Replacement stone for a salt-weathered Renaissance facade in Úbeda

Quarry A is a clay-bearing calcarenite, a clastic/bioclastic limestone with a matrix-supported fabric. Its 24% porosity and clay matrix give high capillary absorption, moderate permeability, low strength and strong sensitivity to salt crystallization and frost. It is physically closest to the historic stone if the source is similar. Quarry B is a spar-cemented bioclastic limestone with low porosity. It is stronger and less absorptive, but its low permeability can trap water and salts at the interface with the old stone, causing spalling of the weaker host. Quarry C is a metamorphic marble with almost no porosity. It is strong but essentially impermeable and rigid, so moisture and salts concentrate at the joints; its thermal expansion is also different from the porous limestone. In a salt-laden, condensing cornice, A will decay most rapidly if unprotected, but B and C create a worse compatibility problem at the interface. The key prediction is not simply durability but where water and salts will accumulate.

First choice is Quarry A, provided its salt content, clay mineralogy and pore-size distribution match the historic calcarenite and it is pre-treated by desalination. Compatibility matters more than strength: a porous, weak stone allows water to migrate and evaporate at the surface, as the original fabric does. Second is Quarry B, acceptable for hidden or load-bearing elements but risky for the decorated cornice because the permeability contrast can concentrate salts at the interface. Third is Quarry C; despite its high strength, it is the worst choice for patching a porous limestone. Its low porosity and stiffness make it an impermeable barrier, and thermal expansion mismatch can open joints. The cement mortar from the previous repair is part of the problem: it is rigid and low-permeability, trapping salts in the limestone. The repair should use a lime-based mortar with similar porosity and flexibility, not a stronger cementitious one. Replacement should follow desalination and water control, otherwise the new stone will decay in the same way.

Water enters the cornice by condensation and capillary rise. It dissolves salts from the stone, mortar and ground; as it evaporates, salts crystallize in pores. Sodium sulfate and magnesium sulfate can hydrate and dehydrate, producing crystallization pressure; calcium sulfate forms gypsum. The black crust is gypsum produced by reaction of atmospheric sulfur dioxide with calcite, and it also traps soot. Cement mortar worsens the cycle by reducing evaporation and concentrating salts in the limestone. The treatment sequence should be: (1) reduce water ingress by repairing the roof and gutters and installing drainage; (2) apply poultices to extract soluble salts, repeating until conductivity falls; (3) remove cement mortar and repoint with a breathable lime mortar; (4) replace only severely lost stone with a compatible, pre-desalinated stone; (5) avoid sealants and water-repellent coatings, which trap salts behind the surface; (6) monitor salt content, condensation and stone loss. Replacing stone without controlling water and salts simply moves the decay to the new material.