Hydrophobic hydrogels as internal curing agent for concrete: The double benefit of super high water content and excellent anti-ion permeability

Owing to their high water content, conventional superabsorbent polymer (SAP) hydrogels can serve as internal curing agents for concrete fabrication. However, the hydrophilic network in such hydrogels permits ion diffusion, which induces severe steel corrosion inside concrete in the marine environment. Herein, we introduce a brand-new hydrogel, termed "hydrophobic hydrogels", for concrete curing. On one hand, these hydrogels possess super high water content (up to 99.6 wt%), which is even higher than that in normal SAP hydrogels; on the other hand, the anhydrous inhomogeneous hydrophobic structures can significantly retard or stop the ion permeation, which is significantly superior to SAP hydrogels. With ion diffusion measurement, the hydrophobic hydrogels achieve much better ion resistance than SAP hydrogels, with 2-5 orders' decrease in ion permeation rate. Among diverse hydrophobic hydrogels, the polymer's hydrophobicity and glass transition directly determine the permeation performance. Hydrogels with a more hydrophobic network and higher polymer T-g possess better anti-ion diffusion properties. At the optimal state, the poly(phenyl acrylate) hydrophobic hydrogels can completely stop the permeation process. Owing to the double benefit, the hydrophobic hydrogel cured concretes demonstrate a notable decrease in ion permeation compared to SAP hydrogel cured concrete, with only 1/20 of ion permeation to that in SAP groups with an electric flux test upon coated on the surface layers. Owing to the double benefit from super high water content and excellent anti-ion permeability, these hydrophobic hydrogels may serve as ideal candidates as concrete curing agents in the future.