confusing quicklime with hydrated lime; assuming faster set of hydraulic lime is always preferred; neglecting proper slaking and aging for fat-lime mortars.

Final Answer:

Fat lime (high-calcium, non-hydraulic)