Surface area measurement of C 3 A/CaSO 4 /H 2 O/superplasticizers system

Transcription

1 Surface area measurement of C 3 A/CaSO 4 /H 2 O/superplasticizers system by F. DALAS, S. POURCHET, A. NONAT, D. RINALDI, M. MOSQUET and J.-P. KORB Synopsis : Hydration leads to hydrated phases precipitation which are more or less covered by the adsorbed admixture depending on the added amount and the nature of the surface. To predict the optimum dosage and to understand the fluidity evolution at the early age, it is interesting to study the specific surface area development of cement during this period. It was shown that the workability evolution is mainly due to the tricalcium aluminate (C 3 A) hydration. Here we choose a simplified system: C 3 A/CaSO 4 /calcite/superplasticizer. We measured the specific surface area by two techniques: the BET method using nitrogen as reference and the nuclear magnetic spin-spin relaxation of 1 H water. We highlighted a good correlation between the surface measured by NMR relaxometry and by the BET method. Continuous NMR measurement confirms the point-bypoint BET measurements that are often subject to question. Thanks to both methods, we studied the influence of the dosage in superplasticizer and of the structure in terms of anionic group density on the specific surface area. In the development of the surface area, we demonstrated that the step mainly impacted by superplasticizers is the nucleation and not the growth. When the dosage of superplasticizer increases, the nucleation is promoted and the growth of ettringite is limited. As consequence, the specific surface area of ettringite increases with both the number of charges added and the backbone charge density. The surface area goes up in presence of superplasticizer as consequence of a dispersing effect (hydrates are not attached to anhydrous phases) and of a reduction of hydrates size. Keywords: specific surface area; NMR relaxation; BET method; tricalcium aluminate; comb-type superplasticizers

2 Florent Dalas is a PhD student from the University of Burgundy in the Laboratory Physical chemistry of cement and colloidal media (Dijon France) and Lafarge Central Research Laboratory (St Quentin Fallavier France). He focused his work about superplasticizers/cement interactions. Sylvie Pourchet is in charge of researches at the University of Burgundy in the Laboratory Physical chemistry of cement and colloidal media (Dijon France). Her current research interests are mainly the study of mineral surface interactions with admixtures. André Nonat is a research director at the CNRS/University of Burgundy in the Laboratory Physical chemistry of cement and colloidal media (Dijon France). His research is based on the reactivity of cement with special interest in studying the mechanism of hydration reactions, the thermodynamical and microstructure of hydrates and the mechanism of the setting. David Rinaldi is a research engineer in the group Binder/Admixture interactions of the Lafarge Central Research Laboratory (St Quentin Fallavier France). Martin Mosquet is a senior scientist of the Lafarge Central Research Laboratory (St Quentin Fallavier France). Jean-Pierre Korb is a research director at CNRS in the condensed matter physics laboratory (Palaiseau France). His current research interests are mainly the NMR and dynamics of liquids in the porous media and biological systems. INTRODUCTION Numerous studies have been carried out to contribute to a general understanding of the influence and working mechanism of comb-type superplasticizers on cement suspensions. Yamada 1 pointed out the important factors affecting the performance of superplasticizers: the specific surface area and the solution chemistry. The dispersing effect is expected to depend on the amount adsorbed and more precisely on the adsorption density on the solid surface. Knowing and measuring the specific surface area of solid phase during the early age of cement is a crucial step to enhance the efficiency of comb-type superplasticizers. Several studies showed the impact of admixtures on the precipitation and morphology of hydrates. But few studies report the effect on the intrinsic specific surface area of hydrates. Morphological differences reflect changes in growth in line with superplasticizer adsorption on specific crystal faces. Changes in the morphology have been confirmed on a synthetic ettringite 2 or on an ordinary Portland cement 3. Prince 4 showed that fine needles of ettringite become, in the presence of polynaphtalene sulfonate, shorter and massive crystals. The crystal size reduction has also been observed by Zingg 5 by cryo-microscopy of cement paste with polycarboxylate superplasticizers. In dispersed cement pastes, the interstitial pore space between the larger clinker particles contains well dispersed small crystals. Those small crystals are very numerous and of similar size. The major hypothesis of the growth slowing down is the promotion of nucleation 6. Holzer 7 demonstrated by nanotomography coupled with cryo-fib, a strong increase of the specific surface area with superplasticizers during the first 6 minutes linked to a significant precipitation of crystals inferior to 500nm. To simplify our cement model system, we decided to focus our attention only on the tricalcium aluminate (C 3 A) hydration. Its reactivity is the cause of the workability evolution during the first hours with ettringite and AFm formation 8. Studies on pure cement phases, for example Zingg et al. 9, showed that ettringite is the cement phase which most adsorbs superplasticizers. A system with tricalcium aluminate, gypsum/hemihydrate and calcium carbonate, an inert supplementary cementitious material, calcite, to replace silicate phases was chosen. To avoid the initial AFm precipitation (monocarbonate) instead of expected ettringite, we used a mixture of gypsum and hemihydrate as sulfate source and a synthetic pores solution containing 60mmol/L of sulfate 10. Aqueous calcite (CaCO 3 ) suspensions can provide an inert system model for early age cementitious materials by simulating superplasticizer/cement interactions and their consequences on rheology without the chemical reactivity of cement and especially silicate phases 11. Calcite in saturated lime water solution has electrical surface properties and a specific surface area (both main parameters governing the adsorption) similar to those of unhydrated cement. Various methods have been already used to evaluate the specific surface area of a hardened cement paste as reviewed by Thomas 12 : gas sorption, neutrons or x-rays small-angle scattering, nuclear magnetic resonance and mercury porosimetry. These techniques are well adapted for measuring hydrated pastes when the porous network is already partially formed, but superplasticizers enhance the properties of fresh concrete (0 3 hours) such as the workability. The challenge of this work is to study the surface at the fresh state. During the early age, concrete is a mix of liquid water, anhydrous phases and first hydrates as ettringite, AFm, C-S-H and portlandite.

3 Among the above techniques, we decided to evaluate the ability of the gas sorption (BET method using N 2 ) and NMR to monitor the surface area during the period of workability. The BET method consists in adsorbing a monolayer of nitrogen gas on the material surface. As shown by Yamada 13, the effective specific surface area of a cement paste may change significantly with the sample preparation and measurement conditions. In general, samples must be pre-heated to dry and remove all the liquid water. The measurement is performed under vacuum. Cement hydrates are very sensitive to drying, for example, ettringite can decompose as a function of water pressure and temperature 14. Vacuum drying of cement pastes for BET analysis should be done below 60 C (140 F) to avoid changes in specific surface area by thermal decomposition. Nuclear magnetic resonance and nuclear spin relaxation can be used for continuous characterization of the evolving microstructure of various cementitious materials as reviewed by Korb 15. This method is based on the clear separation of surface and bulk contributions of the overall 1 H nuclear spin-lattice (or spin-spin) relaxation rate of water confined within the hydrated cement. Nuclear magnetic relaxation has the advantage of being a non-invasive and continuous technique contrary to the BET method which can perturb the microstructure at early age. For example, Holly 16 published a study about the C 3 A/Gypsum hydration without superplasticizer by NMR relaxometry. The author observed the formation of ettringite and its conversion in AFm phases. The purpose of this paper is to follow the specific surface area development in the early age in presence of superplasticizers. Nitrogen BET was chosen as reference measurement. The NMR relaxation allows a continuous monitoring. Relaxometry will be compared with the point-by-point BET often subjected to question because of a destructive behavior for hydrates. By using this multi-method approach, two examples about the interaction C 3 A/superplasticizer are presented. The first example is the influence of the superplasticizer dosage and the second example concerns of the anionic group density on the specific surface area development. EXPERIMENTAL Materials Polycarboxylate superplasticizers To study the effect of the anionic group density, two different PCP were synthesized by conventional radical polymerisation of PEO methacrylate and methacrylic acid (MAA) with an average molecular weight close to g/mol. The length of the side chains was set at a constant value of 1100 g/mol. Then the chosen molar grafting ratio (PEO/(PEO + MAA)= XX %mol) was 20% and 38% respectively designated by PCP20 and PCP38. Minerals Cubic C 3 A was prepared by mixing in stoichiometric ratio pure reagent grade Al 2 O 3 and CaCO 3. The mixture was heated 1 hour in air at 800 C (1472 F) and after 6 hours at 1450 C (2642 F) and then crushed. It was finally mixed with 10w% of gypsum and 10w% of hemihydrate and crushed to a BET specific surface area of 0.66 m 2 /g. It gives a SO 3 /C 3 A molar ratio of 0.43 close to a well-sulfated cement. The calcite Betocarb HP-OG was supplied by OMYA France. The specific surface area determined from nitrogen adsorption using the BET equations was found to be equal to 0.70 m 2 g -1. Synthetic pore solution The solution used is saturated with respect to calcium hydroxide to keep the surface charge of calcite unchanged and contained 60mM of sulfate ions. It was prepared by adding 1.8g ( lb) of CaO and 10.8g ( lb) of Na 2 SO 4 to 1L of distilled deionised water. This suspension was continuously stirred at 25 C (77 F) and then filtered through 0.3 µm filters. The ionic concentrations were then determined by atomic emission spectroscopy: [Ca] = 15±1mmol/L, [S] = 60±2mmol/L and [Na] = 152±5mmol/L. Mixture proportions All the experiments were carried out with water-solid ratio equal to The mixture proportions and the different experiments are given in the table 1: Code Type of PCP Table 1 Mixture proportions and codes for this study Dosage of PCP (w% by weight of solid) 0PCP Without PCP 0 0,05PCP ,10PCP20 PCP ,15PCP ,10PCP38 PCP C 3 A/Gypsum/Hemihydrate/Calcite (w%) 16 / 2 / 2 / 80 The paste is prepared using a blender according to the following procedure. The synthetic pore solution (without or with the dosage of PCP) is poured into the blender and then the powder for about 30 seconds. Mixing is then performed: 1 minute speed 5000 rpm, 1 minute without mixing, 1 minute speed 5000 rpm.

4 Methods Stopping of mixture hydration The mixture containing C 3 A and Gypsum/hemihydrate was stopped for DSC and BET nitrogen analysis at different times (5, 10, 20, 30, 60 and 120 minutes). At each time, about 10g (0.022 lb) of paste was taken from the hydrating sample and poured into a beaker. 30 millilitres of acetone were added and the suspension was filtered under vacuum and rinsed three times with acetone 17. The filtrate was then dried at 40 C (104 F) in a ventilated oven for 12 hours. This low-temperature drying method permits to avoid the thermal decomposition of ettringite. Quantification of the amount of ettringite With or without superplasticizer, hydrated phases detected by DSC were only ettringite and gypsum which precipitate from the initial reactants. The amount of ettringite was quantified by differential scanning calorimetry (TA Instruments DSC 2920). Under the experimental conditions, the quantification of the amount of ettringite by X-ray diffraction was not possible because the calcite present in the mixture is well crystallized and masks the signal of ettringite. Differential scanning calorimetry allows having a better accuracy thanks to the sole response of hydrated phases. The stopped sample was heated from 25 C to 220 C (77 F to 428 F) at 10 C/min (18 F/min). The peak of ettringite partially overlaps with gypsum. The quantification of ettringite was then determined after a previous desummation of both peaks. The amount of ettringite in a sample is the ratio of the ettringite peak area per gram of the sample to the peak area of pure synthetic ettringite per gram of ettringite. The accuracy was estimated at ±0.003 g of ettringite/g of dried solid. BET method using nitrogen The specific surface area was measured by the BET method using nitrogen (Micromeritics Tristar II 3020). After the stopping of the hydration, samples were degassed during 18 hours at 40 C (104 F) to remove all molecules adsorbed on the surface. This low-temperature method permits to avoid the thermal decomposition of ettringite as shown by Yamada 13 or by Zhou 14. Surface area was calculated using the BET method of analysis, over a relative pressure range of on the adsorption isotherm. The accuracy was estimated at ±0.1 m 2 /g of dried solid. NMR relaxation A non-invasive «real time» 1 H (proton of hydrogen atom) NMR spin-spin relaxation technique is used to follow the hydration of C 3 A in presence of superplasticizers. Proton transversal relaxation consists in measuring the decay of the transverse magnetization M xy with one or more relaxation times after a multipulses spin echo excitation on the transverse plane. There is a loss of the phase coherence of spins after stopping the radiofrequency excitation. 1 H (NMR) relaxometry experiments were performed using a low field spectrometer Maran-Ultra (0.55T) from Oxford Instruments Molecular Biotools (England), operating at the proton Larmor frequency of 23 MHz. The transverse relaxation decays were measured by the well-known Carr- Purcell-Meiboom-Gill (CPMG) sequence used to avoid the phase shifts due to local inhomogeneities of the magnetic field. This CPMG sequence was performed during two hours of hydration process. Raw Data (normalised by the first magnetization of the Free Induction Decay sequence) were fitted by bi-exponential decay function F(t) with least squares method refinement according to the following equation (1): t t M norm ( t, t ) P ( t ) hyd hyd P ( thyd ) 1 exp + + c T t 2 exp (1) T t = 2, long ( hyd ) ) 2, short( hyd ) ) where t is the time, t hyd is the hydration time, T 2,long and T 2,short are the corresponding transverse relaxation times associated with the two magnetization proton populations P 1 and P 2 (P 1 +P 2 1). We double check this bimodal fitting procedure by the inverse Laplace transform method and found the same relaxation parameters than those of Fig. 1. Proton relaxometry is able to study the specific surface area development 18. The interpretation of the relaxation curves is based on the general biphasic fast-exchange model between the protons spins belonging transiently to the surface and the bulk in a given pore. This model assumes that the molecular exchange between these two phases is faster than individual proton relaxation times. For a given pore size, the overall proton relaxation rate is written as: 1 f f = + (2) bulk surf bulk surf T2 T2 T2 where T 2bulk and T 2surf are the proton relaxation times in the bulk and at the surface, respectively. In Eq. (2), f surf =1 f bulk and f bulk are the volume fractions of the surface and bulk phases, respectively. The bulk relaxation rate is much lower than the surface relaxation rate, so equation (2) thus simplifies to 1 N S 1 (3) surf T2 N T2 where N S /N is the ratio of the number of water molecules at the surface, N S to the total number, N, of exchangeable water molecules in the sample. Thus this measure gives an access to the specific surface area by the equation 4:

5 S ( t ) 1 ( t ) hyd T2 p, NMR hyd = (4) Cte f ( σ s,τ m ) where σ S is the surface density of fixed Fe 3+ paramagnetic impurities in the sample and τ m is the correlation time characterizing the two-dimensional translational diffusion of the proton species at the surface and f a function defined in ref. 19. All the NMR results are expressed in terms of 1/T 2,long because it is directly proportional to the specific surface area. The accuracy was estimated at ± 4% on the relaxation rate (1/T 2 ). RESULTS AND DISCUSSION A non-invasive method to follow the specific surface area development Relaxation rates for the five different experiments are plotted versus the BET surface area on the figure 1. A linear relationship is obtained between the BET and NMR measurements of the specific surface area with a good correlation coefficient (R 2 = 0.92). This correlation is valid for the first two hours of hydration and with or without superplasticizer. Though the specific surface area by NMR ranges within the results of most other techniques as in this case the BET method, there are several advantages in favour of this measurement. (i) Proton NMR relaxometry is neither invasive nor destructive because one measures the response of the mixing water itself in the normal saturated state of cement. No liquid or gas intrusion, no drying or other temperature and pressure modifications are required that risk damaging the microstructure of hydrates. (ii) The measurement is sufficiently fast to be applied continuously during the progressive hydration of the material. Using T 2 instead of other NMR-sensitive relaxation parameters, such as spin-lattice relaxation time (T 1 ), permits to follow the hydration in real time, as it takes only approximately 1 minute to achieve a good signal-to-noise ratio. (iii) The remarkable features of the proton relaxometry at low frequency allow one to probe directly the proton surface dynamics that contribute to specific surface area. There is no size effect of the probe molecule like with gas sorption (water versus nitrogen) that can explain the variability between different techniques. Fig. 1 Comparison between the relaxation rates (1/T 2 ) and the BET surface area for the five experiments at 10, 20, 30, 60 and 120 minutes. As mentioned before, proton NMR relaxometry is a continuous and non-invasive method to evaluate the rate of the surface creation after the introduction of water. But this method can be applied only in a presence of liquid water and especially cannot be applied on dry powder. BET method using nitrogen is often disparaged because it needs a cautious preparation. Thus it is a good method to study the specific surface area of dry powder (for example powder mixture before water introduction) or of samples properly stopped and prepared. We can measure the initial precipitation of the first hydrates during the first five minutes and follow the surface area

6 point-by-point. Here the interest of using a multi-method approach to study the specific surface area development was highlighted. Specific surface area development at the early age in presence of superplasticizers Influence of the dosage Figures 2a and 2b compare the influence of the added amount of PCP20. A reference without PCP is reported on each figure. The BET surface area is reported on the figure 2a. The figure 2b describes the relaxation rate (1/T 2 ) which is proportional to the specific surface area measured by NMR during the first two hours of hydration. The evaluation of the surface by both methods shows the same temporal evolution. The surface area increases continuously during the hydration for each experiment. Curves are slightly bended during the first 5-30 minutes and almost linearly dependent between 30 and 120 minutes. The mixture without PCP has the lowest surface area. With an increasing dosage, the surface is constantly growing during the hydration. There is no cross over between the curves. Fig. 2 Influence of the PCP20 dosage (0.05, 0.10 and 0.15w%) on the specific surface area. (a) BET surface area versus hydration time; (b) Relaxation rate versus hydration time. The surface creation can be divided in three periods. The first part between 0 and 5 minutes corresponds to the nucleation step. According to BET measurements, there is an increase by 25% of the specific surface area without PCP whereas it is between 90 and 120% in the presence of superplasticizer. The higher the dosage, the greater the surface area. The influence of superplasticizer on this step will be analyzed in the next paragraph. The second period can be found between 5 and 30 minutes with an increase of the surface between 20 and 40% for all samples. The growth of surface area is not as strong as in the first period. The added amount of PCP seems to have no influence on the growth rate of hydrates. The last part, between 30 and 120 minutes, is a slow growth regime of hydrates. Here again the presence of PCP has no influence on the increase rate of surface area. Influence of the structure Figures 3a and 3b compare the influence of PCP structure on the evolution of the specific surface area. Results for 0PCP and 0,10PCP20 are presented for comparison. As for the influence of the dosage, the BET surface area point-by-point reproduces the continuous evolution of the relaxation rate. The evolution of the surface area for 0,10PCP38 is linear during the two hours (figure 3a and b). After 30 minutes, the surface area is superposed to the value without PCP.

7 Fig. 3 Influence of the structure, PCP20 and PCP38 at 0.10w% on the specific surface area. (a) BET surface area versus hydration time; (b) Relaxation rate versus hydration time. The PCP structure clearly influences the surface creation when PCP20 is compared with PCP38 at the same dosage 0.10w%. Three periods can be distinguished for the PCP with a highly charged backbone. With a weakly charged backbone, the same initial strong nucleation can be noticed between 0 and 5 minutes. But the surface increase is the highest with a high-charge backbone: 100% for the PCP20 versus 63% for the PCP38. After this first step, the growth with two regimes observed for the PCP20 is replaced by a unique slow regime of growth for the PCP38 (figure 3b). Moreover this growth occurs at the same speed as the regime between 30 and 120 minutes for PCP20. From 30 minutes, a specific surface area equals to without PCP is obtained with PCP38. Considering these results, we can ask the question about the origin of the initial increase (0 to 5minutes) of the surface in the presence of PCP. Is it induced by the dispersing effect of the PCP which prevents the attraction between seeds and anhydrous phases and artificially create surface? Or is there a real impact of the PCP on the specific surface area of hydrates? This increase by a dispersing effect could be seen by the proton relaxometry but not by the BET measurement because it is done on a dried system. Therefore previous results showed that the surface area evolution is the same with both techniques. This means that the surface increase by dispersing effect on hydrates is minor compared with the morphological changes induces by the adsorption of superplasticizers. The specific surface area of ettringite Both previous results confirmed the BET ability to measure the surface area without sample preparation artifacts as it compares to the measurements by NMR. Thus BET and DSC measurements were used to evaluate the influence of the dosage and of the superplasticizer structure on the specific surface of ettringite. Influence of the dosage Figures 4 (a and b) refer to the ettringite amount for experiment on the influence of the dosage. The ettringite amount can be divided in two periods: from 0 to 30 minutes and from 30 minutes to 120 minutes. About 80% of the ettringite precipitates in the first period (figure 4a) whereas about 20% of the ettringite in the second period as shown on figure 4b. The higher the dosage, the smaller the amount of ettringite especially in the first 10 minutes. Between 30 and 120 minutes the amount of ettringite does not change too much in presence of superplasticizers. Figure 5 is the result of the combination between the ettringite amount measured by DSC and the surface area by BET at 5 and 120 minutes. It relates the influence of the dosage and of the structure on the specific surface area of ettringite. At 5 minutes, the higher the dosage, the lower the amount of ettringite (figure 4a) and the higher the specific surface area of ettringite (figure 5).We can conclude that an increasing dosage implies a nucleation promoted compared with the growth step of ettringite. Without superplasticizer, the nucleation and growth mainly occur on the C 3 A surfaces. In the presence of superplasticizer, a large part of the nucleation is probably homogeneous as shown by Zingg 5. These ettringite nuclei are well dispersed in the solution and the growth occurs consequently in the pore solution. The ettringite specific surface area decreases between 5 and 120 minutes for 0,10 and 0,15PCP20. Therefore the reduction of surface area at 120 minutes leads to values in presence of PCP20 (16, 21 and 29m 2 /g) superior to that without PCP (11 m 2 /g). The dosage of PCP20 has an impact on the morphology of ettringite which has a major role on the rheology at the early age. This effect of the dosage is especially high at 5 minutes and then decreases during the hydration

8 (for example at 120 minutes). It is clear that the morphology affects the workability: ettringite particles with a larger surface area will increase the interparticle contacts and attractive interactions between them and reduce the fluidity of the paste. Fig. 4 Influence of the PCP20 dosage (0.05, 0.10 and 0.15w%) on the ettringite amount measured by DSC. (a) Ettringite amount versus hydration time during the first 30 minutes (b) Ettringite amount versus hydration time during the two hours. Fig. 5 Influence of the PCP20 dosage and of the structure on the ettringite specific surface area obtained by combining the ettringite amount (DSC) and the surface area (BET) ( Surface BET(t t 0 )/ g of ettringite(t t 0 )) at t=5min (filled) and t=120min (hatched) Influence of the structure As the figure 6 shows, the amount of ettringite precipitated is slightly influenced by the presence of PCP38 in the first hour whereas the PCP20 clearly decreases this amount. After an hour, the trend for both PCP20 and PCP38 was identical. By combining results from the nitrogen BET and this quantification, the influence of the anionic group density on the surface area of ettringite has been estimated (figure 5). Comb-type superplasticizers have an effect on the nucleation which increases the surface area at 5 minutes in both case PCP20 and PCP38 on the figure 5 (at the same dosage 0.10w%). Ettringite nuclei and crystals were dispersed in the pore solution instead of being attached on the C 3 A. Due to its anionic charge

9 density, PCP38 does not interact with the surface as strongly as PCP20. Then during the hydration, the growth of ettringite with PCP38 is not perturbed. A poorly-charged backbone such as PCP38 does not influence the specific area of ettringite which is nearly the same (~14m 2 /g) as that measured without PCP at 120 minutes (~11m 2 /g). A highly-charged backbone, PCP20, leads to double the surface of ettringite (~21m 2 /g) at the same dosage. Thus the charge density of the backbone and the distance between two COOH have an impact on the ettringite specific surface area and the morphology. In an attempt to find answers to these questions, Yamada 1 proposed the idea that the cement surface area could be related to the amount of COOH added. Fig. 6 Influence of the structure, PCP20 and PCP38 at 0.10w% on the ettringite amount during the hydration. The adsorption capacity of a PCP is related to the number of charges on the backbone. Therefore we tried to correlate the specific surface area of the ettringite to the number of charges of the PCP used for our system C 3 A/Gypsum/Hemihydrate/Calcite. The figure 7 represents the ettringite specific surface area versus the added COOH per g of initial solid at 5 minutes (figure 7a) and 120 minutes (figure 7b). To have the same amount of added COOH, it is necessary to use a two times higher dosage of PCP38 compared with PCP20 with respect to the structure. The modification of the surface area of ettringite and consequently its morphology is closely related to the number of COOH added. There is a linear relationship between the dosage of added COOH and the ettringite specific surface area at 5 and 120 minutes whatever the PCP structure. This effect is more pronounced at 5 minutes (figure 7a) justifying the role of superplasticizers in promoting the ettringite nucleation. The effect of superplasticizers on the ettringite surface area decreases after 120 minutes (figure 7b). Fig. 7 Influence of the added COOH on the ettringite specific surface area at 5 minutes (a) and 120 minutes (b).

10 This study was done by addition of PCP in the mixing water. We can draw some conclusions about the time of addition, often discussed in the literature. Two mechanisms can explain the initial consumption of superplasticizers as mentioned by Zingg 9 : either they are trapped in the structure of the first hydrates 20 to form organo-mineral phases 21, or they tend to increase the initial surface area of the hydrates. We measured and confirmed the hypotheses here that the adsorbed PCP has a major impact on the specific surface area depending on its anionic charge density. In a deflocculated system, ettringite particles are floating in the pore solution instead of being attached to anhydrous phases: the surface area artificially increases but the variation is too low compared with the morphological changes induced by PCP. Thus there is also a true creation of surface. A backbone with a high charge density interacts with the surface of ettringite by promoting the nucleation and by limiting the growth step when the amount of added charges is high. CONCLUSION Through this study, conclusions can be drawn at three levels in our simplified system C 3 A/CaSO 4 /Calcite/Superplasticizer/H 2 O: (i) There is a good correlation between the surface area measurements obtained by proton NMR relaxometry and by the BET using nitrogen although physical mechanisms and sample preparation are totally different. Our study shows the interest of using a multi-method approach to follow continuously the surface area during the hydration. NMR by measuring the transverse relaxation time T 2 is a non-invasive and continuous method. BET using nitrogen, which is a point-to-point measure, permits to evaluate the surface all the time and also the increase between the anhydrous system and the first point after the water introduction. (ii) The growth of the surface occurs in three steps. Between 0 and 5 minutes, the nucleation is responsible for a strong increase of the surface area. The size of nuclei depends on the charge introduced in the solution by superplasticizers. The second period takes place between 5 and 30 minutes. The surface area increases quite strongly for PCP20 but less rapidly for the PCP38. Therefore this period does not exist in presence of PCP38 for which the surface increases directly as in the last regime. This last regime is between 30 and 120 minutes. The surface area increases slowly to tend to a value which reflects the value at 5 minutes. These considerations give some practical interests to put the superplasticizer after the nucleation step which can be considerably modified by the PCP. (iii) In our system C 3 A/CaSO 4, the major hydrate is ettringite which is partly responsible for the evolution of the fluidity of cement pastes at early age. Comb-type superplasticizers impact the amount of ettringite precipitated at a given time. This amount decreases with raising the dosage or the anionic group density of the PCP. By combining results on the ettringite amount and on the surface area, we measured and demonstrated that the specific surface area of ettringite is increased in the presence of superplasticizers. In this study the increase of ettringite surface area is proportional to the superplasticizer dosage in a way that depends on the backbone charge density. The effect of the PCP is lower when there are fewer charges on the backbone. ACKNOWLEDGEMENT Florent Dalas would like to acknowledge Maxime Guérineau at the University of Burgundy for the BET analysis and all the team Analysis & Measures of the Central Research Laboratory of Lafarge for NMR. REFERENCES 1. Yamada, K., "A summary of important characteristics of cement and superplasticizers," Proceedings of the 9th CANMET/ACI International Conference on Superplasticizers and Other Chemical Admixtures in Concrete, SP-262, V.M. Malhotra, ed., American Concrete Institute, Farmington Hills, MI, 2009, pp Cody, A.M., Lee, H., Cody, R.D., and Spry, P.G., "The effects of chemical environment on the nucleation, growth, and stability of ettringite [Ca3Al(OH)6]2(SO4)3 26H2O," Cement and Concrete Research, V. 34, No 5, 2004, pp Prince, W., Edwards-Lajnef, M. and Aïtcin, P.C., "Interaction between ettringite and a polynaphthalene sulfonate superplasticizer in a cementitious paste," Cement and Concrete Research, V. 32, No. 1, 2002, pp Prince, W., Espagne, M., and Aïtcin, P.C., "Ettringite formation: A crucial step in cement superplasticizer compatibility," Cement and Concrete Research, V. 33, No. 5, 2003, pp Zingg, A., Holzer, L., Kaech, A., Winnefeld, F., Pakusch, J., Becker, S., and Gauckler, L., "The microstructure of dispersed and non-dispersed fresh cement pastes -- New insight by cryo-microscopy," Cement and Concrete Research, V. 38, No. 4, 2008, pp

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