5 CHLORIDE BINDING ISOTHERMS - AN APPROACH BY APPLYING THE MODIFIED BET EQUATION

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1 5 CHLORIDE BINDING ISOTHERMS - AN APPROACH BY APPLYING THE MODIFIED BET EQUATION L. TANG and L.-0. NILSSON Chalmers University of Technology, Gothenburg, Sweden Abstract In this study the modified BET theory was used to interpret the data of chloride binding obtained by the authors of this paper and some other researchers. The results show that there is a good correspondence between the theory and the experiment data. The meaningfbl parameters of the binding isotherm give a better explanation to some phenomena found in the chloride binding experiments. Kev-words: Adsorption, Cement, Chlorides, Chloride Binding. 1 Introduction Chloride binding has a great effect on the transport and distribution of chloride ions in concrete [l]. Pereira and Hegedus [2] applied Langmuir equation to describe the chloride binding isotherms. In our previous study [3], we found that the chloride binding obeys Langmuir equation only at low concentrations due to the monolayer adsorption. At high concentrations, the adsorption or binding becomes complex and Freundlich equation was suggested [3]. Although Freundlich equation is practically applicable in many cases [l, 3-51, its parameters have no physical meaning. In fact, its parameters are just coefficients from non-linear regression. This study attempts to apply the modified BET equation to the chloride binding isotherms. 2 Modified BET theory The BET multilayer adsorption theory has widely been used in many fields for the determination of surface specific area. The most usual form of the BET equations is: 36

2 where S: the amount of the adsorbed adsorbate; S,: the monolayer adsorption capacity; a: adsorption constant; X=- P PS p: the vapour pressure of the adsorbate; p,: the saturation vapour pressure of the adsorbate. The adsorption constant a is related to the difference between the adsorption energy at the first layer and those at the second or higher layers. The original derivation of the BET equation assumed that the adsorption energy at the second is the same as at the higher layers and is equal to the heat of liquification, that is, where E: the adsorption energy at the layer as designated by the sub number; Hi: the heat of liquification; R: gas constant; T: temperature. The problem of the original BET theory is that when x + 1, the adsorption becomes infinite. To avoid this infinite adsorption, Brunaer et a1 [6] introduced a parameter K <l, so that x = K- P in Eq. (1). This treatment was based on the fact that even at the PS saturation vapour pressure the number of adsorption layers is still limited. Since in the original derivation x is defined as the ratio of the adsorbed fraction at the first layer to that at the second layer, a very small value of K implies a very small number of adsorption layers. Therefore, this treatment seems contrary to the original assumption of multilayer adsorption, where the number of adsorption layers tends to the infinite. Xu [7] tried another approach by assuming that the adsorption energy at the second layer is not equal to those at the third or higher layers, that is, E2 # E3 and E3 = E4 = = E, = HI, and derived the following modified equation, where 37

3 Although the expression of Xu's modification seems more complicated than the original expression Eq. (l), the parameter P gives a strong physical meaning since it is related to the difference between the adsorption energy at the second layer and the one at the third or higher layers. 3 Chloride binding isotherms It is well known that the hydrated products of cement provide a huge surface area for physisorption. On the other hand, the formation of Friedel's salt implies a chemisorption of chloride ions. Therefore, chloride binding in concrete involves both chemisorption and physisorption. It is possible to apply Eq. (2) to the chloride binding isotherms by C assuming x = -, where c is the free chloride concentration and c, the concentration in a CS saturated solution, for a NaCl solution, c, = 216 gcvl = 6.09 moh. Tang and Nilsson [3] found that when the bound chlorides were expressed based on the weight of hydration gel, the influences of water-cement ratios and aggregate content could be eliminated. In the literature the unit based on the weight of cement are often used. This expression is, in fact, proportional to that based on the weight of hydration gel if the hydration degree is constant. A comparison between Freundlich's equation and the modified BET equation is shown in Fig. 1, where the data reported in [3] were used. Eq.(2), p= 0.6 Eq.(2), p= Eq.(2), p= 0.03 OPC W /c = 0.4 [3] OPC W /c = 0.6 [3] OFC W /c = 0.8 [3] free Cl, moll1 Fig. 1 A comparison between Freundlich's equation and the modified BET equation. 38

4 4 Discussions It can be seen from Fig. 1 that both Freundlich's equation and the modified BET equation correspond very well when the free chloride concentration is lower than 1 mov1, in spite of different P values. Owing to the lack of experimental data, we do not know the tendency of chloride binding at higher concentrations. According to the reported data [g- 101, the rate of chloride binding becomes less and less with the increment of the free chloride concentration. It is reasonable to assume that the value of P should be about 0.3 or less. This P value may have significant importance for the chloride penetration in the splashing zone or in an environment where the de-icing salt is used. The further study is needed to find an appropriate P value. The influences of hydroxides on chloride binding have been reported by among others Tritthart [l l] and Sandberg [12]. In this study, their data were used to evaluate the quantitative effect of hydroxides. Since only the total chloride contents were reported in both [l l] and [12], the bound chloride contents were then calculated by subtracting the free from the total, and the units were converted to mgcvg-gel by using the equations reported in [3], assuming the hydration degree a h equal to 1 for the data in [l l] and 0.8 for those in [12], and pore volume Vp = w/c ah. The available data are shown in Fig. 2. WIC 0.4, ph13 p] o wlc 0.6. ph13 p] A WIC 0.8, ph13 p] o wlc 0.5, ph12.5 [l11 wlc0.5, ph13[11] xwlc 0.5, ph13.7 [11] xwlc0.7, ph125[ll] ~ w l0.7, c ph13 [l11 +wic0.7, ph13.7 [l11 wlc 0.4. ph13 [l21 ~WIC 0.4, ph13.5 (121 ~ w l0.4. c ph13.8 [l21 - free Cl, molll Fig. 2 Effect of hydroxides on chloride binding. It is obvious that the hydroxide content in the pore solution has a great effect on chloride binding. Quantitatively, when ph-value drops from 13.7 to 13 and 12.5, respectively, the monolayer adsorption capacity increases from 7.5 to 12.5 and 14.5 mgcvg-gel, respectively. This clearly shows that at a low OH concentration, more empty sites are available for Cl-. As Tritthart [l l] stated in his paper, there is a competition between these two types of active ions for the adsorption sites. This might be one of the reasons 39

5 why the chloride content in concrete surface increases with time, because the leaching of hydroxides from concrete increases the chloride binding, and the leaching is timedependent! Special attention should be paid to the top curve in Fig. 2, corresponding to a ph-value of In this case, the value of the adsorption constant a has been doubled, implying a very strong adsorption energy at the surface of hydrated gel, probably because of the vanishing of ~ a and ' K'. Page et a1 [8] reported a lot of data of total chloride content and free chloride concentration, as well as hydroxide concentration, in cement paste and concrete with a W/C of 0.5. By assuming the hydration degree ah = 0.8, 0.9 and 1 coresponding to the immersion duration of 6, 12, and 24 months, one can calculate the bound chloride content according to the reported total chloride content and free chloride concentration, where cb: bound chloride content; ct,t: total chloride content, mgcl/gcement; cf: free chloride concentration, rnmollml; V*: pore solution content, Vp = 0.35 mvgcement for the OPC paste [g]; 0 Wn : non-evaporable water, assuming W: = The calculated results are shown in Fig. 3. free Cl, molll Fig. 3 Relationship between bound and free chlorides in OPC paste calculated according to the data reported by Page et a1 [g]. 40

6 It can be seen that the bound chloride content decreases with free chloride concentration! Possible reason is that the free chloride concentration was determined by pore solution expression, which may be higher than that in the equilibrium solution, as Nagataki et a1 [l31 reported. If we assume that the free chlorides in the equilibrium solution is 70% as much as in the expressed pore solution, the points in Fig. 3 will change their positions, as shown in Fig. 4. In this case it appears that the data reported by Page et a1 [S] are also comparable with those reported by othe researchers [3, 1 1, 121. free Cl, molll Fig. 4 Redrawing of Fig. 3 after taking free chloride concentration 70% as much as in the expressed pore solution. 5 References 1. Nilsson, L.-O., Massat, M. and Tang, L., 'The effect of non-linear chloride binding on the prediction of chloride penetration into concrete structures', in ACI Special Publication SP-145, 'Durability of Concrete', ed. V.M. Malhotra, pp , (1994). 2. Pereira, C.J. and Hegedus, L.L., 'Diffusion and reaction of chloride ions in porous concrete', 8th Intl. Symp. Chem. Reaction Engg., Publication Series No. 87, 427 (1984). 3. Tang, L. and Nilsson, L-O., 'Chloride binding capacity and binding isotherms of OPC pastes and mortars7, Cement and Concrete Research, 23(2)(1993) Tang, L. and Nilsson, L-O., 'A numerical method for prediction of chloride penetration into concrete structures', presented at the NATO/RTLEM Workshop, 41

7 July 10-13, 1994, St. Remy-les-Chevreuse, and to be published in 'The Modelling of Microstructure and its Potential for Studying Transport Properties and Durability', ed. H. Jennings et al. 5. Tang, L. and Nilsson, L-O., 'A new approach to the determination of pore distribution by penetration chloride into concrete', Cement and Concrete Research, 25(4)(1995) Brunauer, S., Skalny, J. and Bodor, E.E., 'Adsorption on nonporous solids', J. Colloid and Interface Science, 30(1969) Xu, A., 'The structure and some physical properties of cement mortar with fly ash7, Licentiate thesis, Chalmers University of Technology (1990). 8. Page, C.L., Lambert, P. and Vassie, P.R. W., 'Investigations of reinforcement corrosion. 1. The pore electrolyte phase in chloride-contaminated concrete', Materials and Structures, 24(142)(199 1) Arya, C. and Newman, J.B., 'An assessment of four methods of determining the free chloride content of concrete', Materials and Structures, 23 (1 990) Mangat, P.S. and Molloy, B.T., 'Chloride binding in concrete containing PFA, gbs or silica fhme under sea water exposure', Magazine of Concrete Research, 47(171)(1995) Tritthart, J., 'Chloride binding in cement: 11. The influence of the hydroxide concentration in the pore solution of hardened cement paste on chloride binding', Cement and Concrete Research, 19(5)(1989) Sandberg, P., 'Critical evaluation of factors affecting chloride initiated reinforcement corrosion in concrete', Division of Building Materials, Lund University of Technology, pp , Report TVBM-7088, (1995). 13. Nagataki, S. et al. 'Condensation of chloride ion in hardened cement matrix materials and on embedded steel bars', ACI Materials Journal, 90(4)(1993)

Modeling of chloride penetration into concrete Tracing five years field exposure

Concrete Science and Engineering, Vol. 2, December 2000, pp 170-175 MRS CONFERENCE PAPERS Modeling of chloride penetration into concrete Tracing five years field exposure Tang Luping 1,2 and Lars-Olof