Discussion of the paper “Dissolution theory applied to the induction period in alite hydration” by P. Juilland et al., Cem. Concr. Res. 40 (2010) 831–844

The new mechanism proposed by Juilland et al. [1] to explain the induction period in alite hydration is discussed with reference to the free energy of dissolution under typical hydration conditions calculated from published data. It is concluded that the proposed mechanism is theoretically possible but requires an unusually high interfacial energy for step formation, plus an unusually large increase in dissolution rate over small changes in the degree of solution undersaturation, compared to typical published values for silicate minerals. It is suggested that comparisons with the hydration rates of CaO and MgO might shed further light on the mechanism.     

Analytical Model for the Alite (C3S) Dissolution Topography

The surface topography that develops during the dissolution of alite (or C₃S) has never been considered as an important aspect of the hydration of portland cement. Like many other minerals, alite dissolution results in the formation of etch pits. In this study, a simple model with a handful of parameters is proposed to explore the kinetic consequences of pitting on the dissolution of alite. The first consequence is an accelerating period during which the reactive surface, that is, the pit, expands. This new feature, to the authors’ opinion never reported previously for alite, is supported by experimental data. The mechanisms leading to the activation of the initial dissolution centers and to the growth of pits could be hindered by some inhibitory species such as aluminum ions or organic molecules. The model indicates that only a few assumptions about the formation of pits in the presence of these species are necessary to introduce an initial period of low dissolution prior to the accelerating phase. Such a low early reactivity is similar to the so‐called dormant period observed during portland cement hydration. The implications of this new model in cement hydration could go beyond this article and shed light on still unresolved fundamental questions on hydration behavior.     

Influence of Portland cement composition on early age reactions with metakaolin

The reactivity of two metakaolins, which vary principally in their surface area, and Portland cements of varying composition were examined via isothermal calorimetry for pastes at water-to-cementitious materials ratio of 0.50 containing 8% cement replacement by weight of metakaolin. Both metakaolins examined appear to have a catalysing effect on cement hydration. Calorimetry showed accelerated hydration, a slight increase in cumulative heat evolved during early hydration, and - for some cements examined - apparently an increased intensity of the heat evolved, particularly during the period typically associated with hydration of calcium aluminates. The higher surface area metakaolin had a greater effect. It is proposed that the presence of metakaolin may enhance dissolution of cementitious phases and/or provide additional, well-dispersed sites for nucleation of hydration products, in addition to increasing the early age concentration of solubilized aluminium (due to metakaolin dissolution). The increased intensity of some of the calorimetry data also suggests that some additional exothermic reactions are occurring, which may be related to an increased reactivity of calcium aluminate phases in the cement as well as the reaction of the metakaolin. This effect is apparently increased as the cement equivalent alkali content increases.     

Effect of CuO on the formation of clinker minerals and the hydration properties

The purposes of this study are to explore the mechanisms of Cu element in clinker burning and hydration processes and to make effective use of waste containing copper in cement production. The effect of CuO on clinker mineral composition, C₃S polymorph and size, Cu element distribution and state, compressive strengths, hydration products, non-evaporable water quantity and hydration heat release rate was analyzed by XRD, SEM, DTA, isothermal heat-conduction calorimetry, etc. Results show that as the amount of CuO increases the formation and growth of C₃S grain are accelerated, R C₃S is gradually transformed into M₃ and the content of C₄AF increases; a small quantity of CuO increases the 3-day and 28-day strengths and the hydration degree of clinker, but excessive CuO has adverse effects. Those effects of CuO on clinker burning process are attributed to the formation of low-melting Cu₂O and the dissolution of CuO in C₄AF which decrease the formation temperature of liquid phase and increase its quantity. The effects on hydration process result from the combined action of the following factors: the induction period is prolonged; the hydration reactions in the initial and acceleration periods are accelerated.     

Early C3A hydration in the presence of different kinds of calcium sulfate

Hydration reactions of C₃A with various amounts of calcium sulfate hemihydrate, gypsum or a mixture of the two, were investigated by isothermal microcalorimetry, and a monitoring of the ionic concentrations of diluted suspensions. This study shows that sulfate type used modifies the early C₃A–CaSO₄ hydration products and the rate of this hydration. The fast initial AFm formation observed before ettringite precipitation in the C₃A–gypsum system is avoided as soon as hemihydrate is present in the suspension. This was attributed to higher super saturation degrees and then higher nucleation frequency with regard to the ettringite obtained in the presence of hemihydrate. Moreover, replacement of gypsum by hemihydrate also leads to an increase of the ettringite formation rate during at least the five first hours under experimental conditions.     

Improved evidence for the existence of an intermediate phase during hydration of tricalcium silicate

Tricalcium silicate (Ca₃SiO₅) with a very small particle size of approximately 50 nm has been prepared and hydrated for a very short time (5 min) by two different modes in a paste experiment, using a water/solid-ratio of 1.20, and by hydration as a suspension employing a water/solid-ratio of 4000. A phase containing uncondensed silicate monomers close to hydrogen atoms (either hydroxyl groups or water molecules) was formed in both experiments. This phase is distinct from anhydrous tricalcium silicate and from the calcium-silicate-hydrate (C-S-H) phase, commonly identified as the hydration product of tricalcium silicate. In the paste experiment, approximately 79% of silicon atoms were present in the hydrated phase containing silicate monomers as determined from ²⁹Si{¹H} CP/MAS NMR. This result is used to show that the hydrated silicate monomers are part of a separate phase and that they cannot be attributed to a hydroxylated surface of tricalcium silicate after contact with water. The phase containing hydrated silicate monomers is metastable with respect to the C-S-H phase since it transforms into the latter in a half saturated calcium hydroxide solution. These data is used to emphasize that the hydration of tricalcium silicate proceeds in two consecutive steps. In the first reaction, an intermediate phase containing hydrated silicate monomers is formed which is subsequently transformed into C-S-H as the final hydration product in the second step. The introduction of an intermediate phase in calculations of the early hydration of tricalcium silicate can explain the presence of the induction period. It is shown that heterogeneous nucleation on appropriate crystal surfaces is able to reduce the length of the induction period and thus to accelerate the reaction of tricalcium silicate with water.     

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.     

Pitting on the crevice wall prior to crevice corrosion: Iron in sulfate/chromate solution

Experimental results are presented for the induction period that precedes the onset of crevice corrosion. In situ visual examination and ex situ microscopy revealed that the first corrosive attack on the (passive) crevice wall occurs as a pit near the bottom of the crevice. Additional pits and corrosion product form higher and higher on the crevice wall during the induction period. The measured current is initially low in the passive range (10 μA), increasing gradually, but the measured current does not record the large increases in anodic current that produce the pits. The latter result and the observation that gas bubbles form within the pits indicate that the pitting initially is a local cell process of metal dissolution and cathodic reactions on the sample, including H₂ formation which occurs inside the pits in this iron/sulfate-chromate solution (pH 8.8, room temperature). Then, a transition from merging pits to corrosive attack across the width of the crevice wall occurs accompanied by a much steeper increasing current. This latter morphology (horizontal boundary that moves up the wall towards the crevice opening, heaviest attack just below this boundary and passive crevice wall above this boundary) is representative of stable crevice corrosion of the IR type reported in the literature.     

Early age hydration of rice hull ash cement examined by transmission soft X-ray microscopy

Early age hydration of barium-doped β-Ca₂SiO₄ cement, produced from rice hull ash (RHA), is examined by transmission soft X-ray microscopy. Use of low-energy cements produced from by-product materials, such as the cement considered here, may be economically and environmentally advantageous. However, the hydration kinetics and morphology and composition of the products of RHA-based β-Ca₂SiO₄ cements have not been investigated. Observation of the early age cement hydration shows evidence of cement dissolution and hydration product formation, including the formation of Hadley grains. The rates of the reaction and amount product formed appear to be related to the hydrothermal processing temperature and the chemical composition of the cement. That is, more rapid hydration is observed for barium-doped RHA cements produced at higher temperatures and for cements produced with higher barium contents, within the ranges examined.     

Influence of self heating and Li2SO4 addition on the microstructural development of calcium aluminate cement

Hydrated Calcium Aluminate Cement (CAC) is known to have a complex microstructure involving different phase assemblages strongly dependant on the temperature. This work presents an experimental approach to study the microstructure of CAC pastes from the first minute of hydration with controlled time-temperature histories up to several months of curing. The self heating usually occurring in the CAC concrete is considered and its influence on the growth and assemblage of the hydration products and subsequent space filling is shown. Quantification of the degree of CA hydration by BSE image analysis is used to understand the evolution of phases throughout the hydration process. Lithium sulphate is commonly used to control the setting time of CAC based materials. It is shown that this promotes the formation of more stable hydrates, but slightly reduces the extent of CA hydration.     

环境扫描电镜用于硅酸盐水泥早期水化的研究

采用环境扫描电镜对硅酸盐水泥的早期水化过程进行了连续观察。将硅酸盐水泥早期水化过程分为预诱导期、诱导期、加速期、减速期和稳定期个阶段加以描述。预诱导期阶段水泥开始水解,释放出离子,硅酸三钙(C3S)颗粒表面形成低n(Ca)／n(Si)层,第一批水化产物产生,水泥颗粒表面生成一层水化物的保护膜,使水化反应速度降低。诱导期阶段保护膜逐渐推进直至覆盖整个颗粒表面,膜内外产生渗透压力差。当渗透压力大到足以使薄膜在薄弱处破裂,缺钙的硅酸盐离子就被挤入液相,并和钙离子结合,生成各种不定形的C—C—H。加速期阶段钙离子和硅酸盐离子浓度相对于C—S—H来说达到过饱和,C—S—H高速生长,在颗粒表面附近形成类似于网状形貌的产物(高密度C—S—H),而在颗粒间的原充水空间里形成近球状形貌的产物(低密度C—S—H)。减速期阶段水化产物继续生长,由不定形富水的凝胶状转变为不定形的颗粒状,显微结构继续发展。稳定期阶段水化产物颗粒个数几乎保持不变,但单个颗粒均逐渐生长变大,显微结构逐渐致密化。保护膜的形成和破裂分别可以解释诱导期的产生和结束。C—S—H的生长速度是加速期水化反应速度控制的主要因素。     

The influence of an ion-exchange resin on the kinetics of hydration of tricalcium silicate

The addition of a finely-ground ion-exchange resin makes it possible to modify the hydration kinetics of C₃S pastes. Analyses of the liquid phase in pastes and more dilute suspensions show that the resin exchanges calcium ions for sodium ions very rapidly during the early stage of hydration and therefore the concentration of silica in solution increases. The resin impacts the hydration of C₃S by other mechanisms which depends on the resin quantity added. For a high resin quantity, the induction period is very short, but the longer-term hydration is enhanced compared to a reference sample without resin. We hypothesize that the surface of the resin can provide sites for the nucleation and growth of C-S-H hydrates and/or portlandite far away from the surface of the C₃S grains. This consequently increases the quantity of hydrates that can precipitate before a continuous hydrate layer forms over the surfaces of C₃S particles.     

Changes in C3S hydration in the presence of cellulose ethers

The influence of cellulose ethers (CE) on C₃S hydration processes was examined in order to improve our knowledge of the retarding effect of cellulose ethers on the cement hydration kinetics. In this frame, the impacts of various cellulose ethers on C₃S dissolution, C-S-H nucleation-growth process and portlandite precipitation were investigated. A weak influence of cellulose ethers on the dissolution kinetics of pure C₃S phase was observed. In contrast, a significant decrease of the initial amount of C-S-H nuclei and a strong modification of the growth rate of C-S-H were noticed. A slowing down of the portlandite precipitation was also demonstrated in the case of both cement and C₃S hydration. CE adsorption behavior clearly highlighted a chemical structure dependence as well as a cement phase dependence. Finally, we supported the conclusion that CE adsorption is doubtless responsible for the various retarding effect observed as a function of CE types.     

Modeling and simulation of cement hydration kinetics and microstructure development

Efforts to model and simulate the highly complex cement hydration process over the past 40 years are reviewed, covering different modeling approaches such as single particle models, mathematical nucleation and growth models, and vector and lattice-based approaches to simulating microstructure development. Particular attention is given to promising developments that have taken place in the past few years. Recent applications of molecular-scale simulation methods to understanding the structure and formation of calcium–silicate–hydrate phases, and to understanding the process of dissolution of cement minerals in water are also discussed, as these topics are highly relevant to the future development of more complete and fundamental hydration models.     

A remastered external standard method applied to the quantification of early OPC hydration

A promising external standard method, first described by O'Connor [15], was used to determine the quantitative phase composition of a hydrating cement paste. On the basis of the data produced we can conclude that the method used is absolutely to be recommended for the examination of OPC pastes, since it displays many advantages in comparison to internal standard methods and other methods. No reaction of the phase alite could be detected during the initial and the induction periods of the cement hydration. Additionally it was found that the cement phases involved in the aluminate reaction (bassanite, gypsum, anhydrite and C₃A) react successively. The changes detected in the phase composition of the OPC paste could be assigned to the different periods of OPC hydration.     

Mechanisms and parameters controlling the tricalcium aluminate reactivity in the presence of gypsum

To understand the mechanisms and the parameters controlling the reactivity of tricalcium aluminate in the presence of gypsum at an early age, a study of the hydration of the “C₃A-sulphate” system by isothermal microcalorimetry, conductimetry and a monitoring of the ionic concentrations of diluted system suspensions have been carried out with various gypsum quantities. The role of C₃A source and its fineness were also studied. This work shows the fast initial formation of AFm phase followed by ettringite formation during the period when the sulphate is consumed. It has been highlighted that the time necessary to consume all the gypsum varies with the type of C₃A and it has been attributed to the intrinsic reactivity of each one and mainly to the change of fineness from one C₃A to another. Results are discussed alongside hypothesis from the literature to explain the slowing down of C₃A hydration process in the presence of calcium sulphate.     

Structural and hydration properties of amorphous tricalcium silicate

Mechanical milling was carried out to synthesize amorphous tricalcium silicate (Ca₃SiO₅) sample, where Ca₃SiO₅ is the most principal component of Portland cement. The partial phase transformation from the crystalline to the amorphous state was observed by X-ray and neutron diffractions. Moreover, it was found that the structural distortion on the Ca-O correlation exists in the milled Ca₃SiO₅. The hydration of the milled Ca₃SiO₅ with D₂O proceeds as follows: the formation of hydration products such as Ca(OD)₂ rapidly occurs in the early hydration stage, and then proceeds slowly after about 15 h. The induction time for the hydration of the milled Ca₃SiO₅ is approximately one half shorter than that for the hydration of the unmilled one. This result means that the mechanical milling brings about the chemical activity of Ca₃SiO₅ for hydration, and may be particularly useful for increasing the reactivity in the early hydration stage.     

Experimental and modeling analysis of the formation of cuprous intermediate species formed during the copper deposition on a rotating disk electrode

Two comprehensive kinetic models reported in the literature for the copper deposition in sulfate media are modeled and compared with experimental data (linear sweep voltammetry) in order to disclose the role and phase of the cuprous species as intermediates. Differences in the phase of these species are considered for each model, i.e. Cu(I)_ads and Cu⁺. Modeling considers the formation of copper in two mono-electronic steps and account for mass transport by diffusion and convection in a RDE. Reasonable fits were obtained for both models at different experimental conditions (e.g. Cu²⁺ bulk concentration, rotation rate). It was found that this behavior is possible due to the low concentrations of cuprous species and their rapid consumption to form metallic copper, i.e. not rate-controlling step. Further insights of the kinetic and mass transport contributions of this system were obtained for both models by computing some variables that cannot be experimentally measured (i.e. surface concentrations). The first reaction in the mechanism was found to be the rate-determining step. A set of optimum kinetic parameters and constants obtained from the analysis of the models are also reported in this study.