New nano-Mg(OH)2 modified siloxane coating for the preservation of gypsum and gypsum-based plasters artifacts

Gypsum is, today, mainly used for decorative or finishing purposes, but historically it has also been applied as a structural material in many constructions. From the Middle Ages to the Baroque age, gypsum was the preferred binder for masonry mortars and decorative plasters but also as blocks for structural elements. Many examples of great architectural richness made with plaster are found in the Mediterranean areas, where plaster has played an important role in traditional and monumental architecture [1]. Due to its high brittleness, low water resistance and low mechanical strength, gypsum may be subject to physical, chemical and biological degradation. This work discusses the synthesis of a new hydrophobic product based on organically modified siloxane and nanosized magnesium hydroxide, and its possible application in the field of conservation of ancient and contemporary architectures. The hydrophobic product was tested as coating applied on gypsum and gypsum-based plasters, and as consolidant added to building materials as mortars and plasters. The organically modified siloxane was synthesized by acid catalyzed sol-gel process using tetra-ethoxysilane (TEOS) and hydroxyl-terminated poly-dimethylsiloxane (PDMS) as precursors while nano-Mg(OH)2 was prepared by precipitation method in alkaline medium NaOH, using MgCl2 as precursor and urea as templating agent. The new hydrophobic product was obtained by mixing the two components in 1:1 volume ratio. Raman, TEM and XRD analyses confirm the crystalline nature of Mg(OH)2 with crystallite size of about 15-20 nm. The hydrolysis of TEOS and the copolymerization of TEOS/PDMS was assessed by FTIR analysis. The effectiveness as protective coating of the hydrophobic nano-composite was investigated by capillary water absorption, and static contact angle measurements on gypsum: the water capillary coefficient decreased more than 50%, and contact angle values higher than 150° were observed. The total color differences within acceptable limits and the small decrease of the water vapor permeability confirmed the harmlessness of the treatment. The influence of the new hybrid hydrophobic product on the carbonation resistance and on the mechanical properties of mortars and plasters was demonstrated by accelerated carbonation tests and measurements of flexural and compressive strength. The mechanical properties significantly improved, showing an increase in flexural and compressive strength of about 25% and 30%, respectively, compared to rough plaster, and about 10% and 15%, respectively, compared to reference mortar samples. The depth of carbonation (measured after 84 days) in the treated material was a few millimeters, while in the untreated ones it was evident in almost the entire cross section (20 mm). The new nano-composite hydrophobic product, both as coating and as additive, is therefore a promising material for protection and consolidation of monuments and buildings of cultural heritage interest.