Superabsorbent polymers: On their interaction with water and pore fluid

Superabsorbent polymers are hydrogels capable of absorbing considerable amounts of water without dissolving, and may have wide application in building materials. They were initially used in the concrete industry with high performance cement-based materials to limit the drop in internal relative humidity during the hydration process of cement. This assures the optimum development of the material properties and avoids both self-desiccation and autogenous shrinkage. The mechanisms involved in the kinetics between the hydrogel and water or pore fluid within the cement-based system are not fully understood. This paper addresses the absorption kinetics of superabsorbent polymers in water and synthetic pore solution. The physical state of particles selected from a superabsorbent polymer sample with a particle size distribution between 50 and 500 micrometers is examined by means of optical microscopy and subsequent image analysis. This method has proven to be an accurate technique for quantifying the absorption of water or fluid into the cross-linked structure of superabsorbent polymers. As a result, the size of the polymers particles has a significant effect on the absorption kinetics of these polyacrylate-based polymers. A model based on the well-known Fick’s second law has been found to accurately express the physical or volumetric change within the fluid medium, after a spontaneous fluid uptake, which occurs upon submersion. An extension to this model is proposed to calculate the kinetics of absorption for a given particle size distribution.