11572
J. Phys. Chem. B 2002, 106, 11572-11578
A Novel Approach Based on Differential Scanning Calorimetry Applied to the Study of Tricalcium Silicate Hydration Kinetics† Alessio Damasceni,‡ Luigi Dei,*,‡ Emiliano Fratini,‡,§ Francesca Ridi,‡ Sow-Hsin Chen,§ and Piero Baglioni*,‡,§ Department of Chemistry and CSGI, UniVersity of Florence, Via della Lastruccia 3, Sesto Fiorentino, I-50019 Florence, Italy, and Department of Nuclear Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ReceiVed: January 23, 2002; In Final Form: July 22, 2002
Complex reactions taking place between Ca3SiO5 and water can be considered the main cause for setting and hardening of Portland cement pastes. The “in situ” kinetics of the “hydration” process has been recently studied, as a function of time, using neutron scattering to obtain the trend of the fraction of free water (free water index, FWI), i.e., the fraction of unreacted water. In this study we developed a novel and simple approach on the basis of differential scanning calorimetry to follow the hydration kinetics of Ca3SiO5 in real time from the decrease of the free water index. FWI has been obtained by measuring the fraction of water in the Ca3SiO5 paste that can solidify and melt. We report the trend of FWI as a function of Ca3SiO5 paste hydration and the results are compared to recent quasi-elastic neutron scattering experiments. The results account well for the hydration process described by two kinetic stages, the first according to an Avrami-Erofeev nucleation and growth law, and the second one to a three-dimensional diffusion equation. Activation energies and rate constants were also computed. Moreover, the hydration process was followed in the presence of an organoaza-phosphonate additive, which is known to inhibit nucleation and growth of solid phases from aqueous media. We found that this additive is very powerful in retarding the hydration process even at very low concentration, by increasing the activation energy of the Avrami-Erofeev stage. Although quasi-elastic neutron scattering and NMR are more powerful and provide additional information on the water dynamics, we believe that this new simple method will become very popular in all applied investigations, where the knowledge of FWI is essential.
Introduction Cement is ubiquitous and probably one of the most important materials in the human activity. The key mechanism of the setting and hardening process is the hydration of calcium silicates present in the cement powder. The study of this process attracts every year an increasingly number of researchers. The overall process is oversimplified with the word “hydration”; indeed, water produces several subsequent reactions that originate (CaO)x‚SiO2‚(H2O)y, the gel that confers to the cement its mechanical properties. However, despite the very extensive literature on cement,1-6 the hydration mechanism and the influence of additives, in particular organic additives that are largely used in the preparation of high-strength concrete with superior workability at low water/cement ratio, are still poorly understood. Several papers have been published concerning the mechanism of cement hydration,1-6 with particular emphasis on the kinetic aspects.7-11 The term hydration refers to several complex reactions that produce (CaO)x‚SiO2‚(H2O)y, the socalled CSH12 gel containing water with restricted mobility that is responsible for the excellent mechanical properties of cement.13-15 Tri-calcium silicate, Ca3SiO5, usually abbreviated † Paper presented in part at the 22nd AICAT-GICAT Congress, Camogli, Genua, Italy, Dec 13-16, 2000. * To whom all correspondence should be addressed. Fax: + 39 055457-3033. E-mail:
[email protected]. www.csgi.unifi.it (for Piero Baglioni). ‡ Department of Chemistry and CSGI. § Department of Nuclear Engineering.
as C3S, is the major component of a cement paste and it has been used as a model for the study of the cement setting and hardening processes.16 Recently, neutron scattering experiments have shown15-21 that it is possible to evaluate the kinetic mechanism “in situ” over the whole hydration period by determining the fraction of un-reacted water or free water index (FWI) as a function of time. The results obtained are very important and provide a complete picture of the process for over 30 days. Quasi-elastic neutron scattering (QENS) is probably the best technique to follow cement hydration since provide water relaxational dynamics and a complete description of the cement paste hydration.19-21 However, neutron scattering measurements have limited access, and a deep physicochemical background is required for a correct interpretation of the experimental results. The calorimetric method, proposed here, is very simple and would help in providing an alternative way to determine the FWI, particularly useful for applied research. The main advantage in respect of isothermal calorimetry is the intrinsic possibility to follow hydration kinetics at very long time (more than 28 days against 40-50 h). Moreover, our approach allows to have a complete hydration profile up to 28 days using the calorimeter for 6 h only (30 measurements each lasting about 12 min). In particular, we develop a novel approach based on differential scanning calorimetry (DSC) to follow the reaction between C3S and water. We determine the FWI parameter from the amount of water that can solidify and melt, the so-called
10.1021/jp020211l CCC: $22.00 © 2002 American Chemical Society Published on Web 10/10/2002
Tricalcium Silicate Hydration Kinetics freezable water,22 by measuring the enthalpy of fusion as a function of time. According to this new method, the fraction of reacted C3S, R, can be calculated from the DSC curves of freezable water melting, being R inversely proportional to the experimental enthalpy of fusion of freezable water present in the C3S/water paste. From this index the kinetic mechanism of the cement hydration process has been obtained, and the results compared with recent QENS experiments performed on C3S samples.15,17,19-21 Moreover, we investigated the effects of an antiscaling additive,23-25 the 1,5,9,13-tetra-azatridecane-1,1′,5,9,13,13′-methylenphosphonic acid sodium salt, on the kinetic mechanism of C3S hydration. To have morphological information on the reacting C3S grains, scanning electron micrographs were also collected at different hydration time to compare the texture changes with the stages of the kinetic mechanism. Experimental Section C3S was supplied by CTG-Italcementi Group; the powder had a surface area of 0.94 ( 0.08 m2‚g-1 BLAINE with an average radius of 3.9 µm. Water was purified by a Millipore Organex system (R g 18 MΩ.cm). The antiscaling additive used was 1,5,9,13-tetra-azatridecane-1,1′,5,9,13,13′-methylenphosphonic acid sodium salt, Dequest 2086 from P. Carini S.p.A., Milan, Italy. The compound Dequest 2086 was purified according to the procedure reported in the Appendix 1 to give the acid form. All the measurements were carried out on a C3S/ water system with a w/c weight ratio of 0.65 ( 0.03 (w ) water; c ) C3S). The C3S/water paste was prepared by manually mixing (about 30 s) the appropriate amounts of water (or a 0.2 wt % Dequest 2086 water solution) and C3S onto a glass surface. DSC measurements have been performed in 60 µL stainless steel pans (Perkin-Elmer). Each pan was sealed with the appropriate cover, equipped with a neoprene O-ring to avoid water leaking. The weight of the C3S/water paste (about 40-60 mg depending on the measurement) was checked during the whole kinetic period (28 days) to determine possible water evaporation. The accuracy on the weight was (0.02 mg. The sealed pans were kept in a thermostatic bath at temperature constant within 0.5°C. The hydration kinetics was followed at 10, 20, 30, and 40 °C. The DSC curves were carried out by means of a Perkin-Elmer (PE) DSC 7 power compensation differential scanning calorimeter equipped with PE pc Pyris 3.52 software. Measurements were performed in a dry nitrogen flow of 16.0 ( 0.5 cm3‚min-1 with the following temperature cycle: from room temperature down to -30 °C at 40 °C‚min-1, isotherm at - 30 °C for 4 min, from -30 °C up to - 12 °C at 20 °C‚min-1, and from - 12 °C up to 14 °C at 4 °C‚min-1. Using these conditions each DSC measurement requires less than 12 min. We carried out also additional measurements on a collection of 30 identical samples prepared at the same time and kept at the same temperature. The samples were numbered and the ith sample was frozen only once at ith time, spanning the whole kinetic process. In this way, we avoided possible effects due to repeated freeze-thaw cycles. The results showed that the kinetic data were not affected by our experimental procedure. The enthalpy change relative to the water melting at 0 °C was used to evaluate the Free Water Index (vide infra) as defined in previous papers.15,17,18 The calibration of the DSC apparatus was made through the melting of biphenyl and tin and then tested with the NH4NO3 transition at 125 °C and the melting of indium. The FWI parameter was calculated from the following formula:
J. Phys. Chem. B, Vol. 106, No. 44, 2002 11573
FWI ) ∆Hexp/0.394∆Hinit
(1)
where ∆Hexp is the enthalpy change (in J‚g-1 of C3S/water paste) of the water melting determined by the DSC experimental curve, 0.394 is the average weight fraction of water in the C3S/water paste, and ∆Hinit is the theoretical value of the specific enthalpy of fusion of water at 0 °C in the C3S/water paste determined considering that at the beginning of the hydration reaction FWI ) 1. The accuracy in the determination of ∆Hexp was ( 0.5 J‚g-1. The FWI data obtained by this method were plotted as a function of time and several kinetics laws26 describing different heterogeneous solid-state reactions were tested to obtain the best fit. For neutron scattering, the C3S paste (0.65 in w/c) was prepared as already described19-21 and spread uniformly into a rectangular aluminum cell making a layer of 0.5 mm thickness. The cell was reassembled and sealed by means of an indium wire gasket. This ensured a negligible loss of water as well as no contamination of the paste by carbon dioxide during the experiment. To prevent interference with the hydration process of the cement, the cell was coated with a thin layer of Teflon to prevent contact of the very basic paste with aluminum sample holder. Experiments were performed at three different temperatures: 15, 20, and 30 °C averaging the signal every hour and following the kinetic for at least 1 day at each temperature. The quasi-elastic neutron scattering experiment was carried out at the Laboratoire Le´on Brillouin (LLB) in Saclay, France using a high-resolution time-of-flight (TOF) spectrometer MIBEMOL. The incident neutron wavelength was chosen at 9.0 Å (1.01 meV) to achieve a good energy resolution at the elastic peak position (fwhm about 28 µeV). The sample cell was placed at an angle making 45° from the direction of the incident neutron beam in such a way that detector banks were in a reflection geometry spanning a scattering vector range from 0.55 to 1.24 Å-1. A complete and detailed description of the QENS experimental set up and data analysis has been already reported in previous papers.19-21 It is important to remark that working in the low scattering vector region permits to reduce the contribution coming from rotational motions and for this reason it is reasonable to consider only the translational contribution. Scanning electron microscopy (SEM) observations were carried out by means of a JEOL-JSM 5600, working at 8-10 kV of acceleration potential. Samples were coated by a gold film using a JEOL Jee 4B apparatus. The preparation of the samples for SEM measurements was performed according to the following procedure: the mixed C3S/water paste (or 0.2% (w/w) Dequest 2086 aqueous solution) prepared as for the DSC experiment was deposited in a Teflon moulds as in Figure 1. This was sealed with glass cover slips treated with a very thin layer of silicon grease to avoid water evaporation, and to reproduce the same environmental conditions of DSC measurements and neutron scattering experiments. The samples were kept in a thermostated Plexiglas box to allow hydration at the constant temperature of 20 °C, and controlled atmosphere. SEM micrographs were collected at 2 h, 1 day, 2 days, 7 days, 14 days, and 28 days after the mixing of the C3S/water paste. SEM micrographs were performed on C3S/water paste taken from the middle part of the mould and deposited with a biadhesive scotch tape on the SEM aluminum stub. The inner part of the paste for SEM has been chosen since the surface was not representative of the bulk hydration, due to surface effects.
11574 J. Phys. Chem. B, Vol. 106, No. 44, 2002
Damasceni et al.
Figure 1. Scheme of the sample container to prepare the hydrated samples of the C3S/water paste for SEM measurements.
Figure 3. FWI parameter as a function of time for a C3S/water sample (w/c ) 0.65) hydrated at 10 and 40 °C (the curves at 20 and 30 °C are in between). ti ) end of the induction period or beginning of the acceleration stage; td ) end of the acceleration stage or beginning of the deceleration mechanism. Symbols are the experimental points.
down of the hydration rate, which takes place at time above td. The ti value was calculated according to the procedure reported in the literature15 at the time when the differential of the FWI shown in Figure 3 sharply increases from zero. The td value was determined from Figure 3 at the discontinuity in the behavior of FWI versus time.15 The kinetics of the hydration process have been obtained from the fraction, R, of reacted C3S. Considering the stoichiometry of the cement hydration as reported by Fuji and Kondo,30 a simple expression for R has been reported:17 Figure 2. DSC curves of a C3S/water sample (w/c ) 0.65) showing the peak relative to the melting of water as a function of the hydration time. The curves refer to a C3S/water sample (w/c ) 0.65) hydrated at 20 °C: (A) 2 h, (B) 24 h, (C) 5 days.
Results and Discussion Figure 2 shows three melting curves of ice recorded at different times of hydration: the three curves refer to a C3S/ water sample cured at 20 °C for 2 h (A), 1 day (B), and 5 days (C). It is evident that the enthalpy of fusion strongly decreases during the hydration in agreement with a consistent decrease of the free freezable water.22 Since during the whole induction period the hydration degree reaches a maximum value of 2-3%,15,27 we carried the first DSC measurement on C3S/water samples immediately after the mixing. This allows calculation of the enthalpy of fusion of the water in the paste. ∆Hinit in formula 1 is approximately 85% of the value of pure water (333 J/g), in agreement with the influence of the samples heterogeneity on the enthalpy changes as determined by DSC.28 With this assumption, the hydration kinetics of C3S is determined by measuring the enthalpy of fusion of ice in the C3S/water paste as a function of time, and the FWI is obtained from formula 1. Figure 3 shows the behavior of the FWI parameter as a function of time at 10 and 40 °C, the two extreme temperatures investigated (the curves at 20 and 30 °C are located between). The trend matches the same curves obtained by neutron scattering experiments.15,17,19-21,29 The kinetic is characterized by an induction period lasting ti, an acceleratory stage with a consistent decrease of FWI as a function time (the region between ti and td in Figure 3), and a third stage with slowing
R ) 3.26(1 - FWI)(w/c)
(2)
With this relationship between the fraction R of reacted C3S and the FWI parameter experimentally obtained from the DSC curves, we deduced the kinetic mechanism of cement hydration. We tested many kinetic equations relative to solid-state reactions,26 and we found that the Avrami-Erofeev model31 accounts very well for the acceleratory stage. According to this model the cement hydration proceeds by a nucleation and growth mechanism. The basic Avrami-Erofeev31 equation is
R ) 1 + Ro - exp[-k(t - to)M]
(3)
where Ro is the fraction of C3S reacted at to, k is the rate constant, and M is the exponent associated with the nucleation type (dimensionality of the product phase, type of growth, and nucleation rate). In our case to was identified with ti, the beginning of the nucleation reaction. To obtain the kinetic parameters we considered the linearized form of eq 3:
ln[ln(Ro + 1 - R)-1] ) ln k + M ln(t - to)
(4)
and we plot ln[ln(Ro + 1 - R)-1] as a function of ln(t - to). Figure 4 shows the results of these least-squares fits at the four temperatures investigated (correlation coefficients always > 0.985). All the other kinetic equations tried (i.e., power law; first-, second-, and third-order laws; contracting area; and volume laws) gave fits with correlation coefficients below 0.92. The kinetic parameters we deduced are reported in Table 1. Both ti and td are in good agreement with the data obtained from the
Tricalcium Silicate Hydration Kinetics
J. Phys. Chem. B, Vol. 106, No. 44, 2002 11575
Figure 4. Fit of the Avrami-Erofeev type behavior at various temperatures in the time range ti - td (to ) ti and Ro is the value of R at to ) ti).
TABLE 1: Kinetic Parameters for the Two Stages of the Cement Hydration Deduced from the DSC Measurementsa temperature (°C)
ti (h)
td (h)
ln kA-E
MA-E
Kd × 1015 (m2‚h-1)
10 15b 20 20b 30 30b 40
4 3.5 2 2 1.5 1.5 1
35 29 24 25 14 15 8
-4.12 -3.99 -3.70 -3.61 -3.35 -3.39 -2.84
1.1 1.2 1.2 1.2 1.3 1.2 1.1
0.60 7.49 5.46 8.13
a
For the meaning of ti and td, see the caption of Figure 3. k and M are the rate constant and the exponent, respectively, for the AvramiErofeev (A-E) law, and Kd is the diffusion constant for the threedimensional diffusion kinetic law. b Results obtained from quasi-elastic neutron scattering data.
p parameter deduced from a detailed analysis of QENS spectra in terms of the relaxational dynamics of water in C3S.19-21 These QENS experiments have been performed on the same C3S sample used for DSC measurements. Since the ti-values follow an Arrhenius-type behavior, ti-1 ) t0-1exp(-Ei/RT), it was possible to calculate the activation energy, which is Ei ) 32.9 ( 0.4 kJ/mol and 34.5 ( 0.5 kJ/ mol from DSC and QENS data, respectively. These values are close to the value reported in the literature.15 A comparison of the rate constant k values and of the exponent M relative to the Avrami-Erofeev and obtained from DSC with the results obtained (with the same C3S samples) from quasi-elastic neutron scattering shows the excellent agreement (see Table 1) between the two methods. However, in the literature15,17 are reported also slightly different values for C3S. In particular. (i) the rate constants deduced from DSC or the QENS experiments of this work are slightly greater than those reported in refs 15 and 17, and (ii) the exponent M is lower (if compared with data of refs 15 and 17) but in better agreement with previous experiments with isothermal calorimetry, thermogravimetry, and Raman scattering.33-35 The main cause of the discrepancy in the rate constants is related to the higher specific surface of C3S investigated in the present work (about 2.5 times that of Fitzgerald et al.15 and Berliner et al.17). Another difference concerns the value of the M exponent that is very
Figure 5. Arrhenius plot in the temperature range 10-40 °C for the Avrami-Erofeyev stage.
close to 1, in agreement with previous kinetic analysis.32-34 This value of M is related to the mechanism of the reaction through the P, Q, and S coefficients:17
M ) (P/S) + Q
(5)
where P ) 1, 2, or 3 fibers, sheets, or polygonal forms growth; S ) 1 corresponds to phase boundary growth, and S ) 2 to diffusion of the component through a liquid phase; Q ) 0 means no nucleation, whereas Q ) 1 indicates constant nucleation. The value M ≈ 1 (obtained from DSC and QENS) confirms previous results33-35 and is consistent with two possibilities: (i) sheets-diffusion-no nucleation, and (ii) fibers (needles)-phase boundary-no nucleation. The SEM results (vide infra) indicated that the mechanism consisted of growth of fibers at the phase boundary with no nucleation. From Figure 3 and Table 1 it is evident a clear dependence of the kinetic mechanism on temperature indicating a thermally activated process. By simple application of the Arrhenius equation:
ln kA-E ) ln A - EA-E/RT
(6)
we found an activation energy EA-E ) 30.7 ( 0.3 kJ/mol and EA-E ) 30.5 ( 0.3 kJ/mol from DSC and QENS data, respectively, in perfect agreement with the data reported in the literature.30,35 Figure 5 shows the Arrhenius behavior in the temperature range 10-40 °C. This match of the activation energy value with those determined with other techniques15,30,35 and with QENS (performed on the same C3S batch19-21) indicates that our novel approach based on estimating the FWI index from freezable water by DSC calorimetry is very reliable and leads to the same results achievable with other techniques. In fact, the activation energies found in the literature are spread over a broad range of values, i.e., from 30 to 50 kJ/mol.36 Moreover, the discrepancy with data of Fitzgerald et al.15 and Berliner et al.17 and the agreement with QENS performed on the same C3S sample, clearly indicate the excellent sensitivity of QENS in the investigation of cement pastes hydration process, which can discriminate between C3S samples slightly differing for the average particle size distribution and surface area. Following the same procedure we determined the best fit for the last part of the C3S hydration, i.e., the deceleratory stage. We found, in agreement with the literature,15,17 that the
11576 J. Phys. Chem. B, Vol. 106, No. 44, 2002
Figure 6. Fit of the three-dimensional diffusion kinetic law at various temperatures above td.
mechanism is well-described by a three-dimensional diffusion equation:
Damasceni et al.
Figure 7. FWI parameter as a function of time for a C3S/water sample (w/c ) 0.65) hydrated at 20 °C without any additive (open circle) and in the presence of 0.2 wt % of the antiscaling molecule Dequest 2086 (full circle). Symbols are the experimental points.
(1 - R)1/3 ) [-(2Kd)1/2/R](t - to)1/2 + (1 - Ro)1/3 (7) where Kd is the diffusion constant, R is the mean radius of the C3S grains, and Ro is the value of R at to ) td. Figure 6 shows the results of the fit at the four temperatures investigated; from the slope of the straight lines and given R ) 3.9 µm it was possible to calculate the values of the diffusion constants that are reported in the last column of Table 1. As for the first part of the hydration process, we performed a QENS investigation for the diffusion period, but the allocated QENS beam time was not long enough to cover the last part of the hydration process. However, DSC results can be qualitatively compared to the results reported by Fitzgerald et al.15 and Berliner et al.17 Considering both the higher w/c ratio and the different characteristics of the C3S grains, we can conclude that our Kd data are in good agreement with QENS data. In particular, we found that the diffusion-limited behavior did not show definite temperature dependence, as found for QENS. The explanation of such a behavior has already been extensively discussed.15 Only the lowest temperature (10 °C) seems to strongly affect the three-dimensional diffusional stage with a consistent decrease of Kd. In particular we found higher Kd as compared to those of QENS experiments15,17 (about 5-8 (×1015 m2/h, at w/c ) 0.65) against the value of ca. 2 (×1015 m2/h at w/c ) 0.40),15 or 0.7-0.8 (×1015 m2/h, at w/c ) 0.65) against the value of 2 (×1015 m2/h, at w/c ) 0.40)17) in agreement with either a great content of water in the paste, or a larger reactivity due to the higher specific area and smaller radius of the grains. As a first application of this new method, we investigated the effects of the antiscaling additive Dequest 2086 on the kinetics of C3S hydration. Figure 7 shows the behavior of the FWI parameter as a function of time for this system at 20 °C. It is evident a very strong retarding effect,24 as compared to the pure water system, resulting in a dramatic increase of the induction period (ti > 1 day against ti ) 2 h without the additive), in agreement with QENS results performed on the same C3S sample19,20 where 2000 ppm of additive were able to stop the kinetic for about 2 days. The kinetic analysis demonstrates that the nucleation mechanism does not change upon the additive addition. In fact, the first two stages can be still described by an Avrami-Erofeev law, and the third one is
Figure 8. SEM micrographs of a C3S/water sample (w/c ) 0.65) hydrated at 20 °C without and with the additive Dequest 2086 (same concentration as in Figure 7) taken as a function of the hydration time: (A) pure dry C3S powder, (B) 2 h no additive, (C) 1 day no additive, (D) 1 day with additive, (E) 14 days no additive, and (F) 14 days with additive. Bar ) 2 µm.
well represented by the same three-dimensional diffusion kinetic model. Two main differences were found between the C3S/water and C3S/water/Dequest 2086 systems: (i) the activation energy for the induction period in the presence of the additive was Ei ) 52.5 ( 0.5 kJ/mol against the value of Ei ) 32.9 ( 0.4 kJ/ mol found for the pure C3S/water paste, (ii) the additive increased the activation energy for the Avrami-Erofeyev stage EA-E ) 30.7 ( 0.3 kJ/mol (no additive) of a factor 3 (EA-E ) 96.4 ( 1.3 kJ/mol). Both of these results confirm a strong effect of the retardation of the beginning of the cement hydration and the dramatic influence of the antiscaling additive toward the nucleation and growth of the CSH gel phase. Interestingly, no particular differences have been observed for the third stage of the process (i.e., the three-diffusion mechanism), suggesting that
Tricalcium Silicate Hydration Kinetics the action of these additives is limited to the inhibition of the nucleation process of a new phase in the microheterogeneous system. The morphology of the C3S grains during hydration was also followed as a function of time by SEM microscopy preparing the samples according to the procedure reported in the Experimental Section. Figure 8 shows a collection of micrographs taken at various time intervals with and without the antiscaling additive. After 1 day, for the sample without the additive, the grains surface is completely covered by very small microfibrils according to the completion of the acceleratory stage (see for comparison Figure 7). It is evident the fiber-like (needles) morphology of the gel growing phase rather than polygonal or sheet forms. The antiscaling additive did not allow the formation of such microfibrils after 1 day (see Figure 8, plate D), whereas after 14 days in some regions of the grain surface microfibrils are present (see Figure 8, plate F, left portion). The sample without additive after 14 days presented the aspect of a fibrils network, typical of the hardened cement. Conclusions The present study showed that the kinetics of cement hydration can be followed with a novel approach based on estimating the FWI from the amount of the freezable water by means of DSC calorimetry. From the enthalpy change associated with the melting of the previously frozen free water it was possible to deduce the FWI and, consequently, the fraction R of C3S reacted. The parameters deduced using this simple and straightforward method are in good agreement with QENS experiments performed on the same C3S sample. The kinetic mechanism of C3S hydration was shown to proceed according to three steps: (i) induction period, (ii) acceleratory, and (iii) deceleratory stages, well-described by the Avrami-Erofeev law and a three-dimensional diffusion equation. The kinetic parameters evaluated from this novel approach agree those obtained with other techniques; in particular, very good agreement was found with the value of the activation energy for the nucleation and growth process determined on the same C3S sample from QENS. This new approach has been also applied to C3S/water pastes containing the antiscaling additive 1,5,9,13-tetra-azatridecane-1,1′,5,9,13,13′-methylenphosphonic acid at the concentration of 0.2 wt %. The additive has been shown to be very effective in retarding the C3S hydration even at this very low concentration. Moreover, the kinetic mechanism remains unchanged, even if a strong increase of the activation energy for the Avrami-Erofeev stage has been observed, showing that the antiscaling additive highly influences the nucleation and growth of a new phase within a solid-liquid system. Finally, SEM micrographs show the effect of the antiscaling additive on the morphology of the C3S reacting grains. The microfibrils typical of the CSH gel appears much later in the presence of the additive than in the case of the pure C3S/water paste. Furthermore, also the shape and size of these microfibrils are strongly modified by the presence of the additive. In conclusion, our study shows that this new experimental approach to determine the kinetics of cement hydration is reliable and powerful, especially in view of possible technological applications to rapidly check the performance of the various additives (fluidizers, plasticizers, etc.) commonly used to upgrade cements for specific industrial applications. Acknowledgment. The authors thank Dr. L. Cassar (CTGItalcementi Group) for invaluable comments and discussions
J. Phys. Chem. B, Vol. 106, No. 44, 2002 11577 and to Prof. Andrea Bencini for the assistance during the purification of the antiscaling additive. Financial support from CTG-Italcementi Group, Ministero dell’Istruzione, Universita` e della Ricerca Scientifica, MIUR, and Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase, CSGI, is gratefully acknowledged. P.B. and E.F. enjoyed the hospitality of the Nuclear Engineering Department of MIT while part of this work is being carried out. Research of S.H.C. is supported by a grant from the Materials Chemistry Program of the U.S. Department of Energy. Appendix 1 The commercial product Dequest 2086 was supplied as an amber-colored aqueous solution (approximately 50 wt %) of the 1,5,9,13-tetra-azatridecane-1,1′,5,9,13,13′-methylenphosphonic acid sodium salt. A few drops of concentrated HClO4 was added to 10 g of this solution up to pH ) 1.The acidic solution was kept under magnetic stirring at room temperature for 30 min. Ethanol was then added up to precipitation of a solid amber-colored product. The compound was filtered and washed several times with ethanol. Finally, the solid was redissolved in water and a white powder has been obtained by slow evaporation of the solution. This white product was filtered, recrystallized from an ethanol-water mixture, and dried under vacuum. Yield ca. 50%, mp 104 °C, elemental analysis C15H42N4O18P6 (MW 752.35): calcd C 23.9, H 5.6, N 7.4, O 38.3, P 24.7, found C 21.3, H 5.5, N 6.6; 1.4% impurity of HClO4. References and Notes (1) Taylor, H. W. F. Cement Chemistry; Academic Press: London, 1990. (2) Parrott, L. J.; Geiker, M.; Gutteridge, W. A.; Killoh, D. Cem. Concr. Res. 1990, 20, 919. (3) Cohen, M. D.; Cohen, R. D. J. Mater. Sci. 1988, 23, 3816. (4) Gartner, E. M.; Jennings, H. M. J. Am. Ceram. Soc. 1987, 70, 743. (5) Bezjak, A. Cem. Concr. Res. 1986, 16, 605. (6) Krstulovi, R.; Dabi, P. Cem. Concr. Res. 2000, 30, 693. (7) Pommersheim, J.; Chang, J. Cem. Concr. Res. 1988, 18, 911. (8) Popovics, S. Cem. Concr. Res. 1987, 17, 821. (9) Schlusser, K. H. Cem. Concr. Res. 1986, 16, 215. (10) Barret, P.; Bertrandie, D. J. Chim. Phys. Phys.-Chim. Biol. 1986, 83, 765. (11) Cohen, M. D.; Cohen, R. D. J. Mater. Sci. 1987, 22, 2032. (12) The well-accepted cement chemistry notation where CdC3S, Sd SiO2, and HdH2O has been adopted. Within this notation Ca3SiO5 is abbreviated as C3S and the correspondent hydrated form, (CaO)x‚SiO2‚ (H2O)y, becomes CxSHy or is shortened as CSH. (13) Diamond, S. Cem. Concr. Res. 1972, 2, 617. (14) Glasser, F. P.; Lachowski, E. E.; Macphee, D. E. J. Am. Ceram. Soc. 1987, 70, 481. (15) Fitzgerald, S. A.; Neumann, D. A.; Rush, J. J.; Bentz, D. P.; Livingston, R. A. Chem. Mater. 1998, 10, 397. (16) Livingston, R. A.; Neumann, D. A.; Allen, A.; Rush, J. J. Mater. Res. Symp. Proc. 1995, 376. (17) Berliner, R.; Popovici, M.; Herwig, K. W.; Berliner, M.; Jennings, H. M.; Thomas, J. J. Cem. Concr. Res. 1998, 28, 231. (18) Fitzgerald, S. A.; Neumann, D. A.; Rush, J. J.; Kirkpatrick, R. J.; Cong, X.; Livingston, R. A. J. Mater. Res. 1999, 14, 1160. (19) Fratini, E.; Chen, S.-H.; Baglioni, P.; Bellissent-Funel, M.-C. Phys. ReV. E 2001, 64, 020201. (20) Fratini, E.; Chen, S.-H.; Baglioni, P.; Bellissent-Funel, M.-C. J. Phys. Chem. B 2002, 106, 158. (21) Fratini, E.; Faraone, A.; Baglioni, P.; Bellissent-Funel M.-C.; Chen S.-H. Phys. A 2002, 304, 1. (22) Shibukawa, M.; Aoyagi, K.; Sakamoto, R.; Oguma, K. J. Chromatogr. A 1999, 832, 17. (23) Benton, W. J.; Collins, I. R.; Grimsey, I. M.; Parkinson, G. M.; Rodger, S. A. Faraday Discuss. 1993, 95, 281. (24) Coveney, P. V.; Humpries, W. J. Chem. Soc., Faraday Trans. 1996, 92, 831. (25) Coveney, P. V.; Davey, R. J.; Griffin, J. L. W.; Whiting, A. Chem. Commun. 1998, 1467.
11578 J. Phys. Chem. B, Vol. 106, No. 44, 2002 (26) Brown, M. E.; Dollimore, D.; Galwey, A. K. In ComprehensiVe Chemical Kinetics; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1980; Vol. 22. (27) Taylor, H. F. W.; Barret, P.; Brown, P. W.; Double, D. D.; Frohnsdorff, G.; Johansen, V. Mater. Constr. 1985, 17, 457. (28) Wendlandt, W. W. Thermal Analysis, 3rd ed.; Wiley-Interscience: New York, 1986; pp 263-264. (29) Thomas, J. J.; FitzGerald, S. A.; Neumann, D. A.; Livingston, R. A. J. Am. Ceram. Soc. 2001, 84, 1811. (30) Fujii, K.; Kondo, W. J. Am. Ceram. Soc. 1974, 57, 492.
Damasceni et al. (31) Avrami, M. J. Chem. Phys. 1939, 7, 1103; J. Chem. Phys. 1939, 8, 212; J. Chem. Phys. 1940, 9, 177. (32) Tarrida, M.; Madon, M.; Le Rolland, B.; Colombet, P. AdV. Cem. Bas. Mater. 1995, 2, 15. (33) Brown, P. W.; Pommersheim, J.; Frohnsdorff, G. Cem. Concr. Res. 1985, 15, 33. (34) Odler, I.; Schuppstuhl, J. Cem. Concr. Res. 1981, 11, 765. (35) Tenoutasse, N.; Donder, A. Silic. Ind. 1970, 35, 301. (36) Thomas, J. J.; Jennings, H. M. Chem. Mater. 1999, 11, 1907.
