Breathable Aerial Lime and Brick Dust Hydraulic Pointing: Mortar Formulation for Bressan Half-Timbering

In the restoration and maintenance of half-timbered architecture (colombage), mortar is not merely an adhesive glue; it is the building's respiratory system. Modern 20th-century construction introduced Portland cement mortars under the false assumption that harder and denser materials are inherently superior. In half-timbered buildings, however, Portland cement acts as an instrument of destruction.

Portland cement cures through rigid tricalcium silicate hydration into an impermeable, rock-hard matrix with high compressive strength (over 30 MPa) and virtually zero vapor permeability (\(S_d > 2.0\text{ m}\)). When applied between flexible oak studs and porous red clay bricks, cement creates an impermeable barrier that traps internal ground moisture and driven rain. Unable to evaporate, trapped water saturates the adjacent oak timbers, initiating catastrophic fungal dry rot (Serpula lacrymans) and spalling the soft brick faces during winter freeze-thaw cycles.

The authentic, scientifically compatible binder for Bressan half-timbering is chaux aérienne éteinte (aerial slaked lime, classification CL-90). Manufactured by burning pure limestone (\(\text{CaCO}_3\)) at 900°C and slaking the resulting quicklime with water, aerial lime consists of pure calcium hydroxide (\(\text{Ca(OH)}_2\)).

Unlike cement, aerial lime does not harden through chemical reaction with water. Instead, it cures exclusively through carbonation—slowly absorbing atmospheric carbon dioxide (\(\text{CO}_2\)) to regenerate natural calcium carbonate:

This crystalline mineral matrix possesses extraordinary microscopic porosity. It yields a vapor diffusion resistance rating of \(S_d < 0.05\text{ m}\), allowing liquid water trapped within timber or bricks to evaporate rapidly into the air as moisture vapor, maintaining the structural framework in a perpetually dry, rot-free state.

While pure aerial lime is the ultimate breathable binder, it possesses one distinct operational vulnerability: because it requires atmospheric carbon dioxide to carbonate, deep mortar joints shielded from open air can take many months to achieve structural set. In the damp climate of eastern France, early autumn frosts could freeze wet lime mortar before carbonation was complete.

Bressan masons solved this through the addition of chamotte (pozzolanic brick dust). When low-fired red clay bricks or roof tiles are ground into a fine powder, their amorphous aluminosilicates become chemically reactive. When mixed into wet aerial lime putty, these pozzolanic particles react with calcium hydroxide in the presence of water to form hydrated calcium silicates and aluminates. This imparts a gentle, internal hydraulic set that cures deep inside the joint within 48 to 72 hours, while preserving the high vapor permeability and elastic flexibility of the aerial lime matrix.

Executing durable lime pointing in half-timbered walls requires precise masonry discipline. Masons begin by raking out all deteriorated mortar to a depth of at least twice the joint width (20 to 25 mm). Crucially, the joints and surrounding bricks must be thoroughly drenched with clean water over 24 hours prior to pointing; if dry, porous bricks will wick moisture out of the fresh lime mortar, causing it to dry prematurely into a weak, powdery paste.

The mortar is packed firmly in thin 10 mm layers using narrow spring-steel pointing irons (fers à jointoyer). To ensure longevity, the outer mortar face is never feathered over the timber flanks; instead, it is finished with a crisp, slightly recessed profile (1 to 2 mm behind the face of the oak stud). This prevents the edge of the mortar from chipping when the oak timber swells during humid seasons. Finally, fresh pointing is shielded beneath damp hessian burlap for seven days, allowing the lime to carbonate slowly into an enduring, breathable mineral bond.