2. IONIC TRANSPORT IN CEMENT-BASED MATERIALS
2.4. Results and discussion
2.4.3. Example 3: Concentrated single-salt leachant
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Figure 2.7: Mass of hydrated PC phases ( [kgsolid/Ltot]) after 280 days of simulated DI water leaching. Note that the first node at depth = 0 corresponds to the external solution.
The main finding from this exercise is that, the LEN model is slower to approach steady state than the Fickian, but at 280 days the differences in transport models have greatly diminished.
This result can be explained through examination of the magnitudes of primary species
concentrations in Figs. 2.4-2.6; the primary ions in solution are the alkalis and OH-, each on the order of 0.1 M. Thus, the charge coupling in the LEN model is achieved primarily through the balance of these three ions, resulting in a system that behaves similar to the binary electrolyte case since Na+ and K+ both possess a single positive charge.
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Fickian model than in the LEN. Similarly, the peaks of Ca, Si, and Al concentrations are nearer to the specimen surface in the Fickian model.
Figure 2.8: Comparison of a) pH, b) Ca, c) Al, and d) Si aqueous concentration profiles predicted by the Fickian and LEN transport models for the case of AN leaching of PC after 280 days of simulated leaching.
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Figure 2.9: Comparison of a) ammonium and b) nitrate primary species concentration profiles predicted by the Fick and LEN transport models for the case of AN leaching of PC.
In Figs. 2.9a and 2.9b the total dissolved concentration profiles of the primary species NH4 and NO3 are plotted, respectively. In the 10 months of simulated time, the Fickian model predicts steady-state concentration profiles of both primary species, but in the LEN model, both NH4 and NO3 fail to reach steady state. Moreover, the NH4 profile falls below the steady-state profile whereas the NO3 profile falls above. To elucidate the reason for these differences, the distributions of NH4 and NO3 species within the depth of the material after seven days of simulation are plotted for the LEN model in Figs. 2.10a and 2.10b, respectively. Note that no solid phases bearing NH4 or NO3 were formed during simulation. The speciation of NH4 at the left boundary consists of only NH4+
and the ion pair NH4NO30
. Upon entering the alkaline PC porewater, appreciable amounts of NH30
and NH4NO30
form, and because these two species are not present at the boundary, they diffuse outward. Thus, the ingress of total NH4 into the external solution is hindered by “back diffusion” of these neutral species with the alkaline porewater acting as a source for NH30
and NH4NO30
. Incidentally, due to their neutrality, both species move according to their own concentration gradient and diffusion coefficient. NO3 also forms ion pairs with both Ca+2 and K+ within the PC porewater, but in general the distribution of NO3 within PC porewater is more similar to its distribution at the boundary than is NH4.
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Figure 2.10: Distribution of predominant NH4 species (a) and NO3 species(b) within the pore solution after 28 days of simulated AN leaching using the LEN model.
As demonstrated in Fig. 2.11, the profiles of primarily cationic primary species also exhibit differences between the Fickian and LEN models. Similar to NH4 and NO3, the concentrations of both Na and K in Figs. 2.11a and 2.11b, respectively, have not reached steady state but are instead biased above the steady state profile. The LEN concentration profile of Mg, shown in Fig. 2.11c, exhibits a peak that is nearer to the left boundary and two times greater in magnitude than the Fickian model prediction.
As illustrated in Fig. 2.1, a number of primary species such as Mg and Ca exhibit solubilities that are highly sensitive to pH in the range of 12-13. Indeed, in the AN leaching case, the solubility of Mg has increased by six orders of magnitude due to the change in porewater pH (Fig. 2.11b).
For these primary species, the choice of transport model can exert considerable influence on leaching behavior because the estimate of ion flux out of the material is determined by the concentration profile near the boundary. As such, the finite difference approximation of the gradient between the first and second nodes for Mg in Fig. 2.11b, is four times greater for the LEN model.
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Figure 2.11: Comparison of typically cationic primary species profiles predicted by the Fick and LEN transport models for the case of AN leaching of PC.
The concentration profiles of both CO3 and S, shown in Figs. 2.12a and 2.12b, suggest a fundamentally similar behavior in the release of these two ions. Interestingly, the CO3 peak is lower in the Fickian case than in the LEN, whereas the peak in S concentration near the boundary is higher in the LEN case. Similar to the other non-precipitating species, Cl (Fig.
2.11c) has not reached steady state in the LEN model after 280 days of leaching.
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Figure 2.12: Comparison of typically anionic primary species profiles predicted by the Fick and LEN transport models for the case of AN leaching of PC.
Fig. 2.13 illustrates the predicted total masses of hydrates along the depth profile of the PC material, and, whereas the trends predicted by both models are quite similar, a distinct peak of reprecipitation at half the material depth occurs approximately 2 to 3 mm farther into the material in the Fickian model simulations at 280 of leaching. Such differences may not only prove to be significant for the prediction of mechanical properties of degraded materials but may also be exacerbated when the effect of porosity and tortuosity change on transport is considered.
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Figure 2.13: Profile of the predicted mass of hydrated PC phases [kgsolid/Ltot] after 280 days of AN leaching.
The major finding of this exercise is that charge-coupling phenomena exhibit greater departures from Fickian diffusion for the AN leaching case as opposed to the DI case, which is perhaps indicative of greater complexity within the most abundant dissolved ions in the AN case. In addition to the high concentrations of NH4 and NO3 primary species and of the alkalis in the native porewater, the aqueous concentrations of dissolved Ca species increases to the order of 0.1 M in the near surface region. Satisfying electroneutrality is thus “complicated” by the presence of the divalent Ca+2 ion as well as Ca(OH)+ and other aqueous Ca complexes and ion pairs.