What actually happens to the stone
Three mechanisms, one page: why limestone fails, why sealing makes it worse, and what the test data shows.
The difference that decides everything
Globigerina limestone is roughly 30% air by volume. That porosity is not a defect — it's how the wall handles moisture. Water enters, and water leaves.
What holds the grains together is a calcite binder. Rain is mildly acidic, and over decades it dissolves that binder out of the outer few millimetres. The grains stay put but nothing binds them. That's the powder on your windowsill.
Two treatments exist for this, and they do opposite things.
A sealer deposits a film across the surface. Water can't get in. Water also can't get out. The pore structure is closed at the face.
A consolidant travels into the damaged zone and re-forms a binder around the loose grains, in place. The pore structure stays open. The stone is stronger and no less breathable than it was.
Consolidate + Protect does the second, and adds water repellency at the pore wall rather than across the surface — so the stone sheds liquid water while still passing vapour.
salts crystallise
Film sits on the face. Pore structure closed.
re-bound zone
Binder re-formed in place. Pores stay open.
Moisture always gets in. What matters is whether it can leave.
Every masonry wall contains moisture — rain that got in before the treatment, groundwater rising through the fabric, vapour from inside the building. There is no realistic condition in which a limestone wall is dry throughout.
While the face is open, that moisture evaporates through it continuously. The wall reaches equilibrium and stays there.
Close the face with a film and the equilibrium breaks. Moisture keeps arriving and can no longer leave. It accumulates behind the film. Dissolved salts travel with it, reach the barrier, and crystallise. Crystallisation exerts pressure — enough to detach the surface layer of stone from the material behind it.
The wall then fails in sheets rather than in dust. Faster, and far more expensive to repair.
This is why the specification for heritage limestone has been "breathable" for as long as anyone has been writing specifications for it, and why we test vapour permeability first.
equilibrium held
zone · surface detaches
What was measured, and what it means
Testing by GFC Chimica on globigerina limestone samples.
| Property | Standard | Result | In plain terms |
|---|---|---|---|
| Water vapour permeability | EN ISO 7783 | Class V1, sd 0.08 m | Highest permeability class. The wall breathes essentially as it did untreated. |
| Capillary water absorption | EN 1062-3 | Class W3, w 0.04 kg/m²·h0.5 | Lowest absorption class. Driven rain runs off instead of soaking in. |
| Fungal growth | EN ISO 846 | Rating 0 | No growth observed on treated samples under test conditions. |
| Penetration depth | Sectioned samples | 12–18 mm | Reaches well past the weathered zone into sound stone. |
| Surface cohesion | Drilling resistance | +180% | Powdering surfaces measurably re-bound. |
| Water absorption reduction | — | 87% | Compared with untreated control. |
| Colour change | ΔE | ≤2.1 typical, 3.4 maximum | Below the threshold of casual visibility in most cases. See below. |
The honest limits
It will not cure damp.
Rising damp, penetrating damp and condensation are water-source problems. This is a surface treatment. Applying it to a wall with an unresolved moisture source will not fix the wall and will not last.
It can lighten very porous stone.
Typically ΔE 2.1 or less, which almost nobody notices. On the most absorbent material it reaches 3.4, which is visible against untreated stone. Always test.
It will not rebuild lost material.
Consolidation binds what remains. Stone that has already gone needs a mortar repair or a new block.
It cannot penetrate a film.
Painted or previously sealed surfaces must be fully stripped first.