Atomistic Simulations of Silicate Species Interaction with Portlandite

Article pubs.acs.org/JPCC

Atomistic Simulations of Silicate Species Interaction with Portlandite Surfaces Sandra Galmarini,*,† Aslam Kunhi Mohamed,‡ and Paul Bowen‡ †

Building Science and Technology, Eidgenössische Materialprüfungs- und Forschungsanstalt (EMPA), CH-8600 Dübendorf, Switzerland ‡ Powder Technology Laboratory (LTP), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland S Supporting Information *

ABSTRACT: Portlandite (Ca(OH)2, CH) is the second most abundant hydrate formed in the reaction of Portland cement with water, making it an important component in the built environment. Formation of CH is closely linked to the growth of the main hydrate phase, calcium silicate hydrate (C−S−H), affecting the microstructure and properties of cement. Understanding the interplay between the growth of CH and C−S−H is the key to comprehend the hydration reaction kinetics. This interplay mainly happens via the interaction of the different species present in the pore solution with the hydrates formed. It has been speculated that silicate species poison the growth of portlandite. In this work, we give evidence and propose a mechanism toward this experimentally observed effect using atomistic simulations. We also study the stability of a Ca−Si complex (CaSiO2(OH)2) expected to exist in pore solution using metadynamics calculations. We find that the adsorption of this stable Ca−Si complex at the (0001) portlandite surface is energetically favorable, contrary to the previously observed adsorption of the CH growth species Ca2+ and OH−. Additionally, the adsorbed complex retains a certain mobility at the surface. Growth poisoning is thus likely to happen by preferential adsorption of Ca−Si complexes. The interplay of CH and C−S−H growth is likely to be enhanced by the easier polymerization of calcium−silicate species adsorbed at portlandite surfaces.

1. INTRODUCTION Cement is the most used material in the world and is essential for the development of the built environment. CO2 is emitted in the production process of the clinker phases and contributes about 5−8% of the world’s carbon dioxide (CO2) emission.1 Hence there is a tendency to replace the clinker phases with supplementary cementitious materials (SCMs) like slag, fly ash, calcined clays, and limestone2−4 which have a lower carbon footprint. The use of the SCMs modifies the cement chemistry and hence the reactivity, which in turn can influence both early and long-term strengths and properties.5−7 To ensure the use of such materials to produce durable, desirable, and sustainable concrete, a better understanding of the underlying mechanisms in cement hydration is needed. The basic understanding of what happens at the atomic scale in this most used material in the world remains a great challenge, with or without SCMs and the modified chemistries and reactivities. For this, one has to know the different aqueous species and phases present and generated during the cement hydration at the atomic and macro level. Alite (C3S), which forms 50−70% of Portland cement (PC), reacts with water to form the main hydrate phases, calcium silicate hydrate (C−S−H), and portlandite (CH). In this work, we concentrate on the interactions of portlandite with the silicate species in solution. Portlandite forms 20−25%8 of the structure and is believed to influence the kinetics of cement hydration significantly. One of the driving mechanisms causing © 2016 American Chemical Society

the onset of acceleration of hydration is believed to be due to the precipitation of portlandite.2,9 It also plays a key role in strength development as it is closely linked to C−S−H growth.10−12 Typical ionic species that can be present in the solution when PC is mixed with water are Ca2+, OH−, 3+ 2− + + 2+ 13 SiO2(OH)2− 2 , Al , SO4 , K , Na , Mg , and a possible charge neutral Ca−Si complex, CaSiO2(OH)2.14 Understanding the interaction of portlandite with this complex mixture of ionic species is an important aspect in cement hydration. In a previous study, a simplified model system was used to experimentally study the effects of individual ionic species (chloride, nitrate, silicates, and sulfates) on portlandite.15 Strong effects could be seen on the morphology, size, and state of agglomeration, but the underlying mechanisms behind these effects remain unknown. Among the main species that are always present during cement hydration are silicates, and these were shown to clearly influence the growth and morphology of portlandite.16 In the model calcium hydroxide precipitation system the uniform faceted portlandite crystals formed were significantly disrupted in the presence of silicates giving poorly ordered irregular aggregates. Such growth poisoning of portlandite by silicates has also been previously observed by other authors,16−19 although the effect has not been studied in Received: July 14, 2016 Revised: September 2, 2016 Published: September 2, 2016 22407

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energies is reasonable and can be described by the empirical error estimate (see Supporting Information) that is used in this work.34 Some care has to be taken for structures and reactions where proton exchange plays a significant role, as this process is not described by the force field.34 Ewald summation was used to take into account the long-range electrostatic forces, and the cutoff between long-range and short-range forces was 8.5 Å. The time step for all calculations was 0.7 fs. For more details, the full set of parameters of the interactions as well as the validation and estimation of the force field error can be found in the supplementary documents of a previous paper.34 Both conventional and well-tempered metadynamics,29,30 sometimes also referred to as the local elevation method, was used to study the stability of the Ca−Si complex, CaSiO2(OH)2, as well as the adsorption of the silicate species on portlandite surfaces. More details on the method can be found in a previous paper.34 Metadynamics allows the fast exploration and the estimation of the free energy landscape with respect to a certain number of system properties called collective variables by the addition of a Gaussian bias potential to the system at different intervals of time. Additionally, depending on the parameters chosen, the free energy profile can be estimated. For three surfaces ([0001], [10−10], and [10−11]), exploratory metadynamics calculations to identify the adsorption sites and the conformation of the adsorbed complex at the surface were done. For all surfaces a periodic calculation cell (22 Å × 23 Å × 109 Å) containing a solid−water−vacuum slab (∼4000 atoms) was used. The calculations were done with the NVT ensemble, with pressure equilibration perpendicular to the surface being possible due to the vacuum gap. For the exploratory metadynamics calculations, the system was described with the help of two collective variables: the distance of the ion from the surface and the local electrostatic interactions with the surface within a gradual cutoff radius of 3.5−8.5 Å. The second collective variable was included to push the system to explore different adsorption sites but was not included for the energy profile calculation at the [0001] surface, as it is not independent from the second collective variable, i.e., the distance from the surface. The movement of the ion of interest was restricted to a region of about 15 Å from the surface. For the exploratory metadynamics calculations, the total simulation time was 650 ps. The stability of the different adsorption sites was then verified by unconstrained NVT simulations over 1.4 ns. For the calcium−silicate complex formation and the [0001] surface, the free energy profile was estimated as a function of the distance between the silicate and calcium ion or between the silicate ion and the surface, respectively, as the only collective variable. For the complex formation a periodic water box in NPT ensemble containing a single CaSiO2(OH)2 complex and 2047 water molecules was used. Both conventional (nontempered) and well-tempered metadynamics calculations were done, and the error of the metadynamics method on the free energy profile was estimated a posteriori by the standard deviation between the nontempered and welltempered metadynamics calculations. The validity of the choice of collective variables was evaluated based on the lack of large fluctuations of the energy profile after convergence of the energy profile as well as by comparison of the a posteriori error with the a priori error according to Laio et al.30 The error bars shown on the energy profiles correspond to the 95% confidence interval on the energy profile plus the estimated force field

detail. To get better insights into the reason behind the drastic effect of silicates on the morphology and growth of portlandite, atomistic-scale simulations will be presented here to elucidate the experimentally observed effect. The first step to understand the influence of silicate species in solution is to know the most abundant species. According to GEMS,20−24 the most abundant species in both the previously reported model systems15 and ordinary Portland cement systems24 is a neutral calcium−silicate complex CaSiO2(OH)2 first reported by Santschi and Schindler.25 However, the actual existence of this complex remains somewhat uncertain.14,26−28 Consequently the stability of the complex has been confirmed using a combination of conventional and well-tempered metadynamics29,30 using a previously validated force field.23 Subsequently the interaction between the neutral calcium− silicate complex as well as the SiO2(OH)2− 2 ion with portlandite surfaces has been studied using the same techniques. A study looking at the adsorption of Ca2+ and OH− ions and growth at portlandite surfaces has been reported previously.21−23 The results reported here will propose an atomistic mechanism for the disruptive effects of silicon species on portlandite growth seen experimentally.15−19,31,32

2. METHODS Thermodynamic Calculations. The thermodynamic calculations were done with GEMS v2 20−22 with the thermodynamic database CEMDATA,23,24 which has been developed for cementitious systems, in combination with the Nagra/PSI chemical thermodynamic database.33 The built-in extended Debye−Hückel equation in Truesdell−Jones form with individual Kielland ionic radii a and a common third parameter βγ = 0.064 has been chosen to calculate the activity coefficients. This model has been used before for cementitious systems24 and should be accurate for ionic strengths up to 1−2 M.22 If nothing else is specified, the thermodynamic calculations were performed for a temperature of 22 °C and a pressure of 1 bar. Atomistic Simulations. Classical atomistic force fields were used to define the interatomic forces in molecular dynamics simulations. The force field has been used previously34 and largely follows the work of Freeman et al.35 who combined the force field originally developed by Lewis and Catlow36 and further developed by others37−41 with the TIP3P force field,42 using a formalism originally developed by Schröder et al.39 However, the force field by Freeman et al.35 did not include silicates. Consequently the silicon−oxygen and oxygen−oxygen terms developed by de Leeuw and Parker41 were introduced. In accordance with the work of Freeman et al.35 the polarizable oxygen shells were removed and replaced by additional Si−O−Si and Si−O−H harmonic angle potentials, fitted to quartz. Finally the TIP3P potential42 was replaced by the TIP4P/2005 potential developed by Abascal and Vega,43 as the TIP4P/2005 gives significantly better results for water.43 The TIP4P/2005 is similar to the TIP3P potential but divides the oxygen into two sites, one with no mass but a charge and the second with no charge but mass and being the origin for dispersion interactions. This logic has been followed for cross-terms as well. The resulting force field has been thoroughly validated based on six different crystal structures containing Ca, Si, O, OH, and H2O and the structure of solvated Ca and OH as well as seven reaction energies.34 The estimated error on distances is in the order of 5%, and the difference between calculated and experimental reaction 22408

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calcium ion occupies the space between the two deprotonated oxygen ions of the silicate tetrahedral to achieve a shorter Ca2+−Si4+ distance of 3.6 Å. It is the first time the structure of this complex is being reported in the literature. However, our results are in qualitative agreement with a previous Hartree− Fock study by Moravetski et al.45 of the KSiO(OH)3 complex structure and NMR data where the K+ was found to bind preferentially to the deprotonated oxygen. The minimum energy corresponding to the complex formation is estimated to be −0.59 ± 0.30 eV. This value is in reasonable agreement with the experimentally determined energy of −0.27 ± 0.01 eV, albeit just outside the estimated error. In conclusion our results indicate that the CaSiO2(OH)2 complex is stable and likely to play an important role in influencing the growth of portlandite. Consequently, the Ca−Si complex was considered as a silicate species present in solution and a possible candidate for portlandite−silicate interactions for further simulations. Silicate−Portlandite Interactions. The different interaction aspects of identified silicate species in the previous section are described here. Adsorption Sites. The results from the surface adsorption sites identified by the exploratory metadynamics calculations can be seen in Figure 3 for the three dominant surfaces in the pure Ca(OH)2 morphology, [0001], [10−10], and [10−11].34 For all the surfaces, hydroxyl groups of both the CaSiO2(OH)2 complex and the SiO2(OH)2− 2 ion are found to orient toward the surface oxygen indicating an adsorption through hydrogen bonding. The residence times of the adsorption sites identified in metadynamics calculations for different surfaces are also verified with an unconstrained molecular dynamics simulation over 1.4 ns at 300 K. The residence time depends on the characteristics of the energy landscape such as the relative energy compared to the solvated ion, the adsorption barrier, and the width of the energy well representing the adsorption site. This means that the residence time can only serve as an estimate to stability and as a weak adsorption site corresponding to a broad minimum, and presenting a large adsorption barrier can have an equally long residence time as a strong one with a deep but narrow energy minimum and no barrier. However, it still gives an estimate of the importance of different adsorption sites. For the [0001] surface the observed adsorption configurations of both the CaSiO 2 (OH)2 complex and the SiO2(OH)2− 2 ion are very similar. Both hydroxyl groups of the two species donate a hydrogen bond each to surface oxygen atoms. However, as seen in the unconstrained MD simulation, both silicate species adsorb and desorb quite rapidly. The adsorption of the Ca−Si complex, with an approximate residence time of about 140 ps, seems to be more stable than that of the silicate ion (approximate residence time: 35 ps). Interestingly both silicate species seem to retain a certain amount of mobility when adsorbed at the [0001] surface. The species seem to perform a type of walking movement with the two hydroxyl groups. The movement starts by the breaking of the hydrogen bond of one of the two hydroxyl groups. The silicate species is then free to rotate around the one remaining hydrogen group, which allows the free hydrogen group to reform a new hydrogen bond with a new neighboring surface oxygen atom. By this alternate breaking and making of hydrogen bonds, the silicate species can move along the surface, as illustrated in Figure 4. For more details, a short video

error. The exact parameters used as well as more details on the method and the error calculations can be found in the Supporting Information as well as in ref 34.

3. RESULTS AND DISCUSSION Silicate Species Present in Calcium−Silicate Systems. According to the thermodynamic calculations with GEMS, at equilibrium a system where 0.1 mol/L CaCl2, 0.2 mol/L NaOH, and 0.001 mol/L Na2SiO3 were added to deionized water, such as the one studied in ref 44, the main silicate species in solution is the neutral CaSiO2(OH)2 complex. Thermodynamic calculations based on the composition measurements of the solution in ordinary Portland cement systems at different times by Lothenbach et al.24 show that this is also the most abundant silicate species in ordinary Portland cement systems, followed, except at very short hydration times, by the SiO2(OH)22− ion.44 However, as mentioned before, the existence and stability of the CaSiO2(OH)2 complex has not been conclusively demonstrated, and it is not included in all thermodynamic databases.26 Consequently, the stability of the complex had to be confirmed. According to the metadynamics calculations, the CaSiO2(OH)2 complex is found to be stable (Figure 1) with a

Figure 1. Estimated free energy of the CaSiO2(OH)2 complex as a function of the distance between the Ca2+ and the Si4+ ion.

well-defined energy minimum observed at a Ca2+−Si4+ distance of dCa−Si = 3.6 Å. In Figure 1, the energy profiles from the welltempered and nontempered metadynamics calculations as well as their average are plotted. The observed minimum distance corresponds to a configuration where the calcium ion is associated with one of the deprotonated oxygens of the SiO2(OH)2− 2 (see Figure 2). For shorter Ca−Si distances the

Figure 2. Typical complex configuration at a Ca2+−Si4+ distance of 3.6 Å. View along the deprotonated oxygen−silicon bond (a) and along one of the hydroxyl oxygen−silicon bonds (b). The size of the different spheres is according to the van der Waals radius of the ion: yellow, Si; red, O; white, H; turquoise, Ca. 22409

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Figure 3. Adsorption sites for CaSiO2(OH)2 and SiO2(OH)2− 2 ions at different portlandite surfaces identified by metadynamics calculations. Approximate residence times of each species on that specific surface from MD simulations are also indicated. At the (0001) surface both the species are found to have some mobility, while residence time of >1.4 ns indicates a stable and strongly adsorbed site. b1 is the strongly bound water site discussed in the text. Yellow, Si; red, O; white, H; turquoise, Ca.

found. Both of these configurations are validated by unconstrained MD simulations and are found to be stable for 1.4 ns. Contrary to the first site no mobility was observed for either of those more strongly bound adsorption sites. These adsorption configurations have not been observed for the SiO2(OH)2− 2 ion. Finally, for the [10−11] surface there are two adsorption ion and one for the sites identified for the SiO2(OH)2− 2 CaSiO2(OH)2 complex. The main adsorption site is again very similar for both the species. Both species adsorb by forming a hydrogen bond between the hydrogen of one of the hydroxyl groups of the solution species and a surface oxygen. The oxygen of the same hydroxyl group is observed to form a weak bond with a neighboring surface Ca2+ ion. No mobility was observed at this site since only one of the hydroxyl groups was involved in bonding resulting in no breaking and making of hydrogen bonds. Adsorption of the complex does not affect the configuration of the strongly bound water. For the calcium− silicate complex, the species remained adsorbed for 1.4 ns of unconstrained MD simulation. For the silicate ion the complex desorbs after about 350 ps, followed by a Ca−Si complex formation. For the SiO2(OH)22− a secondary adsorption site was observed (Figure 3). For this secondary site both a strongly bound surface water and a surface hydroxyl group were desorbed and replaced by the two hydroxyl groups of the ion adsorption. Each of the hydroxyl groups SiO2(OH)2− 2 donates a hydrogen bond to the surface, and one of the deprotonated oxygen atoms of the ion accepts a hydrogen bond from the surface. However, this adsorption site is observed toward the end of the simulation when a sufficient amount of energy was already added to the system, which questions if this adsorption site is energetically favorable. During an unconstrained MD simulation, the silicate ion moves to the first adsorption site after 420 ps. Free Energy Profiles. The free energy profiles calculated for the silicate species on the [0001] surface can be seen in Figure 5. Despite the similarity of the minima (expected adsorption sites), the free energy curves obtained for the two silicate

sequence of the process can be found in the Supporting Information.

Figure 4. Consecutive snapshots of the CaSiO2(OH)2 complex adsobed at the [0001] portlandite surface, illustrating the mobility of the complex at the surface. Yellow, Si; red, O; white, H; turquoise, Ca.

For the [10−10] surface there are two adsorption sites identified for the CaSiO2(OH)2 complex and one for the SiO2(OH)2− 2 ion. The main adsorption site is again very similar for the two species and similar to that observed at the [0001] surface (Figure 3). However, unconstrained MD shows that whereas the silicate ion desorbs almost instantaneously the calcium−silicate complex remains stably bound to the surface for the full 1.4 ns of total simulation time. Again we observe a certain mobility at the surface by breaking and making of the hydrogen bonds with the surface oxygen. However, the mobility seems to be slightly reduced compared to the [0001] surface, probably due to the lower surface oxygen density at the [10−10] surface (as shown in ref 34). For the CaSiO2(OH)2 complex a second adsorption site at the [10−10] surface observed is shown in Figure 3. At this site, two of the previously identified34 strongly bound surface water molecules (b1) are found to be desorbed, the hydroxyl groups of the complex replacing them at the surface. This indicates a stronger bonding of the CaSiO2(OH)2 complex to the surface, restricting its mobility. A third configuration, where only one strongly bound surface water molecule is replaced, is also 22410

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10] surface and are immobilized. As the adsorption of the calcium−silicate complex is preferential compared to the adsorption of calcium and hydroxyl, and the immobilization at edge sites would additionally block potential growth sites, such a mechanism would lead to the inhibition or poisoning of the growth of portlandite. Experimental results indeed show such a trend. Tadros et al.17 observed that the addition of silicates notably slows down the growth of portlandite. Additionally, the proposed mechanism from our metadynamics calculations could help explain why portlandite growth is not observed in cement hydration during the induction period of the hydration reaction although the solution is supersaturated with respect to portlandite. The poisoning of portlandite growth by silicates as a reason for the absence of portlandite growth during the induction period has been previously proposed,18,19 but this is the first time a precise mechanism for the poisoning effect has been put forward. Additionally, our results indicate that the portlandite surfaces could also serve as a template for C−S−H growth. If the mobile, adsorbed calcium−silicate species, that all share the same orientation on the portlandite surface, encounter each other, polymerization may occur relatively easily, potentially leading to precursors for the C−S−H structure. The above observations are also consistent with the experimental observations for model systems.15 Galmarini et al. observed that the addition of silicates reduces the primary particle size and produces agglomerates of portlandite with a small amount of C−S−H at the surfaces and between the particles. With the help of the mechanism proposed in this work, we can put forward an explanation for the reduction of the primary particle size by silicate poisoning of the portlandite growth. The observed portlandite−C−S−H agglomerates with the secondary phase (C−S−H) forming preferentially at the surfaces and in between the portlandite particles also supports our hypothesis of portlandite surface serving as a template or at least heterogeneous nucleation site for C−S−H formation.

Figure 5. Adsorption energy profiles for CaSiO2(OH)2 and SiO2(OH)2− 2 ions at the [0001] portlandite surface.

species are quite different. In the case of the Ca−Si complex, two minima at 7 and 4 Å are observed which corresponds to the outer and inner Helmholtz plane,46 respectively, i.e. the minimum approach distance of the hydrated and bare ion, respectively. The distances are large compared to other ions34 due to the large size of the complex. The energy of the SiO2(OH)2− ion on the other hand is found to increase 2 continually as it approaches the surface, with a slight minima at the outer Helmholtz plane with an energy close to zero. This indicates that the interaction of the calcium−silicate complex with portlandite−water [0001] surface is stronger than the silicate ion. We can compare these results to the observations of the unconstrained MD simulations. For the pure silicate species, the silicate will associate with the calcium ion after the first desorption event, which then makes it difficult to compare further than the respective approximate residence times estimated from the simulation (see discussion above). For the calcium−silicate complex on the other hand, different adsorption−desorption events were observed during the simulation. In order to get somewhat accurate statistics, a second 1.4 ns simulation was performed. During the two simulations, 0.54 ns was spent at the inner Helmholtz adsorption site (9.8 Å from the surface). This seems consistent with the narrower less deep minima at the inner Helmholtz plane and the wider more pronounced minima at the outer Helmholtz plane. We can conclude, that according to our calculations, the adsorption of the calcium−silicate complex at the inner Helmholtz plane of the [0001] surface is energetically favorable compared to the bulk state of the solvated calcium−silicate complex, contrary to the adsorption of Ca2+ and OH− ions (Figure 434), although adsorption at the outer Helmholtz plane is slightly more stable. Additionally, as mentioned above, the calcium−silicate complex does seem to retain some mobility at planar portlandite surfaces until such features as protruding surface calcium atoms are encountered as in the case of the [10−11] surface where no mobility was observed. Insights into Experimental Observations from This Work. With these results one can imagine how the calcium−silicate complex might adsorb at the flat surfaces [0001] or [10−10], move around until they encounter an edge or a [10−11] surface, or replace a strongly bound surface water at the [10−

4. CONCLUSIONS In this paper, using atomistic-scale simulations, we have investigated the adsorption of silicate species at three different portlandite surfaces ([0001], [10−10], and [10−11]) to gain further understanding of the growth of portlandite in cementitious systems. The two main silicate species as identified by thermodynamic modeling, namely, the charge neutral CaSiO2(OH)2 complex and the SiO2(OH)2− 2 ion, were investigated. The stability of the Ca−Si complex, which has previously been debated, was confirmed by metadynamics simulations. The calculated free energy of formation of the complex is in reasonable agreement with the experimentally determined energy. Metadynamics calculations indicate that both species adsorb at the portlandite surfaces by hydrogen bonding, one or both of the hydroxyl groups donating a hydrogen bond to surface oxygen atoms. The residence time of the neutral Ca−Si complex however is significantly larger than that of the silicate ion, indicating a stronger interaction of the Ca−Si complex with the portlandite surfaces. At the atomically flat surfaces ([0001] and [10−10]) both silicate species seem to retain a certain amount of mobility while remaining adsorbed at the surface, although at the [10−10] a more strongly bound adsorption site, where the silicate species remains immobile, exists as well. On the [10−11] surface the silicate species adsorb to the step-like exposed Ca ions. The adsorption free 22411

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Mechanisms of Cement Hydration. Cem. Concr. Res. 2011, 41 (12), 1208−1223. (4) Crow, J. M. The Concrete Conundrum. Chem. World 2008, 62− 66. (5) Papadakis, V. G.; Antiohos, S.; Tsimas, S. Supplementary Cementing Materials in Concrete. Part II: A Fundamental Estimation of the Efficiency Factor. Cem. Concr. Res. 2002, 32 (10), 1533−1538. (6) Lothenbach, B.; Scrivener, K.; Hooton, R. D. Supplementary Cementitious Materials. Cem. Concr. Res. 2011, 41 (12), 1244−1256. (7) Targan, C.; Olgun, a.; Erdogan, Y.; Sevinc, V. Effects of Supplementary Cementing Materials on the Properties of Cement and Concrete. Cem. Concr. Res. 2002, 32 (10), 1551−1558. (8) Diamond, S. Calcium Hydroxide in Cement Paste and Concretea Microstructural Appraisal. Mater. Sci. Concr. Spec, 2001, 37−58. (9) Barnes, P.; Bensted, J. Structure and Performance of Cements, Second Edition, CRC Press, 2002. (10) Groves, G. W. Microcrystalline Calcium Hydroxide in Portland Cement Pastes of Low Water/cement Ratio. Cem. Concr. Res. 1981, 11 (5−6), 713−718. (11) Richardson, I. G.; Groves, G. W. Microstructure and Microanalysis of Hardened Ordinary Portland Cement Pastes. J. Mater. Sci. 1993, 28, 265−277. (12) Chen, J.; Sorelli, L.; et al. A Coupled Nanoindentation/SEMEDS Study on Low Water/Cement Ratio Portland Cement Paste: Evidence for C−S−H/Ca (OH) 2 Nanocomposites. J. Am. Ceram. Soc. 2010, 93 (5), 1484−1493. (13) Emeritus, T.; Taylor, H. F. W. Cement Chemistry; Thomas Telford, 1998; Vol. 20. (14) Hummel, W.; Berner, U.; Curti, E.; Pearson, F. J.; Thoenen, T. Nagra/PSI Chemical Thermodynamic Data Base 01/01. Radiochim. Acta 2002, 90, 805−813. (15) Galmarini, S.; Aimable, A.; Ruffray, N.; Bowen, P. Changes in Portlandite Morphology with Solvent Composition: Atomistic Simulations and Experiment. Cem. Concr. Res. 2011, 41 (12), 1330− 1338. (16) Berger, R. L.; McGregor, J. D. Influence of Admixtures on the Morphology of Calcium Hydroxide Formed during Tricalcium Silicate Hydration. Cem. Concr. Res. 1972, 2 (1), 43−55. (17) TADROS, M. E.; SKALNY, J.; KALYONCU, R. S. Early Hydration of Tricalcium Silicate. J. Am. Ceram. Soc. 1976, 59 (7−8), 344−347. (18) YOUNG, J. F.; TONG, H. S.; BERGER, R. L. Compositions of Solutions in Contact with Hydrating Tricalcium Silicate Pastes. J. Am. Ceram. Soc. 1977, 60 (5−6), 193−198. (19) Odler, I.; Dörr, H. Early Hydration of Tricalcium Silicate II. The Induction Period. Cem. Concr. Res. 1979, 9 (3), 277−284. (20) GEM Software Main Page http://gems.web.psi.ch/ (accessed Jan 19, 2016). (21) Wagner, T.; Kulik, D. A.; Hingerl, F. F.; Dmytrieva, S. V. GemSelektor Geochemical Modeling Package: Tsolmod Library And Data Interface For Multicomponent Phase Models. Can. Mineral. 2012, 50 (5), 1173−1195. (22) Kulik, D. A.; Wagner, T.; Dmytrieva, S. V.; Kosakowski, G.; Hingerl, F. F.; Chudnenko, K. V.; Berner, U. R. GEM-Selektor Geochemical Modeling Package: Revised Algorithm and GEMS3K Numerical Kernel for Coupled Simulation Codes. Comput. Geosci. 2013, 17, 1−24. (23) Kulik, D. A.; Kersten, M. Aqueous Solubility Diagrams for Cementitious Waste Stabilization Systems: II, End-Member Stoichiometries of Ideal Calcium Silicate Hydrate Solid Solutions. J. Am. Ceram. Soc. 2001, 84 (12), 3017−3026. (24) Lothenbach, B.; Winnefeld, F. Thermodynamic Modelling of the Hydration of Portland Cement. Cem. Concr. Res. 2006, 36 (2), 209−226. (25) Santschi, P. H.; Schindler, P. W. Complex Formation in the Ternary Systems Ca II-H4SiO4-H2O and Mg II-H4SiO4-H2O. J. Chem. Soc., Dalton Trans. 1974, 2 (2), 181. (26) Parkhurst, D. L.; Appelo, C. A. J. Water-Resources Investigations Report 99-4259, User’s Guide to PHREEQC (Version 2): A Computer

energy profiles calculated for the [0001] surface showed energy profiles that were significantly different for the neutral CaSiO2(OH)2 complex compared to the SiO2(OH)2− ion. 2 Two clear minima were seen for the Ca−Si complex, whereas no such clear minima were seen for the silicate ion. The free energy profile, as well as unconstrained MD simulations, confirmed the stability of the mobile Ca−Si complex at the surface. This indicates that the Ca−Si complex adsorbs preferentially with respect to calcium and hydroxyl ions, which have been studied previously.27,28 With these observations a mechanism for reduced portlandite growth and poisoning in the presence of silica was made, whereby the Ca−Si complex adsorbs preferentially to the growth species and can migrate along surfaces until a step-like growth feature is encountered where they are immobilized. The proposed mechanism is able to explain previously reported experimental observations. In conclusion the combined use of molecular dynamics and well-tempered as well as conventional metadynamics has allowed a deeper insight into the interaction of silicate species with portlandite surfaces and how these may influence growth and consequently cement hydration kinetics. In addition, the methodology reported here is applicable to other systems, such as the predominant phase in hydrated cement, calcium silicate hydrate (C−S−H), once its atomic structure has been elucidated, which will allow further fundamental insight into the growth kinetics of cementitous systems, the most abundant material used on earth.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07044. Material containing parameters used for metadynamics calculations and error calculations (PDF) Short video sequence showing the alternate breaking and making of hydrogen bonds, allowing the silicate species to move along the surface (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +41 58 765 4066. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Barbara Lothenbach from EMPA Dübendorf for her help with the thermodynamic simulations as well as Ellis Gartner from Lafarge, Prof. Karen Scrivener from EPFL, Patrick Juilland from SIKA, Steve Parker from the University of Bath, and Uli Aschauer from ETH Zurich for their input and helpful discussions. Additionally, the authors would like to thank the industrial-academic research network Nanocem for the funding.

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