The re-swelling mechanism of superabsorbent polymers (SAP) in the SAP voids of cement-based materials

The re-swelling of SAP in SAP voids can significantly influence the effect of SAP in improving the freeze-thaw resistance of cement-based materials. In this paper, the morphology and the re-swelling ratio (eta, the volume ratio of reswollen SAP to dry SAP) of SAP in SAP voids were investigated with a self-made experiment. The 3D morphology of small SAP after re-swelling was investigated by an X-ray computed microtomography technology. The microscopic morphology and elemental composition of SAP after soaking were also analyzed. The results indicate that the cement paste with a higher water-binder ratio (w/b) has a coarser pore structure and the SAP in it can rapidly re-swell and partly seal the SAP voids. The eta value in the cement paste with a low w/b ratio is small and will increase after vacuum saturation. Additionally, the re-swelling capacity of large SAP is greater than that of small SAP, which may be because of the much reduction in the mass ratio of small SAP after dry-wet cycles and the low probability of small SAP to contact with large capillary pores. After soaking in pore solution, the Ca content in the SAP matrix rises while the K content decreases. Moreover, a new phase with a loose structure appears in the SAP matrix. This study reveals that the pore characteristics of cement-based materials and the particle size of SAP are crucial factors affecting the re-swelling behavior of SAP in SAP voids.