Ab- and Desorption Behavior of Superabsorbent Polymer in Cement Pastes

Abstract

Internal curing cement-based materials utilizing superabsorbent polymer (SAP) demonstrates a significant mitigation of shrinkage effects. And the water ab- and desorption behavior of SAP provides critical insights for optimizing the mix proportion design in internal curing applications of cement-based materials. Herein, the ab- and desorption curves of SAP with different parameters in real cement pastes were quantitated through X-ray radiography. The influence laws and mechanisms of SAP size, SAP dosage and water-to-cement ratio (w/c) on the curves in cement pastes were clarified. The experimental results demonstrate that both the maximum water absorption capacity and desorption rate of SAP exhibit a positive correlation with w/c at early age, owing to more free water content and lower Na⁺/K⁺ concentrations for higher w/c. However, at later age, the desorption rate of SAP becomes inversely proportional to w/c owing to the humidity. Besides, the water retention capacity of SAP at later age shows a direct proportionality to SAP dosage. And a quite unexpected finding is the reabsorption behavior of SAP in high w/c (over 0.42) after 7 hours of cement hydration. The mechanism behind the reabsorption phenomenon is the deduction of calcium ions concentration in cement-based environments, which make SAP exhibit calcium ion leaching behavior.