Measuring absorption of superabsorbent polymers in cementitious environments

Abstract

Prior to implementing superabsorbent polymer (SAP) in cement-based materials, the absorption capacity in this environment needs to be known. Two simple methods, the tea-bag method and the filtration method, are often used to measure the SAP absorption capacity in liquid environments. Typically, the two methods give results that do not agree. The scope of this work is to explain the discrepancy between results from the two methods and to examine how well the methods predict the SAP absorption capacity in concrete. The tea-bag method is modified with different convection conditions during the test, and with various water/cement ratios and hydration times during preparation of cement slurry filtrate that is used as test liquid. The filtration method is modified with different liquid/SAP ratios. Air void analyses of cement paste and concrete are used to assess the SAP absorption capacity in hardened samples. The results reveal that a decrease in absorption capacity observed with the tea-bag method is due to properties of the test liquid and ion exchange between SAP and test liquid. The difference in 24-h absorption capacities given by the two simple methods is likely due to unequal amounts of interstitial water. It is advised not to use these simple methods as means of estimating SAP absorption capacity in concrete, since the results from these methods do not properly represent the absorption behavior in concrete. Instead, air void analysis of hardened cement paste seems to be a reliable method to register the SAP absorption capacity in cement-based materials.