Hydration Of Tricalcium Silicate Research Articles

A soft X-ray microscope investigation into the effects of calcium chloride on tricalcium silicate hydration

Calcium chloride (CaCl 2) is one of the most recognized and effective accelerators of hydration, setting, and early strength development in portland cement and tricalcium silicate (C 3S) pastes. The mechanisms responsible for this acceleration, as well as the microstructural consequences, are poorly understood. Soft X-ray transmission microscopy has recently been applied to the study of cementitious materials and allows the observation of hydration in situ over time. This technique was applied to the examination of tricalcium silicates hydrating in a solution containing CaCl 2. It appears that CaCl 2 accelerates the formation of “inner product” calcium silicate hydrate (C-S-H) with a low-density microstructure.

Revisiting the microstructure of hydrated tricalcium silicate––a comparison to Portland cement

The microstructures of 0.40 w/s-ratio pastes of tricalcium silicate (C3S), alite and Portland cement have been studied at the hydration times 24 h and 1 month. A field-emission SEM (FE-SEM) was used to obtain high-resolution backscattered electron images. Comparison revealed no microstructural differences between C3S and alite, but there were considerable differences in microstructure between C3S and Portland cement pastes. The microstructure of the C3S paste was simpler than that of the Portland cement, and could be described by a few characteristic features. Distribution of the reaction products differed substantially in the two systems. While hollow shells (Hadley grains) were a prominent feature of the Portland cement paste, their occurrence was more limited in the C3S and alite pastes. Hollow shells were restricted to grains smaller than about 5 μm in the C3S and alite pastes, and gapped hollow shells were not a common feature.

Structural studies of hydrated tricalcium silicate by neutron diffraction

Structural studies of hydrated tricalcium silicate by neutron diffraction

Ion beam analysis of the hydration of tricalcium silicate

Tricalcium silicate is the major constituent of Portland cement, and the kinetics of its hydration is a major topic in concrete technology. Nuclear resonance reaction analysis using 15N has been applied to measure the evolution of the hydrogen profile. During the first few hours, the induction period, the hydrogen diffusion is controlled by a 10–20 nm thick surface layer. To observe this layer, the beam energy resolution must be on the order of 10 keV or less. This has been achieved at the dynamitron tandem accelerator at the Ruhr Universität Bochum.

The effect of a retarder on the early stages of the hydration of tricalcium silicate

Cement is used in the oil industry to line oil wells. The major component of oilwell cement is tricalcium silicate (C3S), which is responsible for the initial thickening of the cement slurry. It is important to control the time that it takes for this slurry to thicken, and this is achieved in practice by the addition of chemical retarders, which delay the onset of thickening. In this paper, the action of a retarder whose main effect is to form a complex with calcium ions is investigated by use of a model for the hydration of C3S previously investigated by Preece, Billingham and King (2001). It is found that such a retarder can significantly increase the thickening time of pure tricalcium silicate.

Evaluating cement hydration by non-conventional DTA; An Application to Waste Solidification

This paper presents a method to study cement hydration at ambient temperatures by using a micro processed non-conventional differential thermal analysis (DTA) system, which was used to evaluate the solidification/stabilization process of tannery wastes produced in the leather industry. The DTA curves of pastes composed by slag cement, Wyoming bentonite and waste are obtained in real time and used to analyze the heat effects of the reactions during the first 24 h of hydration. By applying a deconvolution method to separate the overlapped DTA peaks, the energy released in the several hydration stages may be estimated and consequently, the effects of each component on the solidification process. The highest separated DTA peak occurs during the several early stages of cement hydration and is due mainly to tricalcium silicate hydration. Very good correlation shows that the greater is the waste content in the paste composition, the higher is its effect on the rates of reactions occurring during the induction (dormant) period of cement hydration. The presence of bentonite used as a solidification additive in the stabilization process has a similar but less dramatic effect on the dormant period.

Glassy dynamics of water in hydrated cement paste

Understanding the state of the hydration water in cement and the ways to control it are likely to be the keys to the improvement of its ultimate strength and durability. In this study we investigate the diffusional dynamics of water molecules in hydrated tri-calcium silicate, a major component in ordinary Portland cement. The spectra of incoherent quasi-elastic neutron scattering from hydrogen atoms were measured using a high-resolution chopper spectrometer having an energy resolution of 28 μeV. The spectra were analyzed by using two different methods: (i) an explicit dynamical model, recently proposed by us, taking into account the existence of two types of water: “immobile water”, presumably water bound inside the colloidal particle component of the cement paste, and “glassy water”, water imbedded in the gel-like component filling the spaces between the colloidal particles, and (ii) a new method in which we analyze the susceptibility function in the frequency domain instead of dealing with the dynamic structure factor as in (i). We extract three important parameters: the Q-independent fraction of immobile water p;the Q-independentstretch exponent β (or b) and the Q-dependentaverage relaxation time \(\bar\Box\tau\Box\) (or 1/ωp) of the glassy water. Both methods account very well for the cement hydration process, but the second method is intuitively more appealing and simpler.

A Novel Approach Based on Differential Scanning Calorimetry Applied to the Study of Tricalcium Silicate Hydration Kinetics

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 organo-aza-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.

A neutron scattering study of the role of diffusion in the hydration of tricalcium silicate

Quasi-elastic neutron scattering was used to monitor the temperature-dependant hydration of tricalcium silicate and Portland cement. Results show that for some samples the degree of hydration is in fact higher at lower curing temperatures. To investigate this effect further, we performed a series of experiments in which samples are initially hydrated at one temperature and then the diffusion process limiting the long-term curing is monitored at a different temperature. The results confirm that the higher the initial curing temperature the more impervious are the product layers to later diffusion. In addition, it was found that the intrinsic activation energy for this diffusion process is much greater than the traditional values obtained using samples initially cured at different temperatures.

Initial hydration of tricalcium silicate as studied by secondary neutrals mass spectrometry: II. Results and discussion

The hydration of tricalcium silicate (C 3S) was studied by secondary neutrals mass spectrometry (SNMS), a method that enables determination of the Ca/Si ratio of the formed calcium silicate hydrate (C-S-H) phase with an extremely low information depth. It was found that the magnitude of this parameter within the hydrate layer formed at the surface of the nonhydrated C 3S is not constant and increases with increasing distance from the liquid–solid interface. It was also found that, at a constant distance from the surface, the Ca/Si ratio declines with hydration time. The kinetics of the hydration process is characterized by a very fast initial reaction, followed by a dormant period and a subsequent period of renewed hydration. The rate of hydration becomes distinctly accelerated by elevated temperature and retarded by the presence of sucrose, while NaCl affects the initial hydration kinetics only to a small degree.

Initial hydration of tricalcium silicate as studied by secondary neutrals mass spectrometry: I. Sample preparation and calibration

Secondary neutrals mass spectrometry (SNMS) is a method that enables to establish the chemical composition (stoichiometry) at the surface of solids with an extremely low information depth and thus without interference from deeper regions of the material. The method may be also applied for analyzing the composition of the material as a function of distance from the surface. This is done by gradual material removal from the analyzed surface by sputtering. For these reasons, SNMS is particularly suitable for the study of the hydration of tricalcium silicate (C 3S) in its initial stage. In this first part of the paper, the principle of the method, the sample preparation, and the calibrations for the measurements to be performed are described.

Hydration of tricalcium silicate (C 3 S) at high temperature and high pressure

Hydration products of tricalcium silicate (C3S) are the calcium silicate hydrates (C-S-H) and Portlandite. Silica fume, added to anhydrous cement in industrial formulations reacts with Portlandite and leads to C-S-H different from the previous one. C3S hydration with and without silica fume has been studied under high pressure (1000 bar) and high temperature (160°C) by numerous techniques (29Si and 1H NMR, XRD, Thermal analysis, SEM) for different hydration times. In these conditions, high temperature more stable crystalline phases are formed and their kinetics of formation is dependent on pressure. Besides, electrical conductivity measurement on hydrating cement under pressure have been carried out in order to evidence the great dependence of hydration kinetics with pressure. This study proposes a practical phase diagram which allows on a thermodynamical base to understand the change of equilibrium temperature with pressure. The kinetics of reaction has been studied and mechanisms of reaction proposed to explain the results.

Complementary analyses of a tricalcium silicate sample hydrated at high pressure and temperature

A cement slurry is universally used to support the casing of oilwells. Cementing operations consist in placing an appropriate cement slurry in the annulus between the walls of the hole and the casing that has been run in. This slurry is pumped down the steel casing of the well and then placed in the annular space between the casing and the surrounding rock. Nowadays, wells become deeper and deeper—at least 6 km length—so the setting conditions of the cement paste are drastic. Pressure and temperature may reach 1000 bars and 350 °C at the bottom of the well. Hydration of synthetic tricalcium silicate—main component of oilwell cements—was performed at high temperature under a high pressure to simulate the oilwell conditions. In these unusual setting conditions, a mixture of different silicate hydrates was synthesized and was rather difficult to analyse and quantify. So, DTA, TG, NMR, XRD and SEM techniques were used to study the cement hydration process. The complementarily of all these techniques was pointed out for the investigations on cementitious materials, more specially, on a sample hydrated at 200 °C under a pressure of 450 bars during 2 months.

Quasi-Elastic Neutron Scattering Study of Translational Dynamics of Hydration Water in Tricalcium Silicate

We investigate single-particle dynamics of water molecules in hydrated tricalcium silicate, a major component in ordinary Portland cement, as functions of temperature and aging and in the presence of an additive that retards the curing process. Spectra of incoherent quasi-elastic neutron scattering from hydrogen atoms were measured using a four-chopper spectrometer having an energy resolution of 28 μeV, thus probing mainly the translational dynamics of water molecules. The spectra were analyzed with an explicit dynamical model. The model takes into account the existence of two types of water: “immobile water” (type one), presumably water bound inside colloidal particle component of the cement paste, and “glassy water” (type two), water imbedded in gellike component filling spaces between the colloidal particles. The model fits very well all normalized spectra in an absolute scale over a wide range of spectrum covering an energy transfer range of at least 300 μeV. We deduced, from these fits, three important parameters as functions of temperature and aging and in the presence of an additive: (1) the Q-independent fraction of the immobile water (p); (2) the Q-independent stretch exponent (β), and (3) the Q-dependent average relaxation time (τ̄) of the “glassy water”. From trends of the age dependence of these three parameters, we obtain a quantitative picture of the kinetics of the hydration process and the structural relaxation of the glassy water. We also conclude that there is no so-called “free” water in a cement paste (at least in the experimental conditions used in our experiments) at any time 1 h after its initial mixing with water.

Hydrated Layer Formation on Tricalcium and Dicalcium Silicate Surfaces: Experimental Study and Numerical Simulations

In this paper, an original approach is used to study the calcium silicate hydrate (C−S−H) layer formation on the surface of grains of anhydrous silicate during tricalcium and dicalcium silicate hydration from the variation of the rate of hydration with lime concentration. The effects of C−S−H nucleation and growth on the curves for the degree of reaction against time have been separated in both experimental study of the rate of hydration in controlled conditions and numerical simulation of the growth of C−S−H on a surface from a particle aggregation model. The influence of the number of nuclei and of the different growth modes has been quantified.

Dynamic scaling of QENS spectra of glassy water in aging cement paste

We propose a new method for the analysis of high-resolution incoherent quasi-elastic neutron scattering spectra of water in porous media and apply it, in particular, to a case of water in hydrated tricalcium silicate. We plot the normalized frequency-dependent susceptibility as a function of a scaling variable ω/ ω p , where ω p is the peak position of the α-relaxation. In this way, we are able to scale QENS data from a range of Q into a single master curve. We then fit this master curve with an empirical formula for the susceptibility proposed by Bergman (J. Appl. Phys. 88 (2000) 1356) to obtain three independent parameters previously introduced by us to describe the dynamic state of hydration water in a curing cement paste. Specifically, we extract q, β, and τ ̄ where, q is the fraction of glassy water, and β and τ ̄ the two parameters associated with the α-relaxation of the glassy water.

State of Water in Hydrating Tricalcium Silicate and Portland Cement Pastes as Measured by Quasi‐Elastic Neutron Scattering

Quasi‐elastic neutron scattering (QENS) was used to monitor the state of water in portland cement and tricalcium silicate pastes during the first 2 days of hydration at three different temperatures. By applying a double‐Lorentzian rather than a single‐Lorentzian fitting function, the QENS signal from water at a given hydration time was divided into three separate populations arising from liquid water, chemically bound water, and constrained water. The constrained water population consisted of water adsorbed on surfaces and contained in very small (<10 nm) pores, and could be associated primarily with the calcium‐silicate‐hydrate (C‐S‐H) phase. The rate of increase in the chemically bound water population closely followed the exothermic heat output, while the constrained water population increased more rapidly during the first several hours of hydration and then leveled off.

Age-dependent dynamics of water in hydrated cement paste.

Self-dynamics of water molecules has been studied in hydrated tricalcium silicate as functions of temperature, aging, and in the presence of an additive. A dynamical model taking into account the existence of "immobile water" and "glassy water" has been used to analyze quasielastic neutron spectrometer spectra. We deduced the fraction of the immobile water (p), the stretch exponent (beta), and the average relaxation time (tau) of the glassy water. A quantitative picture for an aspect of the kinetics of the curing process and the structural relaxation parameters of the glassy water have been established.

Characterization of the induction period in tricalcium silicate hydration by nuclear resonance reaction analysis

Nuclear resonance reaction analysis has been applied for the first time to measure the development of the hydrogen depth profile in the early stages of hydration of tricalcium silicate using the 1H(15N,αγ)12C reaction. The surface layer had an H concentration and thickness consistent with a few unit cells (1.1 nm) of tobermorite-like material. The inner regions exhibited diffusion-controlled growth with time until the hydrogen concentration approaches that of the surface layer at 4.25 ± 0.07 h. This event marked the end of the induction period and the onset of the rapid hydration reaction period.

On the initial stages of cement hydration

After the initial mixing of cement, an induction period occurs during which its consistency remains constant. Thickening occurs at the end of this period when the consistency is observed to increase very rapidly. In this paper we propose a reaction-diffusion model for the hydration of tricalcium silicate, a principal constituent of cement, which is believed to be responsible for the initial development of its strength. Our model is based on the assumption that the hydration of cement can be described as a dissolution -precipitation reaction. The mathematical solutions enable us to determine some of the factors that control the length of the induction period and make predictions of the ionic concentrations which are in agreement with experimental data.