J. Phys. Chem. B 2000, 104, 12143-12148
12143
Effects of Temperature and of the Addition of Accelerating and Retarding Agents on the Kinetics of Hydration of Tricalcium Silicate Dimas R. Vollet* and Aldo F. Craievich† Departament of Physics, IGCE/UNESP, POB 178, 13500-970, Rio Claro, SP, Brazil, and Institute of Physics, USP, POB 66318, 05315-970, Sa˜ o Paulo, SP, Brazil ReceiVed: May 26, 2000; In Final Form: October 3, 2000
The formation of calcium silicate hydrates (C-S-H) during the hydration of tricalcium silicate (C3S) in pure water and in water solutions containing 1% CaCl2 (accelerator) and 0.01% saccharose (retarder) was studied by small-angle X-ray scattering (SAXS). SAXS measurements were performed under isothermal conditions within the temperature range 25 °C < T < 52 °C. The experimental results indicate that the time variation of the mass fraction of the C-S-H product phase, R(t), can be fitted, under all conditions of paste setting, by Avrami equation, R(t) ) 1 - exp(-(kt)n), k being a rate parameter and n an exponent depending on the characteristics of the transformation. The parameter n is approximately equal to 2 for hydration of C3S in pure water. Depending on temperature, n varies from 2 to 2.65 for hydration in the presence of CaCl2 and saccharose. The value n ) 2 is theoretically expected for lateral growth of thin C-S-H plates of constant thickness. The time dependence of SAXS intensity indicates that the transformed phase (C-S-H) consists of colloidal particles in early stages of hydration, evolving by two-dimensional growth toward a disordered lamellar structure composed of very thin plates. The activation energy ∆E for the growth of C-S-H phase was determined from the time dependence of X-ray scattering intensity. These data were obtained by “in situ” measurements at different temperatures of hydration. The values of ∆E are 37.7, 49.4, and 44.3 kJ/mol for hydration in pure water and in water solutions containing CaCl2 and saccharose, respectively.
Introduction The mechanism of hydration of tricalcium silicate (C3S, Ct CaO, and StSiO2) and the structure of the resulting gel hydrates (CsSsH; HtH2O) have received the attention of many authors.1-8 Chemical reactions involving C3S and water were accompanied by recording the heat of hydration as a function of time under isothermal condition.8 The calorimetric curves are composed of (i) a sharp peak within the first minute of hydration, (ii) an almost athermal plateau up to about 2 h, the dormant or induction period, and (iii) a broad peak spanning several hours and corresponding to the setting and hardening of the paste. Just after the induction period, the setting exhibits a rapid acceleration. During this period, the rate of hydration grows rapidly with time.3 This stage corresponds to the process of nucleation and growth of the product C-S-H phase. The kinetics of hydration under isothermal condition of C3S has been described in a previous work8 by fitting Avrami equation9 to the degree of hydration determined from calorimetry measurements. The results indicated that the reaction is controlled by heterogeneous nucleation at the surfaces of C3S grains. On the other hand, it was established that the addition of C-S-H accelerates the reaction.3 This suggests that the formation of the C-S-H phase is an autocatalytic process. The degree of hydration of anhydrous C3S is assumed to be proportional to the fraction of volume of C-S-H formed. A previous work5 established that the chemical evolution of the system controls lime concentration in the solution and determines the physical evolution of the paste. * Corresponding author. E-mail:
[email protected] † E-mail:
[email protected].
Small-angle X-ray scattering (SAXS) has in the past been used to study the nanostructure of C-S-H gels.10,11 In the present work, we studied by SAXS the process of formation of C-S-H occurring during the hydration process of initially anhydrous C3S. Hydration was promoted using pure water and water solutions containing 1% CaCl2 (accelerator) and 0.01% saccharose (retarder). SAXS intensity curves, J(q,t), were determined as functions of the modulus of the scattering vector, q, and of the time, t, during which the samples were held at a constant temperature. The temperature of hydration ranges from 25 up to 52 °C. The structural characteristics of the C-S-H gel phase were inferred from the shape of J(q,ti) curves corresponding to different ti values. The study of the kinetical aspects of the transformation process was mainly based on the shape of the time dependence of the SAXS intensity at a single q value J(q1,t). This SAXS study aims at giving a contribution to the understanding of the kinetics of chemical reaction and structural development during the setting of a fresh C3S-water mixture (pure water and water with additives). The chemical evolution was quantified by the degree of hydration (mass fraction of consumed C3S phase). The structural study refers to the time variation of the morphology of the product C-S-H phase. Experimental Samples. Pastes composed of a fine C3S powder (14 µm) mixed with (i) pure water, (ii) a water solution containing 1% CaCl2, and (iii) a water solution containing 0.01% saccharose (weight percent) were prepared. Water (or water solutions)/solid weight ratios were kept equal to 0.5 for all samples. The pastes were molded inside a sealed cell to avoid water evaporation
10.1021/jp001944d CCC: $19.00 © 2000 American Chemical Society Published on Web 11/30/2000
12144 J. Phys. Chem. B, Vol. 104, No. 51, 2000
Vollet and Craievich
during in situ SAXS measurements and X-ray diffraction (XD) studies. The volume change during sample setting is negligible. Experimental Methods. The structures of the different pastes and their time variation were studied “in situ”, during the isothermal hydration process at temperatures ranging from 25 to 52 °C. The temperature of the sample cells, used for SAXS measurements, was maintained constant by means of a circulating water bath. SAXS measurements were performed by means of a conventional Kratky camera (Rigaku). Ni filtered Cu KR radiation (λ ) 0.154 nm) was used. The direct X-ray beam had a “linear and infinite” cross section.12 A TEC 1D gas position sensitive X-ray detector was employed to record the SAXS intensity, J(q), as a function of the modulus of the scattering vector q ) 4π sin θ/λ, θ being half of the scattering angle. The experimental setup allowed us to cover a q range from 0.09 up to 5.4 nm-1. The intensity of the parasitic scattering produced by collimating slits and air was determined and subtracted from the experimental SAXS curves. The intensity functions J(q) were determined, in arbitrary units, for different time periods during “in situ” sample hydration. All curves were normalized to account for differences in sample absorption and thickness. Therefore, the SAXS curves corresponding to different samples can be subjected to quantitative comparisons. The degree of hydration was also determined for a set of samples hydrated at 35 °C by quantitative X-ray diffraction analysis using pure silicon as internal standard. This study was carried out in order to verify the eventual correlation between the time evolution of the degree of hydration determined from XD and those based on SAXS measurements. The eventual equivalence of both results, obtained from SAXS and XD, was used as a checking of the robustness of the structural model proposed for C3S hydration. The experimental results presented in this article are a part of a larger project of application of SAXS to the study of the kinetics of structural transformations in C3S. Other techniques, such as XD and isothermal microcalorimetry8 can also be used for the same purpose. We have chosen SAXS because of the availability of instrumentation. Furthermore, we have compared the experimental SAXS results corresponding to one set of samples with those of XD and demonstrated their equivalence within the experimental errors. Minor differences observed between the results derived from our SAXS study and from previous calorimetric measurements will be discussed. Structural features of fracture surfaces of dried pastes, previously hydrated using pure water and water solutions containing CaCl2 and saccharose, were also analyzed by scanning electron microscopy (SEM). SAXS Basic Theory. The intensity of X-ray scattering at small angles exhibits an asymptotic trend at high values of q given by Porod’s law.12 This law applies to two-electron density systems, which may be either a dilute or a concentrated system provided the particles are not very thin cylinders or plates. Under the “linear and infinite” collimation condition, the asymptotic dependence of the scattered intensity, J(q), is given by
J(qf∞) ) π2(∆F)2S/q3
(1)
where ∆F is the difference in electronic density between particles and matrix and S the interface area. Porod’s law holds within the q range defined by q >> 1/L, where L is the minimum dimension of the particles. In the case of thin plates of lateral dimensions L and thickness T (T > 1/T.12 This high q range is not attained in typical experiments because the intensity for q
Figure 1. SAXS intensity for different time of hydration of C3S in pure water at 35 °C (∆, 1.0 h; 0, 2.5 h; ], 4.5 h; x, 8.8 h; O, 18 h). The arrow indicates the q value (q1) selected for the determination of the fraction of transformed phase.
>> 1/T is usually very weak. The scattered intensity within the interval 1/L
