Internal chloride binding of OPC pastes: modelling using binding isotherms

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1 Internal chloride binding of OPC pastes: modelling using binding isotherms M.V.A. Florea 1, L.C. Quaas 1, H.J.H. Brouwers 1 1 Department of the Built Environment, Eindhoven University of Technology, P.O. Box 513, 5 MB Eindhoven, the Netherlands Keywords: chloride binding, cement hydration, MSWI residues. Presenting author m.v.a.florea@tue.nl Abstract Purpose and methods The main contaminants of municipal solid waste incineration (MSWI) residues which do not comply with the environmental legislation are Sb, Cd, Mo, Cu, Cl - and SO -. However, it is known from literature that inorganic binders, such as OPC and slag- and fly-ash blended cements, can bind these ions into their hydration products. In this study, the main contaminants from MSWI residues (chlorides and sulphates of the above-mentioned cations) were immobilized in binder pastes (w/c=.5). The contaminants were added to the mixing water, as solution mass concentrations between and 5%, and the samples were then leached using a static leaching test. Results and conclusions With input from XRD results, the amount of formed hydration products can be modelled and the chloride binding capacity can be predicted. In the hydration model, the chemical compositions of the used cement are inserted and additional parameters are filled in as well, such as the binding constants and an estimation of the hydration degree. The basis for this model was a model developed previously for external chlorides. The same binding isotherms were considered, and their role re-evaluated for the specific case of internal chlorides. It was clear that the original chloride binding isotherm overestimated the chloride binding in the current samples. Therefore, a number of factors were taken into account in order to mitigate the differences. The final internal chloride binding isotherm was proposed as the sum of the binding of SO - AFm and C-S-H phases and was deemed a good fit to experimental data. Keywords: chloride binding, contaminants, binding isotherms, cement hydration products Introduction The main contaminants of municipal solid waste incineration (MSWI) residues which do not comply with the environmental legislation in the Netherlands are Sb, Cd, Mo, Cu, Cl - and SO - [1]. It is known from literature that inorganic binders, such as OPC and slag- and fly-ash blended cements, can bind these ions into their hydration products [, 3]. However, the chloride leaching often remains one of the largest challenges. In order to better design immobilisate or building material recipes incorporating MSWI residues, it is important to know the maximum contaminant binding capacity of such binders. A part of the available chloride ions are chemically bound to hydration products of the binder or physically held to the surface of hydration products. These chloride ions are generally referred to as bound chlorides. The chloride ions that are not bound by hydration products travel through the pore solution of the binder matrix and are called free chlorides. There are two main chloride binding mechanisms: binding through physical adsorption on the surface of the hydration products and through chemical reactions regarding the two hydrated phases that bind chlorides. Two main phases are known to use these mechanisms to bind chlorides: Physical binding: the C-S-H phase is known to physically bind chlorides to the surface of the hydration products due to its high specific surface values. Chemical binding: the AFm phase binds chlorides chemically. Other hydrated phases have also been considered for external chlorides binding, such as ettringite and the newly formed Friedel s salt [1]. However, their influence was found to be minimal so they will not be considered further. This study aims to investigate the binding of different types of chlorides and the associated cations (different salts, contamination) in increasing concentrations in cement pastes. The selected contaminants were NaCl -and CaCl -since 1

2 Leached Cl - (mg/l) these salts occur in vast amounts in MSWI residues [1]. Furthermore, SbCl 3 and CuCl were selected from the heavy metal contaminants of MSWI residues because of their good solubility. It was investigated how the associated cation affects the leaching of the chloride ion and XRD was used to obtain information about the interaction between contaminant and cement hydration products. It is attempted to couple this data with a hydration model to predict the chloride binding of the hydrated phases. In this way it can be investigated if there is a connection between the chloride binding and the hydration products. The most likely hydrated phases are identified with XRD to validate the hydration model and eventually be able to clarify how the chlorides are bound. Materials and methods Four chlorides (NaCl, CaCl, SbCl 3 and CuCl ) were used as internal chloride source. Samples with, 1, 3 and 5% Cl in the mixing water from both sources were mixed using w/c=.5. The cement used is a CEM I.5 N from ENCI. The mixing procedure was similar to the one of standard mortars, prescribed by EN The fresh paste was poured into standard mortar xx1mm prism moulds and kept covered for hours, after which demoulding took place. The samples were cured in humidified air in a closed container, but not in direct contact with water. The strength of the samples was measured according to EN 19-1, after which they were further crushed to particles under mm using a laboratory jaw crusher and subsequently placed in water with an L/S ratio of 5. The samples were stirred daily, manually, to ensure the homogeneity of the leachate, and samples were taken after 1, 5, 1, 1 and days in order to verify the needed diffusion time. During the sampling, 5 ml leachate was taken with an Eppendorf pipet. After the sampling, also 5 ml of distilled water was added to keep the L/S ratio steady. The leaching results were corrected since this step is basically a dilution of the leachate. The sampled leachates were filtered with disposable syringe filters (VWR international, 5 mm Syringe Filter, w/. µm Polyethersulfone Membrane). Leached contaminants were quantified with Ion Chromatography (IC) (Dionex 11) after the preparation. An ion exchange column AS9-HS ( x 5mm) with eluent 1 mm sodium carbonate (Na CO 3) / 5 mm sodium bicarbonate (NaHCO 3),with an isocratic flow of.5 ml/min has been used. Detection of ions was done by suppressed conductivity (used suppressor: Dionex AERS 5 mm). The pastes were manually ground in an agate mortar. For the analysis a Bruker D phaser with a cobalt tube (wavelength 1.79 [Å], lynxeye detector 1D mode) was used. Results and discussion Leaching results Figures 1- show the results of these chloride leaching tests for the OPC pastes. 1 1 Ref. 1% NaCl 3% NaCl 5% NaCl Added salt 1 d 5 d 1 d 1 d d 35 d d Figure 1. Leached chlorides from an OPC paste with added NaCl

3 Leached Cl - (mg/l) Leached Cl - (mg/l) d 5 d 1 d 1 d d 35 d d Ref. 1% CaCl₂ 3% CaCl₂ 5% CaCl₂ Added salt 1 Figure. Leached chlorides from an OPC paste with added CaCl Leached Cl - (mg/l) 1 1 d 5 d 1 d 1 d d 35 d d Ref. 1% SbCl₃ 3% SbCl₃ 5% SbCl₃ Added salt Figure 3. Leached chlorides from an OPC paste with added SbCl d 5 d 1 d 1 d d 35 d d Ref. 1% CuCl₂ 3% CuCl₂ 5% CuCl₂ Added salt Figure. Leached chlorides from an OPC paste with added CaCl. A chloride content of 5% Cl with NaCl as source results in the highest chloride leaching in OPC-containing pastes. In literature, it was reported that Ca + increases the chloride binding capacity significantly compared to Na + due to a more rapid formation of chloroaluminates in the case of CaCl exposure [1]. Recipes containing SbCl 3 result in a high leaching at an early stage for OPC, which decreases over time again. This could be due to a low ph of the sample immediately after water immersion, since SbCl 3 will form HCl in these conditions; 3

4 Intensity [a.u] therefore, any unhydrated SbCl 3 will lower the ph temporarily. After equilibrium is reached, this influence is no longer observed. XRD results Figures 5 to 9 present the diffractograms of the reference plain OPC paste and the compared results of each of the four chloride-containing samples at the three chosen chloride concentrations, after days of curing CH CH Etr CaCO₃ CH Etr Position [ᵒTheta] Figure 5. XRD result OPC reference From the results of the pastes that contain NaCl it was observed that CH gives the highest peak for 1% Cl, a slightly lower peak for 3% Cl and a much lower peak for 5% Cl (Figure 7). Next to the first ettringite peak another prominent peak is visible. This peak is not very stable and shifting to higher angles with increasing chloride content. This phase is likelyan AFm phase, it is very likely that this phase is a chloride containing AFm phase that is similar to Friedel s and Kuzel s salt but does not exactly fit either pattern. This phase, identified as AFm in Figures -9, is considered to contain chlorides, based on the fact that it cannot be identified in the reference chloride-free sample (Figure 5), and that its peak itensity increases with the increase in chloride content From the CaCl -containing samples, the most portlandite was formed in the paste recipes containing 5% Cl. From the leaching data could be observed that a chloride content of 5% resulted in the highest binding capacity for chlorides. Similar to the pastes with NaCl, there is also an AFm phase that shows the same trend (Figure). Figure emphasizes that the amount of formed portlandite is much higher at 5% Cl than for 1 and 3% Cl in the case of pastes mixed with added CaCl. In the case of SbCl 3 the chloride content of 5% results in the highest amount of ettringite (Figure ). All paste recipes containing copper chloride show unreacted clinker (C 3S). It is remarkable that a chloride content of 1 and 5% result in more unreacted clinker than paste recipes with a chloride content of 3%. Furthermore, it can be observed that the amount of ettringite remains constant with increasing chloride concentration. The calcite in the samples is formed through the carbonation of portlandite, (Figure 9Σφάλμα! Το αρχείο προέλευσης της αναφοράς δεν βρέθηκε.), so as a consequence of a higher calcite content (at ᵒTheta = 3), the amount of portlandite is decreasing (at ᵒTheta = 55). This phenomenon can be observed throughout all the samples. With input from the XRD results and employing a hydration model, the amount of formed hydration products can be modelled and the chloride binding capacity can be predicted [,3]. A number of factors are taken into account when estimating the composition of the hydrated cement paste: chemical composition of the OPC, w/b ratio, estimated degree of hydration.the amount of SO 3 in the cement composition will determine the composition of the AFm phase, as well as the quantity of AFt phase. The notations from [] are kept the same in this study: CSH Z is the amount of the C-S-H phase, HO-AFm is hydroxyl-afm and SO -AFm is monosulphate. The addition of the individual chloride binding abilities of these hydration products results in the total amount of bound chlorides (C b).the basis for this model was a model developed previously for external chlorides [,3]. The same binding isotherms were considered, and their role re-evaluated for the

5 specific case of internal chlorides. Figure 1 shows an example of a figure dealing with NaCl as admixed salt. It was clear that the original chloride binding isotherm for external chlorides (termed in Figure 9 Cb_external) overestimated the chloride binding in the current samples (termed Cb_internal). Therefore, a number of factors were taken into account in order to mitigate the differences. The obtained experimental results are rather different from the total modelled amount based on external chlorides [] of bound chlorides (C b) for all pastes. It appears that the experimental result is most probably based on the amount of C-S-H gel and the standard AFm phase containing SO - and not on HO-AFm, which would give a higher binding than the experimental values. Therefore, an assumption was made that no HO-AFm is formed. This is also in accordance to literature in which it is described that HO-AFm is unstable in the presence of sulphates and chlorides []. Based also on the XRD results and on the fact that sulphates are abundant in the solution due to ion replacement, the corresponding aluminates are assigned to ettringite. Figure. XRD result of OPC with a) 1% Cl, b) 3% Cl and c) 5% Cl from NaCl Figure 7. XRD result of OPC with a) 1% Cl, b) 3% Cl and c) 5% Cl from CaCl Another parameter taken into account was the amount of Cl needed to satisfy the maximum C-S-H binding capacity, which is considered preferential, and therefore the available chlorides for chemical substitution for the concentrations:. M,. M and 1.33 M (which correspond to 1%, 3% and 5% Cl by wt. added to the mixing water of the pastes). When the amount of chlorides that are needed to form C-S-H are subtracted from the total amount, the amount of chlorides can be determined that are available to substitute in SO -AFm. 5

6 Figure. XRD result of OPC with a) 1% Cl, b) 3% Cl and c) 5% Cl from SbCl 3 Figure 9. XRD result of OPC with a) 1% Cl, b) 3% Cl and c) 5% Cl from CuCl For. M and. M, the available amount of chlorides was lower than the maximum binding capacity for all paste recipes. Therefore, the amount of chlorides that is required for complete SO -AFm conversion, after subtracting the chlorides bound to C-S-H, was divided by two, assuming the formation of Friedel s salt. Furthermore, the following adjustments were made: The degree of hydration was adjusted for CaCl since it is known that this salt acts as accelerator. XRD showed that the degree of hydration should be lower for the pastes containing CuCl since uncreated phases were still visible. However, the compressive strength showed (except for 1% Cl) proper results [1]. HO- AFm does not form in the presents of Cl -, as explained above. Therefore, this part of the model was removed. NaCl samples leached a lot more sulphates than the others (as previously reported), so the amount of SO -AFm in the sample was lowered since not enough sulphates were available to form the amount of SO -AFm that would be modelled without adjustments. Not enough Cl - were available at lower chloride concentrations (. M) to transform all SO -AFm into Friedel s salt.

7 Bound Cl - [mg/ g d.s.] Bound Cl - [mg/ g d.s.] Bound Cl [mg/ g d.s.] Cb internal Cb_external CSH-Z SO₄-AFm HO-AFm,5 1 1,5,5 3 Chloride concentration [M] from NaCl Figure 1. Overview of the modeled hydration products for OPC with NaCl, w/c =.5, crushed after weeks, for an estimated hydration degree of.. Therefore, the final internal chloride binding isotherm, termed Cb internal in Figure 1, is proposed as the sum of the binding of SO -AFm and C-S-H phases. Moreover, the amount of AFm phase was adjusted as explained above. The result was deemed a good fit, and will from now on be termed Cb_final. Cb_final Cb_final ,, 1,33 Chloride concentration [M] from NaCl,, 1,33 Chloride concentration [M] from CaCl Figure 11. Comparison of the modelled amount of bound chlorides (C b_final) and the experimental results of an OPC paste, cured for days, with (left) NaCl and (right) CaCl In Figure 11 and Figure 1, it can be seen that the adjusted model gives quite accurate results when Cb_final is compared to the experimental results. The samples that contain SbCl 3 show that the modelled amount of bound chlorides underestimates the experimental results for all concentrations (Figure 1). 7

8 Bound Cl - [mg/ g d.s.] Bound Cl - [mg/ g d.s.] Cb_final Cb_final ,, 1,33,, 1,33 Chloride concentration [M] from SbCl 3 Chloride concentration [M] from CuCl Figure 1. Comparison of the modelled amount of bound chlorides (C b_final) and the experimental results of an OPC paste, cured for days, with (left) SbCl 3 and (right) CuCl This is also the case for the lowest chloride concentration from CuCl, the higher concentrations show an overestimation of the modelled amount of bound chlorides. However, the overall fit for all the various samples indicates that this model is valid and can be used to predict more complicated systems. Conclusions The OPC paste recipes that contain NaCl leach more chlorides than the paste recipes that contain other chlorides. This is true for all chloride concentrations. Recipes containing SbCl 3 result in a high leaching at an early stage for OPC which decreases over time again. This is different for slag cement, in these pastes the chloride leaching is increasing over time. In slag cement, a percentage of 1% Cl from SbCl 3 results in the lowest leaching of the investigated salts, but a percentage of 5% results in the highest chloride leaching of the investigated pastes. When the XRD results of samples containing 1% Cl from CaCl and CuCl are compared, it becomes clear that less portlandite is formed in the copper chloride samples. However, C 3S peaks are visible due to the fact that there is still unreacted cement. Chlorides are bound by an unidentified AFm phase. This phase appears to be Friedel s salt for higher chloride content. However, more research is required to identify what minerals are formed for lower chloride contents. An internal chloride binding model was proposed, based on an external binding model and the XRD results from this study. The final chloride binding model correctly predicts the bound chlorides from the four sources employed. This study will be extended to consider other binders besides plain OPC, such as slag-blended cement. The binding of the associated cation will be further investigated and correlated with the chloride binding. Acknowledgments The authors wish to express their gratitude to the STW-foundation (project 1331, Development of eco-concretes by using industrial by-products), as well as to the Cement-Concrete-Immobilisates sponsor group at TU Eindhoven. References [1] Quaas, L.C., Florea, M.V.A., Brouwers, H.J.H. (17). Immobilisation of the main contaminants of MSWI residues: Chlorides and their counter ions. In Vlastimil Bílek, Hana Šimonová & Zbynĕk Keršner (Eds.), Non-Traditional Cement & Concrete VI, 19- June 17, Brno, Czech Republic [] Florea, M.V.A., Brouwers, H.J.H. (1). Chloride binding related to hydration products: Part I: Ordinary Portland Cement. Cement and Concrete Research, (), -9. [3] Florea, M.V.A., Brouwers, H.J.H. (1). Modelling of chloride binding related to hydration products in slag-blended cements. Construction and Building Materials,, 1-3. [] Balonis, M., Lothenbach, B., Le Saout, G., Glasser, F.P. (1). Impact of chloride on the mineralogy of hydrated Portland cement systems. Cement and Concrete Research, (7) (1), 19-1