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.     

Hydration of Portland cement with additions of calcium sulfoaluminates

The effect of mineral additions based on calcium aluminates on the hydration mechanism of ordinary Portland cement (OPC) was investigated using isothermal calorimetry, thermal analysis, X-ray diffraction, scanning electron microscopy, solid state nuclear magnetic resonance and pore solution analysis. Results show that the addition of a calcium sulfoaluminate cement (CSA) to the OPC does not affect the hydration mechanism of alite but controls the aluminate dissolution. In the second blend investigated, a rapid setting cement, the amorphous calcium aluminate reacts very fast to ettringite. The release of aluminum ions strongly retards the hydration of alite but the C–S–H has a similar composition as in OPC with no additional Al to Si substitution. As in CSA–OPC, the aluminate hydration is controlled by the availability of sulfates. The coupling of thermodynamic modeling with the kinetic equations predicts the amount of hydrates and pore solution compositions as a function of time and validates the model in these systems.     

Pore solution chemistry of the hydrated system tricalcium silicate/sodium chloride/water

The relative tendency of different Portland cements to remove chloride ions from concrete mix water by forming insoluble complexes is an important determinant of the corrosion behaviourod steel in concrete. Whilst the C₃A phase plays a dominant role in binding chloride ions, other cement minerals may be of secondary importance but their effects are not well established. The reported investigation is an attempt to elucidate the extent to which chloride binding occurs within the hydration products of the C₃S (alite) phase of Portland cement when sodium chloride is present in the mix water.     

The early hydration of Ordinary Portland Cement (OPC): An approach comparing measured heat flow with calculated heat flow from QXRD

Heat flow was calculated from XRD data and compared with measured heat flow from calorimetric experiments. It was shown that the heat released during the hydration of a commercial Ordinary Portland Cement can be assigned mainly to three mechanisms, the silicate reaction (sum of dissolution of alite and precipitation of C-S-H-phase and portlandite), the dissolution of C₃A, and the precipitation of ettringite. The contributions made by anhydrite dissolution and gypsum dissolution to the heat released during hydration turned out to be quite small. It is possible to explain, on the basis of the data produced, the origin of the heat flow curve of the cement used.     

Working mechanism of calcium nitrate as an accelerator for Portland cement hydration

Precast concrete, cold weather concreting, and the emerging technique of concrete additive manufacturing are applications in which the acceleration of cement hydration plays a critical role. To allow precise control of early cement hydration in these applications, a thorough understanding of the working mechanisms of cement hydration accelerators is required. This study contributes to the understanding of the mechanism by which calcium nitrate (Ca(NO₃)₂) influences early cement hydration. The influence of Ca(NO₃)₂ on the hydration of an ordinary Portland cement has been followed by isothermal calorimetry, in situ X-ray diffraction (XRD), quantitative XRD, compressive strength testing, and the analysis of the pore solution composition. Further, the initial pore solution, the initial phase composition, and the phase composition in the fully hydrated cement have been estimated by thermodynamic calculations to corroborate the experimentally obtained results. The results indicate that Ca(NO₃)₂, especially at the highest analyzed dosage of 5 wt.%, enhances the formation of ettringite and a nitrate-containing AFm phase. Furthermore, Ca(NO₃)₂ accelerates alite hydration. Besides the increased Ca concentration in solution, it has been found that a reduction of the Al concentration in the initial pore solution by Ca(NO₃)₂ possibly contributes to the accelerating effect of Ca(NO₃)₂ on alite hydration.     

13C chemical shift anisotropies for carbonate ions in cement minerals and the use of 13C, 27Al and 29Si MAS NMR in studies of Portland cement including limestone additions

¹³C isotropic chemical shifts and chemical shift anisotropy parameters have been determined for a number of inorganic carbonates relevant in cement chemistry from slow-speed ¹³C MAS or ¹³C{¹H} CP/MAS NMR spectra (9.4 T or 14.1 T) for ¹³C in natural abundance. The variation in the ¹³C chemical shift parameters is relatively small, raising some doubts that different carbonate species in Portland cement-based materials may not be sufficiently resolved in ¹³C MAS NMR spectra. However, it is shown that by combining ¹³C MAS and ¹³C{¹H} CP/MAS NMR carbonate anions in anhydrous and hydrated phases can be distinguished, thereby providing valuable information about the reactivity of limestone in cement blends. This is illustrated for three cement pastes prepared from an ordinary Portland cement, including 0, 16, and 25 wt.% limestone, and following the hydration for up to one year. For these blends ²⁹Si MAS NMR reveals that the limestone filler accelerates the hydration for alite and also results in a smaller fraction of tetrahedrally coordinated Al incorporated in the C-S-H phase. The latter result is more clearly observed in ²⁷Al MAS NMR spectra of the cement–limestone blends and suggests that dissolved aluminate species in the cement–limestone blends readily react with carbonate ions from the limestone filler, forming calcium monocarboaluminate hydrate.     

Effect of mixing on the early hydration of alite and OPC systems

The kinetics of hydration of cementitious materials is sensitive to the mixing procedure. High shear mixing conditions lead to an increase in the kinetics of hydration at early age compared to low shearing conditions such as hand mixing. In this study the effect of mixing speed and procedure was studied on alite and Portland cement in the presence or not of aggregates. The kinetics of hydration was monitored using isothermal calorimetry at 20 °C. The early reactivity was enhanced both with an increase in the speed of mixing and the shearing conditions. The principal features are a shortening of the induction period; a higher rate of hydrate precipitation during the acceleration period as well as an increase in the height of the main heat evolution peak. Analysis of the results in terms of dissolution theory, coupled with quantitative simulation with the μic modelling platform indicate different effects of mixing prior to and after the end of the induction period. Before the end of the induction period mixing has an impact on the rate of dissolution in the fast dissolution regime and high undersaturation, which appears to be (at least partially) controlled by the rate of transport of ions away from the alite surface. After the end of the induction period the main effect of mixing appears to be the production of more C-S-H nuclei, due to the possible detachment of the primary C-S-H (metastable) by mechanical action. This higher nucleation density leads to a denser microstructure for systems mixed at high intensities.     

Interactions between alite and C3A-gypsum hydrations in model cements

This paper focuses on the interactions that occur between cement phases during early hydration, especially between the hydration of alite and C₃A-gypsum. This study shows that alite reaction is accelerated in the presence of gypsum. This acceleration was attributed to the interaction of gypsum with Aluminium ions present in alite. Three exothermic peaks attributable to the aluminate reaction could be observed in model cements instead of one in C₃A-gypsum systems. The first corresponds to the C₃A dissolution when the sulfate ions run out, but the product formed is ettringite. The second corresponds to the formation of calcium monosulfoaluminate, but the third could not be assigned to any specific reaction. The effect of alite/C₃A ratio, the dispersion of the phases in the grains and the temperature were also investigated. With respect to temperature, it was found that the activation energy for the C₃A-gypsum reaction was higher than that for alite.     

Change in reaction kinetics of a Portland cement caused by a superplasticizer — Calculation of heat flow curves from XRD data

The hydration process of a commercial Portland cement was followed by means of heat flow calorimetry. The measured heat flow was compared with calculated heat flow curves based on XRD data. Examined in particular was the influence of one selected superplasticizer on the hydration of the Portland cement. It was shown that the superplasticizer in question retards both the aluminate reaction and the silicate reaction. It is certainly conceivable that there are more than only one explanation for the interaction between the superplasticizer and the cement. A complexation of Ca^2 + ions from pore solution by the superplasticizer is as thinkable as the adsorption of the polymer on the nuclei or the anhydrous grain surfaces which in turn might lead to the prevention of the growth of the nuclei or the dissolution of the anhydrous grains.     

Vertical Scanning Interferometry: A New Method to Measure the Dissolution Dynamics of Cementitious Minerals

The aqueous dissolution rate is a key indicator of a portland cement's reactivity, and is relevant in predicting the progress of reactions and property development in cementitious materials. Though a valuable material property, the dissolution rates of the individual cement phases and their mixtures have been seldom determined. This work for the very first time applies vertical scanning interferometry (VSI) as a new method, aptly suited to measure dissolution dynamics of cement relevant minerals. Special emphasis is placed on measuring the first dissolution rate (DR_F), i.e., when water initially and for a short duration (i.e., on the order of tens of seconds) contacts the mineral surface. DR_F, mol·m^?2·s^?1) are measured for a variety of fast and slower dissolving minerals including (1a) natural limestone (CaCO₃), (1b) reagent‐grade gypsum (CaSO₄·2H₂O); (2) alite (impure, MIII‐Ca₃SiO₅); and (3) an ASTM C150, Type I/II ordinary portland cement across a range of solution pHs. Detailed aspects of image acquisition, processing and interpretation are presented to emphasize the methodological and statistical treatment of the results. The outcomes develop quantifications of aqueous dissolution rates—inputs valuable in simulating cement hydration, and forward a new means to study correlations between chemical composition, crystallography, and the reactivity of cementitious materials.     

Modeling the coupled effects of temperature and fineness of Portland cement on the hydration kinetics in cement paste

This paper revisits the coupled impacts of fineness and temperature on the kinetics of Portland cement hydration. The approach consists in i) modeling the impact of fineness on cement dissolution through the hypothesis that the surface dissolution rate of cement particles is independent of their size, in order to, in a second step, ii) model the impact of temperature on the kinetics of cement dissolution. The analysis of the experimental results shows that the effect of cement fineness on the hydration kinetics can be captured by a simple hypothesis: for any age, the reacted thickness of cement grains can be considered independent of the initial cement particle size. In addition, the analysis of the results at different temperatures shows that a constant activation energy can account for the effect of temperature on the hydration kinetics, with an Arrhenius equation applied to the kinetics of surface dissolution. The results from the model give a good agreement with the experimental results in a significant number of combinations (different Portland cements, water/cement ratio from 0.5 to 0.6, cement Blaine fineness from 3500 to 6600 cm²/g, temperature histories between 20 and 60 °C).     

Effect of ferrous and hydrogen ions on the anodic dissolution of iron in nitrate solutions

The effect of Fe(II) and H⁺ ions on the anodic dissolution of iron (adr) in weak acid nitrate solutions is studied by means of rapid polarization plots and anodic galvanostatic charge and decay transients. The parameters of the kinetic equation of the adr are determined. The inhibition effect of Fe(II) and OH^? as revealed by the displacements of the anodic Tafel lines is ascribed to the formation of an adsorption film in the presence of the nitrate ions. The rate of the film formation and its dissolution (desorption) appears to be fast so that the diffusion of the film forming species becomes rate determining. The diffusion control is confirmed quantitatively by the analysis of the anodic transients. The properties of the adsorption film are discussed.     

An X-Ray Diffraction,Fourier-Transform Infrared Spectroscopy,and Scanning Electron Microscopy/Energy-Dispersive Spectroscopic Investigation of the Effect of Sodium Lignosulfonate Superplasticizer on the Hydration of Portland Cement Type V

This study investigated the effects of a common superplasticizer, ligno-sulfonate, on the hydration of Portland cement Type V. Samples of plain cement and superplasticizer-treated cement have been examined by x-ray diffraction, Fourier-transform infrared spectroscopy, and scanning electron microscopy/energy-dispersive spectroscopy. Lignosulfonate has been observed to retard the hydration of cement through specific surface chemical reactions which involve Ca²⁺ ions in pore solution. The admixture has been found to retard the formation of Ca(OH)₂ and stabilize ettringite [Ca₆(Al₂2O₆)(SO₄)₃.32H₂O]. The inhibition of the rate of conversion of ettringite to monosulfate [Ca₄Al₂(OH)₁₂.SO₄.6H₂O] is attributed to charge-controlled reactions caused by large quantities of Ca²⁺ ions from initial hydration reactions. Leaching of the admixture doped cement by water-removed lignosulfonate and caused complete hydration of cement. A charge-controlled-reaction model involving the Ca²⁺ ions is proposed to explain the role of the admixtures during hydration of cement.     

Effects of porosity on leaching of Ca from hardened ordinary Portland cement paste

Aiming at evaluating the effects of porosity in hardened cement paste on dissolution phenomena, we prepared hardened ordinary Portland cement (OPC), with variation in pore volume, and then leached them in deionized water. It was found that the bulk density and pore volume were affected by the dissolution of portlandite. The larger the pore volume of the sample, the more rapidly portlandite is dissolved. An electron probe microanalysis (EPMA) performed on the cross-section of the solid phase showed the ‘portlandite (CH) dissolution front’. As the leaching period became longer, the CH dissolution front shifted towards the inner part. In addition, the movement of the CH dissolution front was described by the diffusion model, with consideration of the dissolution of portlandite. It was concluded that the transport of leached constituents is diffusion controlled, and the major leached constituents of hardened OPC are portlandite and C-S-H gel. Large pore, which was generated associated with the leaching of portlandite, was considered significantly to affect the diffusion of leached constituents.     

Effects of the clinker - gypsum grinding temperature upon early hydration of Portland cement

The temperature at which Portland clinker and gypsum are ground together to produce Portland cement is shown to influence the subsequent early hydration behaviour. Raising the grinding temperature from 90°C to 130°C and thence to 170°C has resulted in increased ettringite being present at all the hydration times examined up to two hours. The results have been interpreted on the basis of a through solution mechanism involving primarily the reaction of tricalcium aluminate from the cement clinker component with available soluble Ca²⁺ and SO₄^2? ions in the aqueous medium.     

钢渣对硬化水泥浆体结构形成的影响研究

研究了钢渣对水泥强度及体积膨胀率的影响,采用SEM和EDXA分析了水化产物的形貌和微区化学成分,并用XRD对水化产物的矿物组成进行了分析研究。研究结果表明,钢渣的掺入会降低水泥净浆的早期抗压强度,但随钢渣水化的进行,掺钢渣的水泥浆体7d以后的强度增长较快,至120d时净浆抗压强度已与纯硅酸盐水泥相近。掺钢渣的水泥的体积膨胀率比纯硅酸盐水泥的体积膨胀率大,钢渣水泥的体积膨胀率主要取决于钢渣中的fCaO含量。掺钢渣水泥的主要水化产物组成和形貌与纯硅酸盐水泥无明显差别,所不同的是C-S-H凝胶中有较多的铁相。掺钢渣水泥的水化产物主要有C2SH(C)、AFt和Ca(OH)2。     

石膏对改性硅酸盐水泥性能的影响

将磷铝酸盐水泥熟料掺入硅酸盐水泥中改性后,运用XRD和SEM等测试技术,研究了石膏对改性硅酸盐水泥性能的影响.结果表明,石膏的掺入可以改善改性硅酸盐水泥的力学性能和抗冻性;在石膏掺量为3.5%时,改性硅酸盐水泥水化速度最快,硬化浆体的结构最致密,强度最高,抗冻性最好.     

Early hydration of white Portland cement in the presence of bismuth oxide

Abstract

Abstract

Mineral trioxide aggregate (MTA) is a clinical product comprising a mixture of 80 wt-% Portland cement and 20 wt-% bismuth oxide, which is used as a root-filling material in dentistry. The influence of bismuth oxide on the hydration reactions of Portland cements is not well understood. In this study, the impact of 20 wt-% replacement of bismuth oxide on the hydration of white Portland cement was monitored by powder X-ray diffraction (XRD), ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) and transmission electron microscopy (TEM). The findings of this research have confirmed that bismuth oxide is an inert additive in white Portland cement, which does not participate in the hydration reactions.     

Structural characteristics and hydration kinetics of modified steel slag

This study investigates the structural characteristics and hydration kinetics of modified basic oxygen furnace steel slag. The basic oxygen furnace steel slag (BOFS) was mixed with electric arc furnace steel slag (EAFS) in appropriate ratios and heated again at high temperature in the laboratory. The mineralogical and structural characteristics of both BOFS and modified steel slag (MSS) were characterized by X-ray diffraction, optical microscopy, scanning electron microscopy, Raman and Fourier transform infrared spectroscopies. The results show that modification increases alite content in MSS and decreases alite crystal size with the formation of C₆AF₂. One more obvious heat evolution peak appears in MSS's heat-flow rate curves in comparison to BOFS, becoming similar to that of typical Portland cement paste. As a result, its cementitious activity is much improved.     

Determining the water–cement ratio, cement content, water content and degree of hydration of hardened cement paste: Method development and validation on paste samples

We propose a new method to estimate the initial cement content, water content and free water/cement ratio (w/c) of hardened cement-based materials made with Portland cements that have unknown mixture proportions and degree of hydration. This method first quantifies the composition of the hardened cement paste, i.e. the volumetric fractions of capillary pores, hydration products and unreacted cement, using high-resolution field emission scanning electron microscopy (FE-SEM) in the backscattered electron (BSE) mode and image analysis. From the obtained data and the volumetric increase of solids during cement hydration, we compute the initial free water content and cement content, hence the free w/c ratio. The same method can also be used to calculate the degree of hydration. The proposed method has the advantage that it is quantitative and does not require comparison with calibration graphs or reference samples made with the same materials and cured to the same degree of hydration as the tested sample. This paper reports the development, assumptions and limitations of the proposed method, and preliminary results from Portland cement pastes with a range of w/c ratios (0.25–0.50) and curing ages (3–90 days). We also discuss the extension of the technique to mortars and concretes, and samples made with blended cements.