Absorbency of Superabsorbent Polymers in Cementitious Environments
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Absorbency of Superabsorbent Polymers in Cementitious Environments Luis P. Esteves1 and Ole M. Jensen1 1 Technical University of Denmark, Brovej, Bygning 118, 2800 Kgs. Lyngby Denmark. ABSTRACT Optimal use of superabsorbent polymers (SAP) in cement-based materials relies on knowledge on how SAP absorbency is influenced by different physical and chemical parameters. These parameters include salt concentration in the pore fluid, temperature of the system and SAP particle size. The present work shows experimental results on this and presents a new technique to measure the swelling of SAP particles. This new technique is compared with existing techniques that have been recently proposed for the measurement of pore fluid absorption by superabsorbent polymers. It is seen that the concentration of Na+, K+, Ca2+, OH-, and SO2-, in the exposure liquid influences the maximum absorption of SAP. Even very low concentrations of these may reduce the absorption to a third of the value measured in pure water at room temperature. Additionally, the influence of the SAP absorption on the ionic composition of the exposure liquid is investigated with atomic absorption spectroscopy. The paper provides the reader with knowledge about the absorption capacity of SAP in a cementitious environment, and how the absorption process may influence the cement pore fluid. INTRODUCTION The use of superabsorbent polymers in cement-based materials is fundamentally dependent on the SAP absorption capacity. Proper measurement of this quantity and awareness of its pitfalls is therefore very important [1, 2]. There are many difficulties involved in the measurement of SAP absorbency in cementitious materials. For example, the tea bag method, which is widely applied in the characterization of absorption capacity of SAP used e.g. in the baby diaper industry, overestimates the real particle absorption, as it includes the interstitial water - water physically held in the interstitial space between SAP particles. Therefore, this technique is generally not appropriate for measurement of SAP absorbency in cementitious environments. In this work, a discussion of methods that can be successfully used in the measurement of the absorption capacity of SAP in cementitious environments is given. Furthermore, a new technique with potential applicability in such studies is explored: laser diffraction particle size analysis. EXPERIMENT The materials used in this work included a suspension polymerized poly acrylic acid acrylamide copolymer, and several exposure liquids with different ionic strengths. The SAP consists in spherical particles, with an average size of 125 µm. For some experiments the SAP was sieved into different size fractions. The exposure liquids are composed as to have similarities with cement paste pore fluid with dominant concentrations of Na+, K+, Ca2+, OH-, and SO2-. The compositions of the exposure liquids are given in table 1.
Table 1. Composition of exposure liquids. C-1 has a composition similar to the pore fluid which may be found in some cement p
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