Cement and Concrete Research 30 (2000) 83–90 0008-8846/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S0008-8846(99)00208-2 Modelling of leaching in pure cement paste and mortar Marc Mainguy a , Claire Tognazzi b,c, *, Jean-Michel Torrenti d , Frédéric Adenot b a LCPC, 58 Boulevard Lefebvre, 75732 Paris Cedex 15, France b DESD/SESD, CEA Saclay, 91191 Gif sur Yvette, France c LMDC, INSA-UPS Génie Civil, Complexe Scientifique de Rangueil, 31077 Toulouse Cedex 4, France d ENPC, 6-8 Avenue Blaise Pascal, 77455 Marne la Vallée Cedex 2, France Received 28 May 1999; accepted 21 September 1999 Abstract The leaching of cement-based materials is analysed through experimental and numerical results. From the experimental point of view, the leaching processes of a pure cement paste and a mortar are characterised by the degraded depths and the cumulative amount of leached calcium at different times. From the mathematical point of view, the leaching is modelled with the mass balance equation of cal- cium in the liquid phase. Material properties useful for a numerical resolution of this equation are presented and the finite volume method is applied for the numerical simulations. Comparisons between experimental and predicted results show the validity of the simple model used to describe the leaching phenomenon and the ability of the finite volume method to support high nonlinearities. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Waste management; Cement paste; Mortar; Degradation; Finite volume analysis 1. Introduction Predicting the long-term behaviour of concrete, particu- larly concrete used for radioactive waste disposal, requires a sound knowledge of the various deterioration mechanisms that will affect the structures over their lifetimes and the ex- tensive use of simulation tools. The object of this paper is to describe a model that predicts the extent of chemical degra- dation by the action of pure water on a cement paste or a mortar. Indeed, constantly renewed water appears to be one of the aggressive agents that attack the chemistry of concrete. Deionized water in contact with cement-based materials create concentration gradients that lead to the diffusion of ions contained in the interstitial solution. These transfers modify chemical balances and induce a total leaching of Portlandite and a progressive decalcification of C-S-H. The two-dimensional model DIFFU-Ca was developed for calculating the variation of the degraded depth and the leached calcium flux with time for cement-based materials. In what follows, DIFFU-Ca is described and validated only for one-dimensional problems by comparing its results with ex- perimental data obtained with both cement pastes and mortars. 2. Experimental program and results 2.1. Materials Two different cement-based materials were investigated: a pure Portland cement paste and a mortar made with the same Portland cement and a quartz sand. The volumetric proportion of paste in the mortar was approximately 50% in volume. The sand was not attacked by leaching on the ti- mescale considered. All the paste and mortar samples were made with the same ordinary Portland cement (OPC). The chemical composition is given in Table 1. The water/cement (w/c) ratio of both materials was 0.4. The mixtures were poured into cylindrical moulds (7  8 cm for paste and 11  22 cm for mortar). All the samples were demoulded after 24 h and then were cured for 6 months under lime-saturated water at 20°C. These curing conditions led to maximum hydration and to partial leach- ing of alkalis. 2.2. Leaching process The samples, protected from atmospheric CO 2 , were im- mersed in water at a constant temperature of 20°C, and kept at pH 7 by small additions of nitric acid. The leaching pro- cess resulted from the concentration gradients between the pore solution of the cement-based material and the deion- ised water that surrounded the sample. These gradients in- duced a diffusion process of all species present in the pore * Corresponding author. ISA BTP, LaSAGeC, UFR Sciences, Allée du Parc Montaury, 64 600 Anglet, France. Tel.: +33-5-59-57-70-62; fax: +33-5-59-57-70-02. E-mail address: claire.lawrence@univ-pau.fr (C. Tognazzi-Lawrence)