Cantera Salt Efflorescence Remediation, Low-Pressure Steam Desalination, and Breathable Alkylalkoxysilane Protection

Due to its high open capillary porosity (often exceeding 30%), Cantera acts as a massive hydraulic pump. In Mexican colonial plazas, rising damp carries dissolved soluble salts—principally sodium chloride ($NaCl$), sodium sulfate ($Na_2SO_4$), and potassium nitrate ($KNO_3$)—upward through foundation footings. When groundwater evaporates at the stone surface, salt concentration increases until saturation is reached.

If evaporation is rapid, salts precipitate beneath the surface within the microscopic pore throats (a phenomenon known as cryptoflorescence or sub-florescence). As anhydrous sodium sulfate hydrates into thenardite ($Na_2SO_4 \cdot 10H_2O$), its molar volume expands by over 300%, generating crystallization pressures exceeding 150 MPa within the pores. Because the tensile strength of porous volcanic tuff is typically only 2 to 4 MPa, these crystallization forces easily shatter the glassy ash matrix, causing the stone surface to disintegrate into loose volcanic sand (arenización) and spall in thick, jagged sheets.

Treating salt-contaminated Cantera requires multi-cycle extraction. Conservators apply a sacrificial poultice formulated from a 1:1 blend of purified Arbocel cellulose fiber pulp and sepiolite (hydrated magnesium silicate clay) slaked with pure deionized water (< 5 µS/cm conductivity). The wet poultice (15 mm thick) is applied across the stone face and covered with polyethylene film for 48 hours to allow water to dissolve crystallized salts deep within the capillaries.

The plastic film is then removed. As the poultice dries from the outside, capillary evaporation draws the salt-saturated water outward into the cellulose fibers where the salts precipitate harmlessly. Poultice cycles are repeated until conductivity measurements of the extracted water drop below 200 µS/cm. Following desalination, biological algae, lichens, and urban soot are cleaned using low-pressure micro-steam (saturated dry steam at 120°C delivered at 0.5 to 1.5 bar pressure). The thermal energy melts organic residues and softens gypsum crusts without eroding the fragile volcanic ash matrix.

Once the stone is clean and desalinated, crumbling volcanic grains must be structurally re-bonded. Conservators saturate the stone with tetraethyl orthosilicate (TEOS). The low viscosity of monomeric TEOS allows it to penetrate up to 40 mm into the porous volcanic tuff. Over a 3 to 4-week reaction period, TEOS hydrolyzes with ambient atmospheric moisture and condenses, depositing a resilient, three-dimensional network of amorphous silica ($SiO_2$) that permanently binds the loose glass shards and phenocrysts back into a cohesive stone body.

To protect the consolidated stone from future driving rain without creating an impermeable vapor barrier, conservators apply a breathable alkylalkoxysilane water repellent (such as isooctyltriethoxysilane). Unlike acrylic sealers that form an airtight plastic film over the surface, silane molecules are tiny enough to penetrate deep into the capillaries, chemically bonding to the silicate minerals inside the pores. The organic alkyl tails project outward into the pore throats, lowering surface energy to make the pores strongly hydrophobic. Liquid rainwater beads up and runs off the surface, while the open pore channels remain 100% breathable, allowing indoor water vapor ($Sd < 0.05\text{ m}$) to vent freely to the atmosphere.