Effects of hexitols on the hydration of tricalcium silicate

Cement and Concrete Research 91 (2017) 87–96

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Effects of hexitols on the hydration of tricalcium silicate Camille Nalet, André Nonat ⁎ Laboratoire Interdisciplinaire Carnot de Bourgogne (ICB), UMR 6303 CNRS/Univ. Bourgogne Franche-Comté, Dijon, France

a r t i c l e

i n f o

Article history: Received 9 March 2016 15 September 2016 Accepted 8 November 2016 Available online 17 November 2016 Keywords: Sugar Dissolution Precipitation Hydration Tricalcium silicate

a b s t r a c t The hydration of tricalcium silicate (C3S) in presence of hexitols (D-glucitol, D-galactitol and D-mannitol) which only differ by their stereochemistry was followed by using calorimetric and conductometric measurements of the pastes and of the diluted suspensions respectively. Hexitols delay the acceleration of the growth of C-S-H and enhance the quantity of C-S-H when the maximum rate of C3S hydration is reached in relation to the stereochemistry of their hydroxyl groups: D-glucitol N D-galactitol N D-mannitol. Their relative retarding effect is shown to change depending on their sensitivity to calcium ions at pH found in cement paste. In diluted suspension in solutions representative to a cement paste, alditols are shown to retard C3S hydration whereas its dissolution is not limited suggesting an impact of the molecules on the precipitation of C-S-H. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Cement set retardation generated by organic admixtures is a practical concern for industrial applications and needs to be controlled if desired or counteracted if undesired. Cement setting results from the nucleation and growth of calcium silicate hydrates (C-S-H) due to the hydration of the major phase of cement which is tricalcium silicate (C3S) [1]. Many authors investigated the retardation caused by carbohydrates, sugar alcohols and phenols on cement hydration and tried to understand the mechanism responsible of this delay [2–16] and the following reviews [17,18]. It is mainly reported that the time at which the acceleration of the growth of C-S-H starts is enhanced with increasing concentrations of retarders but their action mechanism is still not well understood [3,8–12]. The hydration of C3S is a dissolution-precipitation process so the retardation induced by the organic molecules may come from an effect of the molecules on the dissolution of the anhydrous and/or on the nucleation-growth of C-S-H. The retardation generated by organic admixtures was often mentioned to result from a disruption of the nucleation-growth process of C-S-H [4,13,19,20,21] but a polycarboxylate functionalized latex and D-gluconate which also retard C3S hydration were shown to strongly decrease the dissolution rate of the mineral phase [22–24]. Given that the hydration kinetics of C3S results from a chemical coupling between C3S dissolution and C-S-H precipitation, the origin of the effects of the organic molecules on the formation of hydrates is not obvious. Consequently, one can realize ⁎ Corresponding author. E-mail addresses: [email protected] (C. Nalet), [email protected] (A. Nonat).

http://dx.doi.org/10.1016/j.cemconres.2016.11.004 0008-8846/© 2016 Elsevier Ltd. All rights reserved.

that the mechanism(s) by which organic chemicals act(s) on the dissolution-precipitation process of C3S hydration is not clearly identified and prevents us from understanding why and how these molecules really retard cement hydration. This study proposes to investigate the action mechanism(s) of three sugar alcohols which were recently reported to delay the hydration of C3S pastes [3,25]. These polyols that are D-glucitol, D-galactitol and Dmannitol have the same chemical formulae C6H14O6 and only differ depending on the stereochemistry of their hydroxyl groups as shown in Fig. 1. Here, the investigation focuses on the effects of alditols on the advancement of C3S hydration in paste and on their impacts on the dissolution-precipitation process with C3S in diluted suspension. In fact, C3S suspensions are more diluted systems than C3S pastes and allow us to identify the influence of the molecules on the dissolution-precipitation process of C3S hydration from the study of the composition of solutions. During the hydration of C3S in paste, due to the low amount of water in contact with C3S, the dissolution of the powder rapidly increases the calcium hydroxide concentration and saturates the solution with respect to calcium hydroxide. This way, the effects of the calcium concentration and of the pH of the initial solutions on the hydration of C3S in suspension were also considered in presence of hexitols. To proceed, the effects induced by the stereoisomers on the advancement of C3S hydration in paste were first studied by using isothermal calorimetry. Secondly, the hydration of C3S with hexitols in suspension started in solutions with different concentrations of calcium and pH representative of different hydration time in paste, was followed by conductometric experiments. Then, the influence of alditols on the dissolution-precipitation process of C3S hydration were investigated by following the evolution of the composition of the solution with conductometric and ionic measurements of the suspensions. Finally, a

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Fig. 1. Chemical structures of the hexitols studied. Spatial representation and Fischer projections. Threo stands for two hydroxyl groups on opposite sides and erythro stands for two hydroxyl groups on the same side of the carbon chain.

discussion focused on the interactions of the molecules in C3S pastes and on the origin of their retarding effect is proposed. 2. Materials and methods 2.1. Materials The water used for the preparation of the pastes, suspensions and solutions was distilled and deionised. The hexitols studied were Dglucitol (≥ 98%), D-mannitol (≥ 98%) and D-galactitol (≥ 99%) from Sigma Aldrich, all as a powder form. C3S was supplied by Mineral Research Processing. It is a pure triclinic C3S (99.2%) with a granulometric distribution centred around 7 μm. The particle size distribution is given in supplementary material. Calcium oxide used in the different experiments was obtained after decarbonation of calcium carbonate (98.5–100%, VWR AnalaR NORMAPUR) at 1000 °C for 24 h. Sodium chloride (N99.5%, AnalaR NORMAPUR, VWR), calcium chloride (N96%, Acros Organics), sodium hydroxide (N 97%, Merck) used to make salt solutions were dried minimum a day in an oven at 180 °C. Saturated lime solutions were obtained by filtering (0.1 μm cellulose ether, Merck Millipore) saturated lime suspensions made by adding an excess of freshly decarbonated lime to water left to equilibrate minimum a day in a thermoregulated bath at 25 °C. In order to study the effect of seeding during the hydration of C3S with hexitols, C-S-H suspensions were synthesized by mixing calcium oxide, aerosil silica (Evonik) and water. The proportions of calcium oxide, aerosil silica were chosen in order to obtain an initial Ca/Si equals to 1.58 and a Liquid to Solid ratio (L/S) equals to 20. These stock suspensions were stirred for a month at 23 °C, which is long enough to reach equilibrium [26].

2.2. Methods 2.2.1. Calorimetric measurements during the hydration of C3S pastes with hexitols The pastes were made by adding 0.4 mL of aqueous solutions at different concentrations of sugar alcohols (0–11.3 mmol/L) to 1 g of C3S (L/ S = 0.4). The pastes were mixed at 3200 rpm for 2 min with a stirrer. Then, they were immediately put in a TAM AIR isothermal calorimeter at 23 °C where the heat flow was measured during their hydration. The cumulated heat flow was calculated by integrating the measured curve of heat flow. Finally, the advancement of C3S hydration was obtained by the product of the cumulated heat flow and the enthalpy of hydration at 23 °C (Δ H = 460.16 J/g). This enthalpy was calculated like Grant [27] for the following reaction: 5C3S + 12H2O → C8S5H10 + 7CH2 with the reasonable assumption that the stoichiometry of the reaction is not changed by the admixtures.

2.2.2. Conductometric studies during the hydration of C3S suspensions in presence of alditols During the hydration of C3S, the concentrations of calcium, silicate and hydroxide ions present in the suspension evolve over time. Hydroxide ions are particularly conductive and allow us to follow the hydration of C3S with and without organics by measuring the conductivity of the suspensions with an electrode related to a CDM210 conductivity meter (both from Radiometer Analytical). The suspensions with L/ S = 100 (1 g of C3S added to 100 mL of different initial solutions) were stirred in thermoregulated cells at 25 °C. A flow of nitrogen gas was ensured in all the cells to prevent the formation of calcium carbonate.

C. Nalet, A. Nonat / Cement and Concrete Research 91 (2017) 87–96 Table 1 Properties of the solutions used for experiments in diluted suspension. Solution

NaCl

NaOH

CaCl2

Ca(OH)2

Ca(OH)2

Water

Concentration (mmol/L) Ionic strength (mmol/L) pH

44 44 7

44 44 12.55

22 66 7

22 56 12.5

11 29.6 12.2

– – 7

2.2.2.1. Hydration of C3S in suspension started in solutions with different initial pH and calcium concentrations with hexitols. In order to reproduce the calcium concentration and/or the pH of the pore solution of C3S pastes at a given time when hydrating C3S in suspension, C3S was added to different initial solutions composed of water; Ca(OH)2 at 11 mmol/L and Ca(OH)2 at 22 mmol/L. The first is representative of the very early beginning in paste, the second of the beginning of the induction period and the last of most of the hydration [28]. While not representative of paste pore solution, solutions of NaCl and NaOH at 44 mmol/L, CaCl2 at 22 mmol/L have been used to evidence the specific role of ionic strength, pH and calcium concentration separately. The concentration, ionic strength and pH of these solutions are given in Table 1. The amount of alditols added to each initial solution was selected to fix the initial concentration of these molecules at 7.50 mmol/L, a value chosen in order to distinguish the impact of the molecules on C3S hydration within a reasonable time. After 30 min to equilibrate the initial solutions, the C3S powder was added. 2.2.2.2. Delayed addition of C-S-H in C3S suspensions with D-glucitol in saturated lime solutions. Different amounts of the C-S-H suspension were added 30 min after the hydration of C3S has started in saturated lime solutions with a constant concentration of D-glucitol (7.5 mmol/L). The amount of the C-S-H suspension was weighed in order to fix the amount of initial C-S-H added. Due to the low volume of water put in the C3S suspension when adding the small amount of the C-S-H suspension (maximum 19 μL of solution), the total volume of the C3S suspensions was considered as unchanged. 2.2.3. Ionic concentration measurements during the pure dissolution and hydration of C3S in suspension with alditols The pure dissolution and hydration of C3S was followed by continuously measuring the silicon in the aqueous phase of the C3S suspensions using Inductively Coupled Plasma — Atomic Emission Spectroscopy. The hydration of C3S was followed in suspensions with L/S = 100 (L = 100 mL of water) whereas the pure dissolution of C3S was followed in highly diluted C3S suspensions with L/S = 10,000 (L = 100 mL of a solution at 11 mmol/L of Ca(OH)2) in order to obtain solutions under-saturated with respect to C-S-H (no precipitation of hydrates).

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The C3S powder was added to the initial solution put in a stirred reactor which was connected to the spectrometer in a room maintained at 23 °C. The suspension was continuously passing through a filtering device (0.1 μm cellulose ether, Merck Millipore) before measuring the silicates concentration of the solution as described in [29] where a similar experimental set-up was used. After this measurement, the left solution which was not analyzed was brought back in the reactor which was under an argon gas atmosphere.

3. Results 3.1. Effects of hexitols on the advancement of C3S hydration The advancement of C3S hydration calculated from the heat flow obtained by isothermal calorimetry in presence of different initial concentrations of hexitols is shown in Fig. 2. The presence of these sugar alcohols in C3S pastes extends the induction period depending on the stereochemistry of their hydroxyl groups compare to the one of the reference without additive: D-glucitol N D-galactitol N D-mannitol. At the end of the induction period, the strong increase of the advancement of C3S hydration is not affected by the organic molecules given that the slope of the curve do not strongly change compare to the one of the reference. However, after the maximum rate of C3S hydration is reached i.e. the maximum heat on the recorded calorimetric curve (not shown) that corresponds to the inflection point of the advancement, the advancement of C3S hydration is higher in presence of the molecules than without. Similar results were revealed when hydrating C3S with sucrose [12] or cellulose ether derivatives [30–32,21] in solution. The evolution of advancement of C3S hydration and of the time at the inflection point are represented as function of the concentration of hexitols in Fig. 3a and b respectively. The data results from the average measurements of two replicates and the y error bars indicate the standard deviation of the mean. It can be observed that both the advancement of C3S hydration and the time at the inflection point increase with the concentration of alditols. Nevertheless, their impact is different depending on their stereochemistry. For a given concentration of polyols, D-glucitol particularly enhances the advancement of C3S hydration and the time at the inflection point and is followed by D-galactitol and then by D-mannitol. Fig. 4 indicates that the advancement of C3S hydration increases with the time at the inflection point but that the effect seems to be different depending on the hexitols present. For a given time at the inflection point, the advancement of C3S hydration is higher with D-glucitol than with D-galactitol and D-mannitol. This result reveals that the presence of D-glucitol particularly enhances the amount of hydrates compared to two other sugar alcohols when the maximum hydration rate of C3S is reached at least at high concentration.

Fig. 2. Advancement of C3S hydration in presence of different concentrations of hexitols, L/S = 0.4.

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a

b

Fig. 3. Evolution of a) the advancement of C3S hydration and b) the time at the inflection point depending on the concentration of alditols.

3.2. Influence of the calcium concentration and of the pH of the solutions on the hydration of C3S suspensions in presence of alditols Conductivity measurements of C3S in suspension were made over time to follow its hydration with and without hexitols (7.5 mmol/L) in different initial solutions and are shown in Fig. 5. The pH and the calcium concentration of the initial solutions were varied by using water and CaCl2, NaCl, NaOH, Ca(OH)2 solutions at different concentrations. After the addition of C3S in the different aqueous solutions, a sharp rise in the conductivity occurs due to the dissolution of C3S. Then, the conductivity remains stable during the induction period and the precipitation of C-S-H occurs. At the end of the plateau, the conductivity starts increasing due to the acceleration of C3S hydration. In fact, the stoichiometric ratio Ca/Si of C3S when dissolving (Ca/Si = 3) is higher than the one of C-S-H when precipitating (Ca/Si b 2): there is an accumulation of calcium hydroxide in solution which increases the conductivity until portlandite precipitates. Fig. 4 shows that hexitols, whether in water, NaCl solutions at 44 mmol/L or CaCl2 solutions at 22 mmol/L do not have noteworthy effect on the hydration of C3S. In contrast, they lengthen the induction period in Ca(OH)2 solutions at 11 and 22 mmol/L and in NaOH solutions at 44 mmol/L. Hence, whatever the calcium concentration in initial solutions at low pH (≈7), there is no retarding effect induced by alditols on C3S hydration whereas a delay

is generated at higher pH. It can be noted that D-galactitol is the best retarder of C3S hydration in 11 mmol/L Ca(OH)2 solutions and is followed by D-mannitol and D-glucitol which have similar retarding effects. On the other hand, in 22 mmol/L Ca(OH)2 solutions, D-glucitol is the best retarder and D-mannitol is the less retarding molecules, D-galactitol being between them. The relative order of retardation changes depending on the concentration of Ca(OH)2. Nevertheless, when comparing the impacts of the molecules on C3S hydration in Ca(OH)2 solutions at 11 and 22 mmol/L, both the calcium concentration and the pH vary so their retarding effects cannot be related to their interaction with calcium and/or hydroxide ions. To rationalize this, the influence of the calcium concentration on the retardation of C3S hydration caused by alditols is identified at a given pH and vice-versa. D-Galactitol retards more the hydration of C3S in 44 mmol/L NaOH solution than the two other hexitols whereas no retardation is observed in NaCl solutions at 44 mmol/L. D-galactitol is then particularly sensitive to pH in solutions exempted of calcium ions. When comparing the retardation induced by the sugar alcohols in Ca(OH)2 solutions at 22 mmol/L and NaOH solutions at 44 mmol/L, one can observe that D-glucitol has an enhanced retarding effectiveness in the first solution mentioned for a given and high pH (≈12.5). It reveals that D-glucitol is especially sensitive to calcium ions at high pH. On the other hand, this molecule is pretty much insensitive to calcium

Fig. 4. Representation of the advancement of C3S hydration as a function of the time at the inflection point in presence of the different hexitols.

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ions at lower pH (≈7) as the other alditols (solutions at 22 mmol/L of CaCl2). It is worth to notice that the relative retarding effects of alditols on the hydration of C3S pastes and of C3S suspensions in saturated lime solutions are similar. In fact, when hydrating C3S in paste, the solution which is initially water becomes rapidly saturated with respect to calcium hydroxide and allows us to assume that comparable interactions of the molecules with calcium and hydroxide exist in these two systems.

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3.3. Impacts of hexitols on the dissolution-precipitation process of C3S hydration 3.3.1. Effects of alditols on the pure dissolution of C3S The pure dissolution of C3S with and without D-glucitol, D-galactitol and D-mannitol (7.5 mmol/L) in 11 mmol/L Ca(OH)2 solutions was followed to identify their effects on the initial rate of C3S dissolution. This concentration has been chosen because it is representative of the

Fig. 5. Conductivity measurements monitored during the hydration of C3S suspensions in initial solutions at different pH and calcium concentrations with and without hexitols (7.5 mmol/L), L/S = 100.

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concentration of the pore solution of the C3S paste during the induction period. The comparison of the impact of hexitols on the silicates concentration over time is presented in Fig. 6. In presence of D-mannitol in the C3S suspension, the silicates concentration is similar to the one of the reference for a given time. With D-galactitol and D-glucitol, the concentrations of silicate ions of the C3S suspensions are even more important than for the reference sample. These molecules are then supposed to increase the dissolution rate of C3S by complexing with calcium, silicate and/or hydroxide ions in solution. Hence, the pure dissolution of C3S in 11 mmol/L Ca(OH)2 solutions is not limited by the polyols studied but is even enhanced in presence of the strongest retarders that are Dglucitol and D-galactitol.

3.3.2. Effects of hexitols on the precipitation of C-S-H during C3S hydration The hydration of C3S started in water was followed by measuring the silicates concentration of the suspensions over time with and without sugar alcohols present, Fig. 7. Once C3S is added to the aqueous solutions, the concentration of silicates immediately increases due to the dissolution of C3S and reaches a maximum value. Then, it keeps decreasing due to the precipitation of C-S-H consuming silicates which is faster than the dissolution of C3S. The precipitation of C-S-H can be described as a primary and secondary nucleation. The primary heterogeneous nucleation refers to the spontaneous formation of C-S-H at the surface of C3S when the critical super-saturation with respect to the hydrate is reached in solution. The secondary heterogeneous nucleation corresponds to the precipitation of new C-S-H at the surface of initial nuclei. Indeed the true growth of C-S-H nuclei is very limited. Just after the concentration of silicates reaches its maximum value, no plateau of the silicates concentration is observed but a drop of the concentration is identified with and without the molecules due to the primary nucleation of C-S-H [33]. This result indicates that there is no apparent effect of alditols on the primary nucleation of C-S-H in the conditions of the study. However, after 4 min of hydration, the silicates concentrations of the suspensions of C3S with sugar alcohols decrease slowly compare to the one of the reference. Comparable effects on the silicates concentration over time were identified during the hydration of C3S in suspension with retarding organic molecules such as D-gluconate, cellulose ethers and a polycarboxylate functionalized latex [21–23]. This important concentration of silicates could be interpreted as a slowing down effect caused by the organics on the secondary nucleation of C-S-H or/ and it might be related to the complexation of the molecules with silicate, calcium and/or hydroxide ions present in solution.

3.3.3. Effects of a delayed addition of C-S-H on the hydration of C3S suspensions with D-glucitol Already made C-S-H were added to C3S hydrated in saturated lime solutions with D-glucitol (7.5 mmol/L) at 30 min of hydration. The conductometric curves monitored during the hydration of C3S with Dglucitol in presence of an increasing amount of C-S-H are compared in Fig. 8. One can first observe that the time at which the conductivity starts to increase (at the end of the plateau of conductivity) is reduced in presence of increasing C-S-H contents. Hence, the acceleration of C3S hydration occurs earlier when adding new C-S-H in presence of Dglucitol. Moreover, the values of the maximum conductivity revealing the composition of the solution reached before portlandite starts precipitating are really similar in presence of D-glucitol whatever the amount of C-S-H added to C3S suspensions (around 13.1 mS/cm). However, the value of the maximum conductivity is higher in presence of D-glucitol than without (around 12.6 mS/cm). It means that the molecules modify the ionic composition of the C3S suspension and that the supersaturation with respect to portlandite reached is similar whatever the amount of new C-S-H added. These results lead us to suggest that the ionic composition of the solution corresponding to the supersaturation with respect to portlandite is the consequence of the interaction(s) of Dglucitol which occurred before the addition of new C-S-H. These interactions can be a complexation with some ions in solution, an adsorption at the surface of C3S and/or on C-S-H precipitated on C3S. 4. Discussion The retarding effectiveness of hexitols on the hydration of C3S pastes depends on the stereochemistry of their hydroxyl groups: D-glucitol N Dgalactitol N D-mannitol. In addition, after the acceleration of the reaction, the degree of hydration is enhanced in the same way. In order to quantify more precisely these effects, the kinetic curves have been simulated using a mesoscopic model of hydration. 4.1. Simulation of the kinetic curves The hydration model is based on the anisotropic growth of C-S-H onto the surface of C3S [34]. The growth is described with two rates, parallel and perpendicular to the surface and the rate of hydration depends strongly on the number of nuclei precipitated at the very early beginning of hydration. The model captures well the role of the composition of the pore solution (mainly the calcium hydroxide activity) [34] and

Fig. 6. Evolution of the silicates concentration over time during the pure dissolution of C3S in suspensions with and without hexitols (7.5 mmol/L), [Ca(OH)2] = 11 mmol/L and L/S = 10,000.

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Fig. 7. Evolution of the silicates concentration during the hydration of C3S suspensions started in water as a function of the time in presence of hexitols (7.5 mmol/L), L/S = 100.

the effect of the particle size [35]. The evolution of the surface area developed by C-S-H and measured by proton NMR relaxometry is also well described [36]. Taking this growth description combined with the dissolution of the C3S grains of different sizes, allows to perfectly describe the rate of the hydration from the beginning of the acceleration till the complete hydration [37] if two dissolution flows are considered: – the dissolution of the C3S not covered by C-S-H – the dissolution of C3S already covered by C-S-H

the second being slower. The ratio between the two dissolution flows is proportional to DeffSdiff/E where Sdiff is the surface area of C-SH that covers C3S, E the thickness of the C-S-H layer, and Deff an effective diffusion coefficient. The model also successfully describes the effect of accelerators [38]. The simulation proceeds according to a cellular automat algorithm at the mesoscopic scale in which each element (10 ∗ 10 ∗ 10 nm3) dissolves from C3S to precipitate onto its surface. In a way, it is a numerical version of the BNG model that takes into account the diffusion through the already precipitated C-S-H. Both models describe the hydration of pure C3S without introducing an induction time and in both cases, some parameters are covariant, the initial number of nuclei and the lateral

growth rate in the present model, and the three transformed volume fractions in the BNG model [39]. An example of the simulated kinetic curves is given in Fig. 9 in the case of the reference sample and in presence of glucitol at 11.5 mmol/L. The results of the simulations in term of induction time, number of initial nuclei and growth anisotropy (axial/ lateral rate) are given in Fig. 10 and the simulated curves in supplementary information with the details of the calculation. The simulation requires introducing an induction time as soon as hexitols are present in solution even at low concentration. It was the same in the case of the addition of methyl hydroxyethyl cellulose for which the kinetic curves were fitted with the BNG model [31]. This induction time, which can be seen as a measure of the retardation is the same for the 3 hexitols at the lowest concentration but it is clearly greater for glucitol at the highest concentrations. The shortest induction times are found for mannitol. The parameters describing the nucleation and growth of the C-S-H layer around the C3S grains are also influenced by the presence of alditols. The density of initial nuclei (which is in fact here the density of nuclei after the induction time) is greater with alditols for the lowest concentration studied than in the case of the reference and is smaller for the highest concentrations. The curves are fitted with less initial nuclei in case of glucitol and galacticol than for mannitol. Concerning the growth anisotropy of the C-S-H layer, according to the model, the glucitol would strongly favour the growth perpendicular to the surface. The same trend is observed for galacticol and

Fig. 8. Delayed addition of C-S-H to C3S suspensions in presence of 7.5 mmol/L of D-glucitol, [Ca(OH)2] = 22 mmol/L and L/S = 100.

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Fig. 9. Comparison of the evolution of the experimental (full lines) and simulated (open squares) degrees of hydration in function of time in the case of the reference sample (hydrated in water) and in a 11.5 mmol/L glucitol solution.

mannitol but in a much smaller extend. To summarize, alditols not only delay the acceleration of hydration but seem to modify the growth of the C-S-H layer. The induction time, introduced in the simulation can be seen as the time required getting the initial density of nuclei able to grow. 4.2. The nucleation delay The nucleation delay may have to origins, the hindrance of the dissolution and/or the hindrance of the nucleation by the adsorption on C3S and C-S-H respectively. Indeed, we recently showed that D-glucitol and D-mannitol adsorb on C-S-H in systems at equilibrium [40] so these molecules may interact with C-S-H present at the surface of C3S during its hydration. The surface properties of C3S in solution being very close to C-S-H one [41], it is probably the same on C3S. The pure dissolution experiments reported Fig. 6 indicates that in very diluted conditions (L/S = 10,000), the dissolution is not limited in presence of hexitols in a lime solution at 11 mmol/L. When hydration starts in paste (L/S = 0.5), the lime concentration is much lower (0–6 mmol/L) and one can expect that the effect should be smaller. It is the same in the experiments reported in Fig. 5 at lower liquid/solid ratio (L/S = 100); whatever the lime concentration, there is no limitation of the initial dissolution and a retardation of the acceleration of hydration is observed in lime solutions. Several authors which investigated the effect of different retarders such as yellow dextrin, cellulose ethers and hydroxypropylguars on the pure dissolution of cement or C3S also found that there was no limitation of the dissolution process caused by these molecules [4,6,21]. However, there is a delicate question

about the balance between complexation and complexation driven adsorption. This depends on the liquid to solid ratio and makes comparing experiment at different L/S in presence of organic additives very delicate [42]. There is not simple answer as it depends on the specific balance between adsorption and complexation for each compound. Unfortunately, it has not been possible to determine neither surface nor bulk solution complexation for hexitols yet [40]. However, a simulation of the repartition of the adsorbed and complexed species in function of the liquid to solid ratio has been made in the case of sodium gluconate which shows a similar repartition even if both adsorption and complexation are higher [40]. The simulation shows that in this case, even if complexation in solution increases of 3 orders of magnitudes when L/S increases from 1 to 10,000, the adsorption also increases of 2 orders of magnitude (see details in supplementary information). One could probably expect a stronger effect of the adsorption on the dissolution at the highest L/S. In addition, in the specific case of gluconate which adsorbs more than hexitols, a slow down of the dissolution is observed but not a complete hindrance [23]. The impact of these hexitols on the dissolution of C3S does not seem to be at the origin of their retarding effect. In a separate study, we have proposed that hexitols are supposed to retard C3S hydration by impeding the precipitation of C-S-H due to an interaction of the molecules with ions in solution and/or to an interaction with C-S-H [25]. In fact, a slow decrease of the silicates concentrations was identified in solution during the hydration of C3S started in water in presence of hexitols (Fig. 7) whereas no retardation of C3S hydration was noticed by conductometric measurements (Fig. 5). Then, these important silicates concentrations result from the complexation of hexitols with silicate, calcium and/or hydroxide ions in solution and not from the hindrance of the precipitation in theses ionic conditions. The more probable origin would be the adsorption on the C-S-H nuclei, hindering their growth. In this hypothesis, the dissolution-precipitation process should continue at a very small rate (in condition very close to the C3S solubility) until all the molecules would be adsorbed and the next nuclei could grow. This hypothesis is in agreement with the effect of the delayed addition of C-S-H presented in Fig. 8. The added C-S-H bring an extra surface to adsorb and consume the remaining hexitols and so reduce the time needed for the hydration accelerates. Nevertheless, the only way to prove what the limiting step is between dissolution and precipitation is to compare the activity product of the ions in solution with the solubility product of C3S and C-S-H during the induction period: if it is closer to the C-S-H solubility product in presence of hexitols (farer to the solubility product of C3S) than in the case of the reference, that means that the dissolution is the limiting process [43]. But this require to be able to calculate the activity product and for this to determine first the complexation equilibria with hexitols in solution. 4.3. The role of the hexitols stereochemistry The role played by the pH and the calcium concentration on the delay generated by hexitols on the hydration of C3S was pointed out

Fig. 10. Variation of the adjusted parameters used to fit the hydration kinetic curves in function of the concentration of hexitols: a — induction time, b — nuclei density, c — growth anisotropy.

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suggesting an interaction of the molecules with calcium and hydroxide ions. It is known that hexitols form calcium complexes in aqueous solution and D-glucitol is often stated as the strongest calcium complexing hexitol [40,44–47]. Some authors also underlined that an interaction of hexitols with hydroxide ions may exist [40] and that hydroxide ions favour the complex formation of D-glucitol with calcium ions [45]. The configurations of the hydroxyl groups carried by the hexitols studied are described in Fig. 1 thanks to their Fischer projections. Angyal proposed different effectiveness of the complexing sites depending on the configuration of the molecules [48]: threo-threo-erythro N threoerythro-threo N erythro-threo-erythro. The retarding effect caused by hexitols on C3S hydration follows the complexing sites effectiveness proposed by Angyal. Consequently, the surface complexation driving the adsorption seems to follow the same trend. 5. Conclusions The retarding effectiveness of hexitols (D-glucitol, D-galactitol and Dmannitol) on C3S hydration was related to their affinity with hydroxide and calcium ions present in solution. Moreover, the impacts of these organic molecules on the dissolution-precipitation process of C3S hydration were investigated. The study reveals that alditols delay the acceleration of C3S hydration and enhance the degree of hydration when the maximum rate of C3S hydration is reached depending on the stereochemistry of their hydroxyl groups: D-glucitol N Dgalactitol N D-mannitol. The retardation on the hydration of C3S induced by hexitols depends on their sensitivity to both hydroxide and calcium ions in solution but their relative retarding effectiveness in paste is controlled by their affinity with calcium ions. It is also pointed out that the delay generated by alditols does not come from an effect of these organic molecules on the dissolution of C3S but is assumed to result from a hindrance of the precipitation of C-S-H. Acknowledgements The authors are grateful for the financial support from Nanocem under Core Project CP12, the industrial-academic nanoscience research network for sustainable cement and concrete. They would like to warmly thank all partners interested in the project for the fruitful discussions. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi:10.1016/j.cemconres.2016.11.004.

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