Absorption and Desorption of Superabsorbent Polymers for Use in Internally Cured Concrete

Superabsorbent polymers (SAP) have been investigated as an additive for use in the manufacture of internally cured concrete. The ability of SAP to absorb and desorb fluid is important for the design of internally cured concrete mixtures. Internal curing research on lightweight aggregates (LWA) has typically focused on the absorption of water in the LWA internal curing agent. However, when SAP is used, the absorption test should be performed using a pore solution with a defined ionic concentration. To address the effect of the ionic composition of the pore solution on SAP absorption, pore solutions were extracted from fresh cementitious pastes, and their composition was evaluated using X-ray fluorescence. This study characterizes the absorption and desorption of a commercially available SAP, using both simulated and extracted pore solutions with a range of ionic concentrations. The teabag method was implemented to measure the absorption of the SAP. As expected, the absorption of the SAP decreased in solutions with higher ionic concentrations. In addition to studying solutions extracted from ordinary portland cement pastes, the effects of the inclusion of supplementary cementitious materials on the SAP absorption were studied. Results showed that the inclusion of supplementary cementitious materials had a relatively minor impact on the SAP absorption, primarily due to a dilution of the ionic concentration of the pore solution. This article examined the desorption of the SAP in two conditions: a reduction in the ambient relative humidity and after exposure of the SAP to solutions with a higher ionic concentration. It was observed that SAP-containing solutions with a higher ionic concentration had a reduced rate of desorption and a reduced overall desorption at a given relative humidity. In addition, moving the SAP from a solution with a lower ionic concentration to a more highly concentrated solution resulted in desorption. An equation was developed that expresses the SAP absorption as a function of the pH of the soaking solution. The expression was used to predict the desorption of SAP due to an increase in the ionic concentrations in a hydrating system. This equation was used to show that the desorption of SAP due to changes in the pore solution ionic concentration were significant during the first 72 hours. The findings and the techniques used in this study are meant to be used as an example for the characterization of SAP in concrete internal curing applications.