Why is lime-putty mortar preferred over cement-based mortars when repointing historic brickwork?

• A. Lime-putty is softer and more permeable, preventing damage from rigid cement mortars and preserving moisture movement.
• B. Lime-putty is harder and less permeable, increasing strength.
• C. Lime-putty dries faster and provides better water resistance.
• D. Lime-putty is cheaper and readily available.

Answer: (A)

When repointing historic brickwork, you want the mortar to be part of a breathable, flexible system that can move with the masonry without trapping moisture. Lime-putty mortar fits that need because it is softer and more permeable, so moisture can pass through and move through the joints as it did with the original historic construction. This compatibility helps prevent moisture buildup behind the mortar, which can lead to brick spalling, cracking, or other water-related damage over time.

Cement-based mortars, by contrast, are harder and far less permeable. They resist moisture movement and can trap moisture inside the wall, creating stress that can damage bricks and joints, especially during freeze-thaw cycles. Lime mortar cures by carbonation and remains relatively flexible, which aligns with the historic brickwork’s behavior and makes repairs easier in the future.

So the preferred choice is lime-putty mortar because its softness, permeability, and breathable nature preserve the masonry system and reduce the risk of moisture-related damage, unlike cement mortars which can cause such problems.