Ammonium hydrogen phenylphosphonate forms a stable calcium phenylphosphonate dihydrate coating on marble, enhancing cohesion, reducing porosity, and restoring strength with minimal color changes, promising for the consolidation of carbonate stones.

Buildings of historic neighborhoods of Santiago de Chile are protected by a coating system composed of different layers of earth-based mortars, as part of a building culture that has been neglected and forgotten since the introduction of industrialized materials but still exists in many buildings. This study presents preliminary results from ongoing research that explores the hygroscopic capacity of this vernacular coating system and the impact of incorporating recent finishing layers into traditional construction practices. The investigation focuses on identifying materials and techniques typical of traditional Chilean coatings, highlighting their role in enhancing the durability of historic buildings, improving user comfort, and promoting environmental sustainability. It contributes to the conservation of historic buildings and their reuse, as well as to the health of its inhabitants, due to its contribution to hygrometric regulation. This article focuses on this last purpose, through the identification and characterization of the coating system and its finishing layer materials, and a comparative sorption/desorption test of four case studies with these vernacular coatings. This study began with the sample extraction in situ, followed by its observation and cataloguing. Stratigraphic and stereo microscope analysis of the finishing layers were carried out to identify them. The characterization of the finishing materials was performed using FTIR-ATR and SEM-EDX tests. The sorption/desorption test was performed with a set of original complete samples of the four case studies. Subsequently, another set was prepared with the removal of the finishing layers in order to compare their influence on the hygroscopicity of the coating systems. The results elucidate the variety of materials employed on the finishing layer of these coatings, which are often superimposed, revealing renovations and reparations over time. The influence of these finishing materials on sorption properties of the coating system (the scratch and base coats) is exposed by comparing the samples with and without them.

Ammonium N-ethyloxamate (AmEtOxam) was synthesized, fully characterized by microanalytical and spectroscopic means, and assayed as a precursor of calcium oxalate, acting as a protecting agent for white Carrara marble. The monohydrate form of AmEtOxam shows a water solubility of 1.5 mol·L–1 (~23% w/w), significantly higher than that of common calcium oxalate precursors (CaOx), such as ammonium oxalate (0.4 mol·L–1, ~5% w/w). While AmEtOxam is stable in water solution and in the solid state in its monohydrate form, , during the application on carbonate stone it undergoes a complete hydrolysis resulting in the formation of an uniform weddellite layer (CaC2O4·2H2O) on carbonate stone surfaces. Application of 5% w/w aqueous solutions by spraying, brushing, and immersion resulted in different effects. Spraying yielded the most balanced performance, improving mechanical strength, reducing water absorption, recovering superficial tension, and limiting chromatic alteration. Brushing achieved significant gain in surface hardness with minimal aesthetic impact. Immersion most effectively reduced porosity and increased surface tension. These results, coupled with the negligible chromatic changes induced in all cases, make AmEtOxam a promising candidate for the conservation of stone cultural heritage.

Ammonium aminomethylphosphonate monohydrate was synthesized via aqueous neutralization of aminomethylphosphonic acid with ammonium carbonate. The compound was fully characterized by FT-IR and NMR spectroscopy, as well as single-crystal X-ray diffraction (SC-XRD). Structural analysis confirmed the zwitterionic nature of the aminomethylphosphonate anion and revealed an extensive 3D supramolecular network of charge-assisted N–H···O and O–H···O hydrogen bonds.

Ammonium N-ethyloxamate (AmEtOxam) was synthesized, fully characterized by microanalytical and spectroscopic means, and assayed as a precursor of calcium oxalate, acting as a protecting agent for white Carrara marble. The monohydrate form of AmEtOxam shows a water solubility of 1.5 mol·L⁻¹ (~23% w/w), significantly higher than that of common calcium oxalate precursors (CaOx), such as ammonium oxalate (0.4 mol·L⁻¹, ~5% w/w). While AmEtOxam is stable in water solution and in the solid state in its monohydrate form, during the application on carbonate stone it undergoes a complete hydrolysis resulting in the formation of a uniform weddellite layer (CaC₂O₄·2H₂O) on carbonate stone surfaces. Application of 5% w/w aqueous solutions by spraying, brushing, and immersion resulted in different effects. Spraying yielded the most balanced performance, improving mechanical strength, reducing water absorption, recovering superficial tension, and limiting chromatic alteration. Brushing achieved significant gain in surface hardness with minimal esthetic impact. Immersion most effectively reduced porosity and increased surface tension. These results, coupled with the negligible chromatic changes induced in all cases, make AmEtOxam a promising candidate for the conservation of stone cultural heritage.