AMPS-based superabsorbent polymers for internal curing in concrete: Ion effects on absorption-release behavior and performance

Aimed at overcoming the shortcomings of current single-component internal curing agents, two superabsorbent polymer (SAP) systems (AA-AMPS and AM-AMPS) were synthesized using three monomers: 2-acrylamido-2-methylpropane sulfonic acid (AMPS), acrylic acid (AA), and acrylamide (AM). The absorption and release behaviors of these systems and their effects on autogenous shrinkage and compressive strength were systematically evaluated. The pore-forming absorption ratio ( Q pf ), defined as the SAP absorption at the final setting time, was measured using fluorescence impregnation and digital image processing. The influence of SAP chemical composition on internal curing performance was analyzed in terms of ionic crosslinking and repulsion effects. In terms of the 7-day and 28-day compressive strengths, Q pf and the pore-forming effective water-to-cement ratio (i.e., effective water-to-cement ratio at final setting) are identified as the main influencing factors. In the AA-AMPS series, as the content of AMPS ( m AMPS ) increases, the ionic crosslinking effect weakens while the ionic repulsion effect strengthens. This leads to an upward trend in Q pf , resulting in a higher porosity induced by SAP and a subsequent decrease in compressive strength. In the AM-AMPS series, Q pf exhibited a non-linear trend, peaking at 50 % m AMPS . Regarding autogenous shrinkage, Q pf and the water release rate of SAPs were determined to be the main determinants. In the AA-AMPS series, owing to the ionic crosslinking effect of AA, the water release rate of this series of SAPs is relatively faster. The increase in m AMPS weakens the ionic crosslinking effect and enhances the ionic repulsion effect, leading to an increase in Q pf , which results in more water available for reducing autogenous shrinkage and an expansion of cement paste. In the AM-AMPS series, the ionic crosslinking effect of AMPS is weaker, resulting in a slower water release rate for this series of SAPs. As m AMPS increases, the ionic crosslinking effect is enhanced, which improves the water-release capability of SAPs and strengthens their effect on reducing autogenous shrinkage. This study elucidates the mechanism by which the ion effect influences the internal curing performance of AMPS-based SAPs, thereby providing valuable insights into the relationship between their chemical composition and performance.