Novel calcium carboxyphosphonate/polycarboxylate inorganic-organic hybrid materials from demineralization of calcitic biomineral surfaces

Profiling Novel, Multifunctional Silane-Phosphonate Consolidants for the Mitigation of Gypsum Stone Deterioration via Concerted Autocondensation/Surface Complexation Processes

Mineral gypsum (selenite) stones have been used extensively by ancient Cretans in the Minoan Palace of Knossos (Crete, Greece), mostly for building and ornamental purposes. Exposure of mineral gypsum to environmental stresses (temperature fluctuations, rain, air-borne pollutants, soluble salts, etc.) causes solubility-driven degradation, and loss of cohesion of the crystal aggregates, with ensuing aesthetic degradation. In this work, the efficiency of four consolidants for artificial gypsum specimens is presented and evaluated based on drilling resistance measurements [drilling resistance measuring system (DRMS)]. Two of them (commercial names RC-70 and RC-90, RC = Rhodorsil Consolidante) are alkoxysilane-based and they are considered as benchmark consolidants. The other two [3-(trihydroxysilyl)propyl methylphosphonate monosodium salt, TRIMEPHONA, and 3-(trihydroxysilyl)propylamino-diphosphonate, TRIPADIPHOS] are multifunctional consolidants because they possess a self-condensable (after hydrolysis) trihydroxysilyl [-Si(OH)3] moiety and phosphonate groups (one in the former, two in the latter). Consolidants RC-70 and RC-90 exhibit rather low consolidation effectiveness. This is not unexpected, as these are alkoxysilane-based and act simply as "fillers" for the pores of the gypsum. Consolidant TRIMEPHONA demonstrates an enhanced level of consolidation action. This is due to its double functionality, i.e., the presence of an anionic phosphorus-based moiety that anchors onto the gypsum surface, and a condensable silane triol [-Si(OH)3] unit. Consolidant TRIPADIPHOS shows excellent gypsum consolidation features and is much more efficient (per unit concentration) than all other tested consolidants. This is assigned to its better gypsum anchoring ability via surface Ca-complexation. Selected compressive strength studies were performed on gypsum samples treated with the phosphorus-based consolidants, and corroborate the findings from DRMS. To shed further light on possible binding modes of the phosphonate moiety on surface Ca2+ sites in gypsum, two model compounds were synthesized and structurally characterized, Ca-C2D and Ca-C3D (C2D = ethylamino-di(methylenephosphonic acid) and C3D = propylamino-di(methylenephosphonic acid).

Crystal structure of aqua-tris-{μ-N-[bis(diethyl-amino)phosphoryl]-2,2,2-tri-chloroacetamidato-κ3O,O':O}calciumsodium

In the mol-ecular structure of the title compound, [CaNa(C10H20Cl3N3O2P)3(H2O)], the Ca2+ ion has a slightly distorted octa-hedral coordination environment defined by six O atoms which belong to the carbonyl and phosphoryl groups of the three coordinating ligands. Two Cl atoms of CCl3 groups and four O atoms form the coordination environment of the Na+ ion: three from the carbonyl groups of ligands and one O atom from a coordinating water mol-ecule. In the crystal, the bimetallic complexes are assembled into chains along the c-axis direction via O-H⋯O hydrogen bonds that involve the coordinating water mol-ecules and the phosphoryl groups.