In architectural restoration there's a mistake that recurs with a frequency wildly out of proportion to its severity: plastering or repairing historic lime masonry with a cement mortar, because it's "stronger" or because it's available ready-mixed in a bag at the nearest building-supplies shop. The intervention seems to work — in fact, it seems to work better than the original, because cement is harder and more waterproof. It's exactly this apparent superiority that makes it a serious technical error, not an improvement.
The difference that matters: vapor permeability
A historic masonry wall of stone, fired brick, or tuff, built before the advent of Portland cement, was conceived — even if not in those technical terms — as a breathable system: the moisture that inevitably enters from the ground through rising damp, from rainfall, or from internal condensation needs to be able to evaporate outward through the pores of the plaster. Air lime, with its high vapor permeability (diffusion resistance, expressed as an equivalent air-layer thickness sd, on the order of 0.01–0.10 m for typical values), lets this process happen without significant obstruction. Portland cement, with its much denser and more closed pore structure, has drastically lower vapor permeability: in practice, it acts as a barrier.
The problem isn't that the wall gets wet — every historic wall gets wet, to a greater or lesser degree, over its lifetime. The problem is what happens to that water once it's inside. With a lime plaster, it evaporates outward. With a cement plaster, it stays trapped in the masonry underneath, often for years, because it no longer has a low-resistance way out.
The damage mechanism: salts and freeze cycles
Trapped moisture isn't itself the direct cause of decay — the two mechanisms it feeds are. The first is salt crystallization: soluble salts present in rising damp or in the masonry materials themselves crystallize near the evaporation surface as the water evaporates. If that surface is blocked by a waterproof plaster, the evaporation point shifts deeper into the masonry, and the salt crystals grow inside the pores of the stone or brick, not on the plaster: it's there, not on the surface, that crystallization pressure breaks down the material's structure, often invisibly until whole sections of wall collapse. The second mechanism is frost spalling: trapped water that freezes expands by about 9% in volume, fracturing the micropores from within at every seasonal freeze-thaw cycle.
The compatibility principle
The correct technical criterion for a restoration intervention isn't "which material is strongest in absolute terms", but "which material is mechanically and physically compatible with the existing masonry". That means three simultaneous conditions: the repair plaster's mechanical strength must be equal to or lower than (never higher than) that of the underlying masonry, so that any movement is absorbed by the sacrificial plaster rather than by the irreplaceable historic masonry; a compatible coefficient of thermal expansion, to avoid cyclical stress at the interfaces; vapor permeability equal to or higher than the original. A plaster that is stiffer and more waterproof than its substrate violates all three conditions at once — which is why its apparent performance advantage translates, in practice, into damage that is delayed but more severe than the damage it was meant to fix.
Diagnose before intervening
The practical corollary is that a serious restoration intervention always starts from a diagnosis, not from a product choice. The source of the moisture must be identified (rising damp, roof infiltration, internal condensation), the existing masonry must be characterized (type of stone or brick, original mortar, presence of salts), and only then is the repair mortar chosen — typically air lime for weak historic masonry, natural hydraulic lime (NHL) at a calibrated hydraulicity for stronger masonry or masonry exposed to more moisture, always checking grain-size and color compatibility with the original. A well-chosen repair plaster doesn't "fix" the wall in any absolute sense — it lets it keep doing what it has always done: breathe.