A Novel Method of Improving the Water Resistance of Gypsum Using Soluble Salts

The poor moisture resistance of gypsum remains one of the main factors limiting its wider application in construction. This study investigates a novel approach to improving the moisture resistance of gypsum through the addition of soluble salts capable of reacting with dissolved calcium sulfate to form insoluble products within the gypsum matrix. The formation of insoluble reaction products was considered as one of the possible mechanisms contributing to this effect. Three salts were examined—trisodium phosphate dodecahydrate (TSP), potassium sodium tartrate tetrahydrate (PS), and sodium oxalate (SO)—each added at 2 wt.% of gypsum mass. The influence of the salts on phase composition, microstructure, setting behavior, density, porosity, mechanical properties, and water-vapor transport was evaluated. The reference gypsum exhibited compressive strengths of 5.18 MPa and 0.79 MPa and flexural strengths of 3.01 MPa and 0.57 MPa after storage in laboratory conditions and water, respectively. The results showed that salt chemistry strongly affected gypsum performance. TSP significantly altered crystal morphology, accelerated the initial setting time from 16.0 min to approximately 4.0 min, and delayed the final setting to the third day after mixing. Consequently, TSP exhibited the poorest mechanical performance, with compressive strengths of 2.77 MPa and 0.09 MPa and flexural strengths of 2.03 MPa and 0.27 MPa in dry and wet conditions, respectively. In contrast, the organic salts PS and SO preserved a gypsum crystal network similar to that of the reference material. PS achieved compressive strengths of 4.89 MPa and 0.79 MPa and flexural strengths of 2.84 MPa and 0.67 MPa, while SO reached 4.43 MPa and 0.34 MPa in compression and 2.59 MPa and 0.55 MPa in flexure. Moreover, PS and SO improved the flexural softening coefficient by 24% and 11%, respectively, whereas TSP reduced it by approximately 30%. Total porosity ranged from 53 to 61% for specimens stored in laboratory conditions and decreased to 35–39% after water storage. Water-vapor diffusion resistance was affected only marginally, and the vapor-open character typical of gypsum materials was preserved. Among the investigated admixtures, potassium sodium tartrate exhibited the most promising overall performance, maintaining compressive strength after water exposure at the same level as the reference gypsum while improving moisture resistance. The results indicate that the selected organic salts represent a promising route for improving the moisture resistance of gypsum-based materials. However, the present results suggest that the observed improvement cannot be attributed solely to the formation of insoluble reaction products, and further research is required to clarify the relative contribution of the underlying mechanisms.