Restoration of old masonry — lime, rewetting and conservation

Much of Danish masonry work is not about new construction, but about keeping the old alive: churches, manor houses, town houses, and protected façades. Different rules apply here than in new construction, because the old walls are built with soft stone and lime mortar and must be able to 'breathe'. If you use modern, strong cement mortar to repair them, you often do more harm than good.

Old masonry is diffusion-open: moisture that enters can evaporate out again through the soft stone and soft lime mortar. Cement mortar, on the other hand, is strong, hard and dense. If you repoint an old wall with cement, the mortar becomes tighter and stronger than the stone. Then the moisture can no longer escape through the mortar — it is forced out instead through the soft stone, where salts and frost then shatter the stone's surface. The result is spalling, damaged stones around hard, intact cement mortar — the exact opposite of what is desired.

Building lime is declared according to DS/EN 459-1. The two main families are air lime and hydraulic lime. Air lime (CL — calcium lime, e.g. CL 90) is pure slaked lime, which only sets by absorbing CO₂ from the air (carbonation); it is the softest and most diffusion-open and sets slowly. Natural hydraulic lime (NHL) also contains hydraulic components and sets both by carbonation and by reaction with water — it comes in three strengths.

Repointing is to replace weathered joints without touching the stones. The old joints are scraped out to a depth of typically at least twice the joint width, so the new mortar gets a proper grip — but carefully, so the stone's edges are not damaged. This is precisely why conservation guidelines warn against scraping joints with an angle grinder: the disc easily cuts into the soft stone and leaves ugly marks. The new joint is laid in a lime mortar, adapted to the original, and compressed and finished so it closes tightly to the stone.