applied sciences Article Swelling Capacity and Hydraulic Conductivity of Polymer- Modified Bentonite under Saline Water Conditions Xin Xu 1 ID , Xiaofeng Liu 2 ID , Myounghak Oh 3 and Junboum Park 1, * 1 Department of Civil and Environmental Engineering, Seoul National University, Seoul 08826, Korea; xuxinsnu@hotmail.com 2 College of Architecture and Civil Engineering, Taiyuan University of Technology, Taiyuan 030024, China; m3835149921_1@163.com 3 Coastal Disaster Prevention Research Center, Korea Institute of Ocean Science and Technology, Busan 49111, Korea; omyhak@kiost.ac.kr * Correspondence: junbpark@snu.ac.kr; Tel.: +82-2-880-8356 Received: 7 June 2018; Accepted: 21 June 2018; Published: 22 June 2018   Abstract: Bentonite is widely used as a waterproof material in civil engineering. The swelling capacity and impermeability will be reduced in saline water conditions. A polymer-modified bentonite was used in this study, and the swelling characteristic and hydraulic conductivity in saline water were investigated. Results show that: the modified bentonite (MB) has good swelling characteristics and low permeability in saline water conditions; the unload swelling strain of MB in saline water conditions increased with rising swelling time; the final unload swelling strain of MB decreased with the increase of vertical pressure and increased with increasing initial dry density; and, as the swelling time increased, the swelling pressure increased rapidly at first and then decreased. Based on experimental results, this study obtained a linear relationship between the ratio of time to unload swelling strain and swelling time, a formula for load swelling strain that was related to initial dry density and vertical pressure, a linear relationship between the ratio of time to swelling pressure and swelling time, and an exponential relationship between swelling pressure and initial dry density. The MB can be used as a waterproof material in seepage-prone applications under saline environmental conditions in civil engineering works. Keywords: bentonite; polymer; saline water; swelling capacity; hydraulic conductivity 1. Introduction Waterproof anti-seepage structures are important in building construction operations such as: roadway, water conservancy, house building, and landfill works. Bentonite, as a waterproof material, is widely used for seepage control applications in foundation, dike construction, hydraulic engineering, and landfill construction [1]. Bentonite is a clay mineral with high swelling properties which results in its low permeability in aqueous solution [2]. The high swelling capacity and low permeability are a result of its main component [3]: montmorillonite. The swelling process and mechanism of montmorillonite crystals have been examined by many scholars [4–8]. The swelling capacity is an important property of bentonite as it affects the self-healing and crack-filling ability of waterproof materials [9,10]. When bentonite is used as a waterproof material, owing to the complexity of most geological structures, seepage-prone structures may be destroyed and bentonite can swell and fill cracks to prevent water penetration of the structure. On the other hand, if the swelling pressure is too strong, then the protective structure is extruded; therefore, the swelling capacity of bentonite is essential to its use as a waterproof material in civil engineering [11]. Disadvantages such as a low swelling capacity and high permeability will be evident when bentonite is used in saline conditions [12–15]. A low hydraulic conductivity and high swelling capacity Appl. Sci. 2018, 8, 1025; doi:10.3390/app8071025 www.mdpi.com/journal/applsci