Enhancement and mechanism of macro-defect free (MDF) gypsum water resistance achieved by hydrophobic modification

Macro-defect free (MDF) gypsum is characterized by high strength and has the potential designed to advanced functionalization materials. However, the application of MDF gypsum is still limited by its poor water resistance. Here, we propose to modify the gypsum surface to hydrophobicity at molecule scale by adding potassium methylsilicate. The compressive strength, softening coefficients, and water absorption were subjected to rigorous testing, while elucidating the underlying mechanisms of water resistance enhancement through micro-structural analysis and organic-inorganic interactions. The results show that when adding 0.25% potassium methylsilicate, the wet compressive strength, softening coefficients are increased by 30.1%, 28%, respectively. And potassium methylsilicate can change the gypsum surface to hydrophobicity, which delaying the initial rapid water absorption stage and decrease both the 2 hours and 24 hours water absorption. The addition of potassium methylsilicate leads to a reduction in the dried compressive strength, attributed to the decrease in intrinsic dihydrate crystal strength. Upon the introduction of potassium methylsilicate, there is a transformation in gypsum crystal morphology from strip-like to lamellar structures. This alteration is accompanied by the formation of a potassium methylsilicate film coats the surface of gypsum, impeding water-gypsum contact. The potassium methylsilicate film coats on the surface of gypsum, which hinders the contact of water and gypsum.