Thermodynamic characteristics of hydrogen bonding formation in gypsum-based materials and its effects on mechanical performance

Using gypsum coatings in vertical sealing systems for internal buildings' environments has become popular in civil construction. Rapid drying, reduced thickness, and the absence of roughcasting or substrate moistening, in addition to the excellent finish, contribute to the growing use of this coating. The main goal of the present article is to evaluate the influence of hydrogen bonds on the mechanical properties of gypsum pastes for coating. Gypsum coatings with a w/g ratio of 0.6, 0.8, and 1.0 were prepared and applied to ceramic and concrete block substrates with three different compressive strength classes: low, medium, and high resistance. Chemical, structural, and energetic characteristics of hydrogen bond complexes involving gypsum pastes were investigated by quantum chemical calculations. The results revealed that higher amounts of water in the coating lead to stronger and more stable hydrogen bonds, where for a/g= 0.9 the enthalpy of hydrogen bond formation is equal to -187.05 KJ/mol, and provides lower mechanical performance, with values of compressive strength, flexural tensile strength and surface hardness equal to 3.16; 1.95 and 7.38 MPa, respectively. When these bonds are weak and unstable, at low w/g ratios, they provide higher tensile bond strength in ceramic blocks and lower tensile strength in concrete blocks, equal to 0.54 and 0.40 MPa, respectively. In conclusion, hydrogen bonds play an important role in the adhesion strength of gypsum coatings on porous substrates, with higher mechanical performances associated with weak hydrogen bonds.