Hydration Of Tricalcium Silicate Research Articles

A Determination of Hydration Mechanisms for Tricalcium Silicate Using a Kinetic Cellular Automaton Model

Reaction mechanisms for the early stages of hydration of tricalcium silicate (Ca3SiO5) have not been agreed upon, although theories have appeared in the literature. In this paper, a mechanistic description is proposed that is consistent with a wide range of reported experimental observations, and which is supported quantitatively by simulations using HydratiCA, a new three‐dimensional microstructure model of chemical kinetics. Rate processes are quantitatively modeled using probabilistic cellular automaton algorithms that are based on the principles of transition state theory. The model can test alternate assumptions about the reaction paths and rate‐controlling steps, making it a kind of experimental tool for investigating kinetics and interpreting experimental observations. It is used here to show that hydration of Ca3SiO5 is most likely controlled by nucleation and growth of a compositionally variable calcium silicate hydrate solid, mediated at very early times by a transient, thermodynamically metastable solid that rapidly covers and sharply reduces the dissolution rate of Ca3SiO5. This proposed mechanism involves important elements of two leading theories of Ca3SiO5 hydration, neither of which alone has been able to capture the full range of experimental data when tested by the model.

Microscopic investigation of modified hydration kinetics in tricalcium silicate paste and mortar strength caused by dicalcium silicate addition

It was recently revealed that some processes of hydrating tricalcium silicate are altered by the addition of dicalcium silicate. Previous neutron scattering results revealed two critical tri/dicalcium silicate compositions. At one composition, changes in the early time hydration kinetics were observed that result in the formation of more products (reflected in increased 28 day strength), despite dicalcium silicate being essentially unreactive at early times. At the other composition, changes in the early-time hydration kinetics were observed that correspond to reduced strength. The current work uses scanning electron microscope analysis with backscattered electron imaging of 50 day hydrated tri- and dicalcium silicate mortars to reveal that at the former critical composition increased hydration of the tricalcium silicate phase occurs, and at the latter critical composition, the amount of dicalcium silicate reacted is decreased.

End of the Induction Period in Ordinary Portland Cement as Examined by High‐Resolution Scanning Electron Microscopy

The early stages of ordinary Portland cement (OPC) hydration were studied by cold field emission scanning electron microscopy and isothermal conduction calorimetry. Particular attention was paid to samples from the end of the induction period, where an additional peak in the tricalcium silicate hydration reaction has recently been discovered. Higher resolution images were obtained than is possible with currentin situimaging techniques, allowing for the observation of features on the 5‐nm scale. The peak in the hydration at the end of the induction period was associated with both the formation of pores in the surface of the cement grains and the formation of nanoscale calcium silicate hydrate (C–S–H) structures on those surfaces. The effects of grinding of OPC and additions of tricalcium aluminate and tetracalcium aluminoferrite on the peak at the end of the induction period were also examined. The results of the study were compared with the models in the literature used to describe the causes of the induction period and its end.

Immobilisation of heavy metal in cement-based solidification/stabilisation: A review

Heavy metal-bearing waste usually needs solidification/stabilization (s/s) prior to landfill to lower the leaching rate. Cement is the most adaptable binder currently available for the immobilisation of heavy metals. The selection of cements and operating parameters depends upon an understanding of chemistry of the system. This paper discusses interactions of heavy metals and cement phases in the solidification/stabilisation process. It provides a clarification of heavy metal effects on cement hydration. According to the decomposition rate of minerals, heavy metals accelerate the hydration of tricalcium silicate (C3S) and Portland cement, although they retard the precipitation of portlandite due to the reduction of pH resulted from hydrolyses of heavy metal ions. The chemical mechanism relevant to the accelerating effect of heavy metals is considered to be H+ attacks on cement phases and the precipitation of calcium heavy metal double hydroxides, which consumes calcium ions and then promotes the decomposition of C3S. In this work, molecular models of calcium silicate hydrate gel are presented based on the examination of 29Si solid-state magic angle spinning/nuclear magnetic resonance (MAS/NMR). This paper also reviews immobilisation mechanisms of heavy metals in hydrated cement matrices, focusing on the sorption, precipitation and chemical incorporation of cement hydration products. It is concluded that further research on the phase development during cement hydration in the presence of heavy metals and thermodynamic modelling is needed to improve effectiveness of cement-based s/s and extend this waste management technique.

Pozzolanic reactivity of nanoscale pyrogene oxides and their strength contribution in cement-based systems

This paper deals with the pozzolanic reactivity of pyrogene oxides (represented by Aerosil) that belong to the colloidal silicas. Pyrogene oxides are highly reactive pozzolanic nanoscale additives. They react much faster pozzolanically than other commonly used pozzolans and in this way they consume the available calcium hydroxide in a much shorter time. It is shown by using in situ X-ray transmission microscopy, pH-change measurements and the strength property results that Aerosil begins to react as early as the first hour of contact with both the added calcium hydroxide in solution and with the products of the tricalcium silicate (C3S) hydration reaction. In comparison with the other commonly used pozzolans, that is, silica fume, pyrogene oxides contribute by developing a better early and final strength. The concrete specimens that were cast by combining silica fume with small quantities of Aerosil were considered to belong to the ultra high performance class and were additionally characterised by finer pores and reduced total porosity.

Tricalcium silicate (C 3S) hydration under high pressure at ambient and high temperature (200 °C)

The hydration of a tricalcium silicate paste at ambient temperature and at 200 °C under high pressure (up to 1000 bar) has been studied. Two high pressure cells have been used, one allows in-situ electrical conductivity measurements during hydration under high pressure. The hydration products were characterized by thermal analysis, X-ray diffraction and 29Si NMR measurements. The pressure has a large kinetic effect on the hydration of a C 3S paste at room temperature. The pressure was seen to affect drastically the hydration of a C 3S paste at 200 °C and this study evidences the competition between the different high temperature phases during the hydration.

A New Approach to Modeling the Nucleation and Growth Kinetics of Tricalcium Silicate Hydration

The hydration kinetics of tricalcium silicate (C3S), the main constituent of portland cement, were analyzed with a mathematical “boundary nucleation” model in which nucleation of the hydration product occurs only on internal boundaries corresponding to the C3S particle surfaces. This model more closely approximates the C3S hydration process than does the widely used Avrami nucleation and growth model. In particular, the boundary model accounts for the important effect of the C3S powder surface area on the hydration kinetics. Both models were applied to isothermal calorimetry data from hydrating C3S pastes in the temperature range of 10°–40°C. The boundary nucleation model provides a better fit to the early hydration rate peak than does the Avrami model, despite having one less varying parameter. The nucleation rate (per unit area) and the linear growth rate of the hydration product were calculated from the fitted values of the rate constants and the independently measured powder surface area. The growth rate follows a simple Arrhenius temperature dependence with a constant activation energy of 31.2 kJ/mol, while the activation energy associated with the nucleation rate increases with increasing temperature. The start of the nucleation and growth process coincides with the time of initial mixing, indicating that the initial slow reaction period known as the “induction period” is not a separate chemical process as has often been hypothesized.

Characterisation of products of tricalcium silicate hydration in the presence of heavy metals

The hydration of tricalcium silicate (C 3S) in the presence of heavy metal is very important to cement-based solidification/stabilisation (s/s) of waste. In this work, tricalcium silicate pastes and aqueous suspensions doped with nitrate salts of Zn 2+, Pb 2+, Cu 2+ and Cr 3+ were examined at different ages by X-ray powder diffraction (XRD), thermal analysis (DTA/TG) and 29Si solid-state magic angle spinning/nuclear magnetic resonance (MAS/NMR). It was found that heavy metal doping accelerated C 3S hydration, even though Zn 2+ doping exhibited a severe retardation effect at an early period of time of C 3S hydration. Heavy metals retarded the precipitation of portlandite due to the reduction of pH resulted from the hydrolysis of heavy metal ions during C 3S hydration. The contents of portlandite in the control, Cr 3+-doped, Cu 2+-doped, Pb 2+-doped and Zn 2+-doped C 3S pastes aged 28 days were 16.7, 5.5, 5.5, 5.5, and <0.7%, respectively. Heavy metals co-precipitated with calcium as double hydroxides such as (Ca 2Cr(OH) 7·3H 2O, Ca 2(OH) 44Cu(OH) 2·2H 2O and CaZn 2(OH) 6·2H 2O). These compounds were identified as crystalline phases in heavy metal doping C 3S suspensions and amorphous phases in heavy metal doping C 3S pastes. 29Si NMR data confirmed that heavy metals promoted the polymerisation of C–S–H gel in 1-year-old of C 3S pastes. The average numbers of Si in C–S–H gel for the Zn 2+-doped, Cu 2+-doped, Cr 3+-doped, control, and Pb 2+-doped C 3S pastes were 5.86, 5.11, 3.66, 3.62, and 3.52. And the corresponding Ca/Si ratios were 1.36, 1.41, 1.56, 1.57 and 1.56, respectively. This study also revealed that the presence of heavy metal facilitated the formation of calcium carbonate during C 3S hydration process in the presence of carbon dioxide.

Hydration of Tricalcium Silicate: Effects of CaCl2 and Sucrose on Reaction Kinetics and Product Formation

The effects of CaCl2 and sucrose on the hydration of monoclinic and triclinic tricalcium silicate were studied using quasielastic neutron scattering and calorimetry. Aside from acceleration and retardation, other effects on the reaction were revealed including changes to the rate constant and the type and amount of hydration product. These effects were measured in terms of specific kinetic parameters for the first time. The phenomenon of “delayed acceleration” caused by sucrose is attributed to the solubilization of the silicate species.

States of water in hydrated C3S (tricalcium silicate) as a function of relative humidity

Quasi-elastic neutron spectroscopy was used to study the changes in the water content of hydrated tricalcium silicate cement paste with decreasing relative humidity (RH). The structurally bound water was divided into water bound in Ca(OH)2 and in calcium-silicate hydrate, or C–S–H-gel, utilizing the inelastic vibrational modes of Ca(OH)2. The quasi-elastic line was analyzed in terms of free and constrained water, and both were observed to decrease as the pores empty of pore-water with drying. An inelastic line related to translational vibrations of immobile water molecules was also extracted from the spectra, and its intensity was found to decrease with lower RH.

Time-resolved quasielastic neutron scattering study of the hydration of tricalcium silicate: Effects of CaCl 2 and sucrose

Time-resolved quasielastic neutron scattering coupled with hydration modeling enabled the interpretation of the hydration processes in a triclinic form of tricalcium silicate when calcium chloride and sucrose were added. Calcium chloride increases the rate of product formation and causes a less dense product to form. Sucrose was investigated at two concentrations. With increasing amount of sucrose, the retardation time and the length of the nucleation and growth period increase, and the product density decreases. The rate of nucleation and growth of products is not linearly dependent on the amount of sucrose added, a result attributed to complexation with Ca 2+ at higher concentrations.

Quasielastic and inelastic neutron scattering study of the hydration of monoclinic and triclinic tricalcium silicate

The hydration of Mg-stabilized triclinic and monoclinic tricalcium silicate samples were studied using quasielastic neutron scattering to follow the fixation of hydrogen into the reaction products and by applying hydration models to the data. The quantity of Ca(OH) 2 produced during hydration was also determined using inelastic neutron scattering. The monoclinic form was found to be intrinsically less reactive that the triclinic form. The monoclinic form was also confirmed to produce more product than the triclinic form after 50 h, a process found to occur through a longer, rather than earlier, nucleation and growth regime. Results indicated an increase in the permeability of the hydration layer product relative to the triclinic form and the increase in the length of the nucleation and growth regime was thus attributed to an alteration in morphology or structure of the hydration layer product, extending the time for diffusion limited mechanics to be reached.

Effect of NaOH on the kinetics of tricalcium silicate hydration: A quasielastic neutron scattering study

The effects of NaOH on tricalcium silicate hydration kinetics were investigated using in situ time-resolved quasielastic neutron scattering. NaOH was confirmed to reduce the induction period. After the induction period the rate of products formation is nearly independent of NaOH concentration. However, NaOH suppresses the quantity of hydration products formed, mainly by decreasing the length of the nucleation and growth period, but also because it may cause morphological changes in the hydration products. These effects are commensurate with NaOH concentration, and explain the previously observed reduction in the early strength of tricalcium silicate pastes hydrated in the presence of NaOH.

Effect of temperature on hydration kinetics and polymerization of tricalcium silicate in stirred suspensions of CaO-saturated solutions

Tricalcium silicate was hydrated at 274, 278, 283, 298, and 313 K in stirred suspensions of saturated CaO solutions under a nitrogen-gas atmosphere until the end of deceleratory period. The suspension conductivities and energy flows were measured continuously. The individual reaction rates for tricalcium silicate dissolution, calcium silicate hydrate precipitation, and calcium hydroxide precipitation were calculated from these measurements. The results suggest that the proportion of tricalcium silicate dissolved was determined by the rate of tricalcium silicate dissolution and the time to very rapid calcium hydroxide precipitation. The time to very rapid calcium hydroxide precipitation was more sensitive to changes in temperature than was the rate of tricalcium silicate dissolution, so that the proportion of tricalcium silicate hydration dissolved by the deceleratory period increased with decreasing temperature. The average chain length of the calcium silicate hydrate ascertained by magic-angle spinning nuclear magnetic resonance spectroscopy increased with increasing temperature.

Structural Studies of Hydrated Tricalcium Silicate by Neutron Powder Diffraction

Structure refinements have been carried out on the pure tricalcium silicate (C3S) and the hydrated C3S with the D2O–C3S mass ratio, which is 0.5, using neutron powder diffraction (NPD). The multi-phase Rietveld analysis of the hydrated C3S revealed the coexistence with the Ca(OD)2 (trigonal phase) and the unhydrated C3S (triclinic one). The Ca(OD)2 phase was hardly observed on the NPD patterns in the first ∼6 h of hydration, while the several Bragg reflections of Ca(OD)2 appeared drastically from ∼6 to ∼24 h, and then the hydration reaction rate was gradually suppressed. We could apply the Avrami-model to the initial hydration reaction process of C3S.

Study of C-S-H growth on C3S surface during its early hydration

A study to understand and quantify the growth mode of C-S-H during the hydration of tricalcium silicate grains is presented here based upon number of complementary approaches: study of C-S-H nucleation during the hydration of tricalcium silicate compared with experimental studies of homogeneous nucleation; experimental kinetics studies of C-S-H formation during tricalcium silicate hydration in dilute suspensions; direct observations of C-S-H growth by Atomic Force Microscopy; Simulation of curves obtained from experiments in dilute suspensions from a model based on AFM observations. With AFM the formation of CSH is observed by the agglomeration of identical elements 60×30×5 nm3 in size on alite surface. This agglomeration takes place perpendicularly and parallel to the surface. Numerical simulation of the experimental curves of the degree of hydration versus time enabled us to quantify the variation of the growth rates parallel and perpendicular to the surface with lime concentration. It is shown that the growth rate of C-S-H only depends on the lime concentration in solution.

The influence of superplasticizers on the first steps of tricalcium silicate hydration studied by NMR techniques

The influence of superplasticizer sulfonated naphthalene formaldehyde (SNF) on the hydration process of tricalcium silicate (C3S) paste was investigated by (1)H nuclear magnetic resonance spin-spin and spin lattice relaxation times. The addition of SNF superplasticizer to C3S paste clearly affects the morphology and growth rates of the hydration products, mainly by increasing the dormant period length, which lasts for several hours more than in conventional C3S hydrated paste, while reducing the acceleration period length. The relaxation data indicated that a pronounced delay occurs in the C3S hardening when sulfonated polymers are added to the makeup water. For all the analyzed samples, prepared with a water-to-C3S ratio of 0.4, the decay of the echo magnetization has been fitted by adopting both a monoexponential and a biexponential relaxation model in order to evaluate the contributions from water in different regimes of hydration.

Adsorption and heterocoagulation of nonionic surfactants and latex particles on cement hydrates

The adsorption of nonionic surfactants of the alkyl-phenol-poly(ethylene oxide) family and of acrylic latex particles on several anhydrous (but hydrating) or fully hydrated mineral phases of Portland cement was studied. No or negligible adsorption of the surfactant was observed. This was assigned to the ionized character of the surface silanol groups in calcium-silicate-hydrates and to the strongly ionic character of the OH groups in calcium hydroxide and in the calcium-sulfoaluminate-hydrates, which prevents the formation of surface-ethoxy hydrogen bonds. In contrast, provided they are properly stabilized by the surfactant, the latex particles form a loose monolayer on the surface of hydrating tricalcium silicate particles. The attractive interaction between the positive mineral surface and the negative latex surface appears to be the driving force for adsorption. In line with this, adsorption is reduced by sulfate anions, which adsorb specifically onto the silicate surface. Compared to tricalcium silicate, portlandite and gypsum interact only marginally with the latex particles. Our results show that the stability of the nonionic surfactant/latex/cement systems is essentially controlled by the latex colloidal stability and the latex-cement interactions, the surfactant having little direct interaction, if any, with the mineral surfaces.

In situ quasi-elastic scattering characterization of particle size effects on the hydration of tricalcium silicate

The effects of different particle size distributions on the real-time hydration of tricalcium silicate cement paste were studied in situ by quasi-elastic neutron scattering. The changing state of water in the cement system was followed as a function both of cement hydration time and of temperature for different initial particle size distributions. It was found that the length of the initial, dormant, induction period, together with the kinetics of hydration product nucleation and growth, depends on the hydration temperature but not on the particle size distribution. However, initial particle size does affect the total amount of cement hydrated, with finer particle size producing more hydrated cement. Furthermore, the diffusion-limited rate of hydration at later hydration time is largely determined by the initial tricalcium silicate particle size distribution.

Room Temperature Solid Surface Water with Tetrahedral Jumps of 2H Nuclei Detected in 2H2O-Hydrated Porous Silicates

The nature of water adjacent to solid silicate surfaces in kanemite, rehydrated zeolite A, rehydrated highly porous glass, rehydrated high surface area silica gel, and the hydration products of tricalcium silicate has been investigated with 2H NMR techniques and calorimetry. The room temperature 2H NMR quadrupole echo spectra of all samples show a sharp central resonance that corresponds to 2H2O. The kanemite, porous glass, and silica gel spectra show in addition powder patterns that were assigned, based on previous findings for kanemite, to silanol −OH groups experiencing rapid 3-fold jumps or rotational diffusion about the Si−O bond. The spectrum of hydrated tricalcium silicate shows in addition to the aqueous peak a rigid powder pattern that is assigned to Ca(O2H)2. Spectra obtained at −120 and −150 °C for the kanemite sample show that the qcc values for the aqueous deuterons range from 180 to 210 kHz, as would be expected for the different water environments observed in the X-ray structure of kanemite...