Measurement and modelling of membrane potentials across OPC mortar specimens between 0.5 M NaCl and simulated pore solutions J.-Z. Zhang a, * , JianYun Li b , N.R. Buenfeld a a Department of Civil and Environmental Engineering, Imperial College, London SW7 2BU, UK b China Building Materials Academy, Beijing 100024, PR China Abstract This paper reports experimental measurement and theoretical simulations of membrane potentials across OPC mortar specimens between 0.5 M NaCl and simulated pore solutions with various OH  concentrations. The effect of OH  counter transport and cation transport on membrane potentials is discussed. Membrane potentials are found to increase with an increase in OH  concentration. Model simulations are consistent with the measured membrane potentials. This study confirms that cations diffuse much more slowly than anions and that OH  ions diffuse significantly faster than Cl  ions. Membrane potentials across OPC mortar specimens between NaCl and simulated pore solutions are mainly determined by OH  and Cl  transport. A simplified Henderson equation is recommended for membrane potential modelling of thin specimens in diffusion cell tests in which simulated pore solutions are used. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Membrane potential; Ion transport; Modelling; Corrosion; Cement; Concrete 1. Introduction Water saturated hardened cement pastes are porous materials containing a concentrated alkaline pore solu- tion. When cement-based materials are exposed to a wet environment, ions in both the pore solution and the environmental solution transport within the material according to their concentration gradients. Because of the different mobility of the ions in a cementitious paste, the charge balance across the ion transport zone may not be maintained, resulting in an electrical field. This self established electrical field, in return, regulates ion transport to achieve electrical neutrality in the sys- tem. Consequently, ion transport becomes dependent on both the concentration gradient and the electrical field. The electrical field is characterised in terms of mem- brane potential [1]. The effect of membrane potential on ion transport has been found to be significant [2]. An early study indicated that OH  counter diffusion may play an important role in determining membrane potentials and hence chloride transport [2]. The alkali content of different cement pastes varies significantly and this may be a factor affecting the resistance of dif- ferent cement pastes to chloride diffusion. This work investigates the effect of the OH  concentration of simu- lated pore solutions on membrane potentials across OPC mortar specimens. It is recognized that cations diffuse much more slowly than anions in OPC pastes [3,4]. Therefore, it can be assumed that the transport behaviour of anions, partic- ularly OH  and Cl  ions, largely determines ion trans- port properties of cement paste. While OH  counter diffusion has been recognised as playing a part in Cl  ion transport, the interaction has not been thoroughly in- vestigated. It is normally assumed in chloride modelling that the diffusion coefficient ratio of OH  and Cl  ions in cement pastes, D s OH =D s Cl , is similar to that in water. However, a diffusion cell study showed that the ratio is much higher [2]. Therefore, this work also examines the value of D s OH =D s Cl and its effect on membrane potentials by theoretical simulations. 2. Experimental 2.1. Specimens 100 mm diameter, 50 mm thick OPC mortar discs were cast using 0.34 free water/cement, 2.5 sand/cement * Corresponding author. Fax: +44-0207-594-6053. E-mail address: j.zhang01@ic.ac.uk (J.-Z. Zhang). 0958-9465/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0958-9465(01)00076-2 Cement & Concrete Composites 24 (2002) 451–455 www.elsevier.com/locate/cemconcomp