Process and mechanisms of multi-stage water sorptivity in hydrophobic geopolymers incorporating polydimethylsiloxane

Geopolymers as emerging green cementitious materials have attracted much attention, while they can be infiltrated by water involving harmful ions due to their hydrophilic matrix and porous structure. In this work, metakaolin-based geopolymers that hydrophobically modified by polydimethylsiloxane (PDMS) were comprehensively investigated in terms of integral wettability, pore structure, and water sorptivity kinetics. Results demonstrate that an ultra low sorptivity stage occurs at the beginning of the water absorption process for hydrophobic geopolymers, which is derived from the change in pressure drop caused by the PDMS layer on pore walls. Although the low sorptivity stage is temporary due to continuous moisture diffusion, it is of great significance for the application of geopolymers in waterproof engineering. The hydrophobic matrix (contact angles: ∼130°) and the improved pore structure jointly contribute to low water sorptivity and transport of modified geopolymers. Moreover, a multi-stage water sorptivity model was established to further explain the sorptivity process and its mechanisms. •Geopolymers modified with PDMS become hydrophobic with contact angles of ∼130°.•An ultra-low sorptivity stage occurs due to the PDMS layer on pore walls.•A multi-stage water sorptivity model is proposed.