The Lime vs. Cement Debate

The Hidden Danger of Cement on Victorian Bricks: Why “Modern” Mortar is Killing London’s Heritage

For any owner of a pre-1930s property in London, the choice of mortar is the single most important factor in the longevity of the building. At Pointing by Tony Wallace, we spend a significant portion of our time performing “remedial restoration”—undoing the catastrophic damage caused by previous builders who used modern Portland cement on historic facades.

The Architecture of Breathability: How London Houses Work

To understand why cement is dangerous, one must first understand the engineering of a Victorian or Georgian terrace. These are “solid wall” constructions. Unlike modern homes built with cavity walls and plastic damp-proof membranes, historic buildings rely on a “Transpiration Cycle.”

In this cycle, the walls act like a sponge. They absorb a controlled amount of moisture from the atmosphere and the ground, and then—critically—they allow that moisture to evaporate freely. The bricks (typically London Stocks, Gaults, or Red Rubbers) and the original mortar were designed to be vapor-permeable.

The Mechanics of Failure: Why Cement is a “Death Sentence”

When a general builder uses a standard 3:1 sand-to-cement mix on an old building, they are effectively wrapping a breathable organism in a plastic bag.

1. Vapor Trapping & Hydrostatic Pressure: Cement is non-porous. When moisture enters the brick (which it inevitably will through microscopic cracks or the ground), it can no longer escape through the mortar joints. It becomes trapped behind the cement “pointing.” As more moisture accumulates, hydrostatic pressure builds up behind the hard cement face.
2. The Spalling Cycle: This is the most visible form of damage in London. During the winter, the trapped water freezes. Because water expands by roughly 9% when it turns to ice, it exerts immense internal pressure on the brick. Since the cement is harder than the brick, the face of the brick is forced to shatter outward. This is called spalling. Once the hard “fire-skin” of the brick is lost, the soft interior of the brick erodes rapidly, leading to structural hollows.
3. Salt Crystallization (Subflorescence): As water is forced to evaporate through the brick instead of the mortar, it carries soluble salts to the surface. These salts crystallize just beneath the brick’s surface. The pressure of these growing crystals (cryptoflorescence) shreds the internal structure of the brick from the inside out.

The Structural Mismatch: Hardness vs. Flexibility

London is built on a massive basin of clay. London clay is notorious for being “active”—it expands when wet and shrinks when dry. Consequently, London terraces are constantly in a state of micro-movement.

Mortar is designed to be the sacrificial element of a wall. It should be slightly softer than the brick so that any movement is absorbed by the mortar joints, causing minor, repairable hairline cracks. Portland cement is incredibly rigid and brittle. When a building moves, a cement joint will not flex; instead, it transfers all the stress directly into the bricks, causing them to snap or shear.

The Solution: Natural Hydraulic Lime (NHL)

At Pointing by Tony Wallace, we exclusively use Natural Hydraulic Lime (NHL) for period restorations.

• Permeability: NHL allows the building to “wick” moisture away from the bricks and into the atmosphere.
• Flexibility: Lime can accommodate the seasonal movement of London’s clay foundations without cracking.
• Self-Healing: Through a process called autogenous healing, lime can actually seal small hairline cracks when moisture reacts with free lime in the mortar.

The Expert Verdict

If your property was built before 1930, your mortar must be lime-based. Using cement is not a “stronger” fix; it is a slow-motion demolition of your property’s value.