Absorption, Desorption and Re-swelling Behavior of Superabsorbent Polymers in Cementitious Materials: a Review
YANG Haitao1,2, BIAN Hongjian1,2, LIU Juanhong3,*
1 Key Laboratory of Roads and Railway Engineering Safety Control, Ministry of Education, Shijiazhuang Tiedao University, Shijiazhuang 050043, China 2 School of Civil Engineering, Shijiazhuang Tiedao University, Shijiazhuang 050043, China 3 College of Civil and Resource Engineering, University of Science and Technology, Beijing 100083, China
Abstract: Cracks and the reduction of water tightness after cracking are the important factors leading to the decrease of durability of cementitious mate-rials. Superabsorbent polymers (SAP) can relieve the self-shrinkage and improve the freeze-thaw resistance, impermeability after cracking, and self-healing ability of cementitious materials. The above functions of SAP rely on its absorption, desorption and re-swelling behavior in cementitious materials. In this paper, the mechanism of the absorption and desorption of SAP and its key influencing factors were first summarized. Then, the evolution mechanism of the re-swelling behavior of SAP in cracks was reviewed. In solutions, the osmotic pressure between the gel and solutions is the driving force for SAP swelling. Ca2+, as the key ion, can complex with the carboxyl group on the polymer chains and increase the ionic crosslinking density of SAP. A high ionic crosslinking density will produce a reverse osmotic pressure and induce the desorption of SAP. In cementitious materials, the key factors influencing the desorption rate of SAP are osmotic pressure and capillary force. Reducing the desorption under osmotic pressure and improving the desorption under capillary force can enhance the internal curing efficiency of SAP. Compared with AA SAP, AA-co-AM SAP has a more excellent internal curing effect due to its stronger water restraint ability. The re-swelling capacity of SAP in cracks of cementitious materials is lower than that in solutions, because the restraint effect of cracks and the high ion concentration in cracks inhibit the re-swelling of SAP. Designing new SAP based on the crack characteristics, mix designs and hydration process of cementitious materials may help to enhance the application effect of SAP.
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