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.     

Microstructure of tricalcium silicate and Portland cement systems at middle periods of hydration-development of Hadley grains

The development of the microstructure of C₃S paste and a Portland cement paste was studied between 7 and 24 h by means of backscattered electrons in a field-emission SEM. The course of hydration was measured by isothermal calorimetry. While the abundant occurrence of Hadley grains (hollow-shells) in Portland cement systems is well documented from a number of SEM and other microscopy studies, some earlier reports have noted that Hadley grains do not form in C₃S or alite paste alone. This report shows evidence of Hadley grains in C₃S paste, and follows their development from middle to late hydration stages. At around 10 h the microstructure with respect to Hadley grains were seen to develop in a very similar manner in C₃S and cement. In both systems, a narrow gap often developed between the receding anhydrous cores and layer of reaction product enveloping the cores. By 1 day, Hadley grains had continued to develop only in the cement paste, where they became a prominent feature. Only small ‘hollowed-out’ hydration shells were observed in the C₃S paste by 1 day. These were presumably reminiscences of the small gapped Hadley grains seen at the earlier hydration stages.     

On the Glass Formation in the Na2O–SiO2–H2O System

The glass formation in the Na₂O–SiO₂–H₂O system is considered in the framework of the free volume theory of glasses. The boundaries of the glass transition range in the given system are determined from the dependences of the specific volume and the degree of connectivity on the water content in the system. The differences in the structure of hydrated glasses prepared by different methods are revealed. It is demonstrated that the molecular packing coefficients of the structures formed by drying of sodium silicate solutions are smaller than those of glasses prepared by hydration of anhydrous glasses. The densities are determined and the molar volumes are calculated for hydrated glasses in the Na₂O–SiO₂–H₂O system with a water content of 12–23 wt % (33–51 mol %) at a constant molar ratio SiO₂ : Na₂O = 2.8. The correlation relationship for the partial molar volume of water as a component of hydrated glasses is proposed as a function of the water content in the system. The use of this dependence allows one to calculate the molar volumes (densities) of hydrated glasses with different water contents.     

Tricalcium silicate (C3S) 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 C₃S paste at room temperature. The pressure was seen to affect drastically the hydration of a C₃S paste at 200 °C and this study evidences the competition between the different high temperature phases during the hydration.     

Early hydration of tricalcium silicate I. Kinetics of the hydration process and the stoichiometry of the hydration products

The initial hydration of C₃S in paste form at room temperature was studied. The process is initiated by a short lasting rapid hydration in which about 1 – 2% of C₃S is hydrated and a hydrate with low C/S and high H/S ratio is formed. After a subsequent induction period of 4 hours a renewed rapid hydration is observed in which a hydrate of constant stoichiometric composition, independent on the time of hydration is formed. This hydrate has a higher C/S and lower H/S ratio than the one formed initially. The liquid phase stays supersaturated with respect to calcium hydroxide for several hours after the induction period is terminated.     

Irradiation Grafted Polymeric Films. II. Gas Transport Properties of Hydrated Potassium Acrylate-Grafted Polyethylene Films

Abstract

The permeability constants of oxygen and carbon dioxide through hydrated potassium acrylate-grafted polyethylene films increase rapidly as the degree of hydration of the films increases above about 28 wt %. Below about 28 wt %, the carbon dioxide permeability constant increases with the degree of hydration. In the case of oxygen, the opposite is true.

The separation factor (CO₂/O₂) increases rapidly with film hydration up to about 28 wt %. Above this degree of hydration, the separation factor gradually approaches the value for pure water. An explanation for the results obtained is presented.     

The effects of limestone type on the sulphur capture of slaked lime

This study examines the effect of the chemical composition and origin on the performance of two calcitic and two dolomitic limestones from different sources in South Africa. The experiments were carried out in a fixed bed reactor maintained at 80 °C. The raw sorbent materials were calcined at 900 °C and the resulting quicklime hydrated to produce the relevant hydrates which were used in the tests. Results obtained show that the maximum temperature rise during the hydration of the samples varied from 5 to 65 °C depending on the chemical composition of the sorbent. Sorbents with higher temperature rise resulted in products with a more porous structure and a better performance in the sulphur capture. The maximum sorbent conversion in terms of mol of SO₂ per mol of sorbent varied from 0.0274 for dolomitic limestones to 0.1823 for the calcitic limestones. The presence of Fe₂O₃ in small quantities was observed to have a positive effect on the performance of the sorbent.     

Evaluation of photovoltaic efficiency of dye‐sensitized solar cells fabricated with electrospun PVDF‐PAN‐Fe2O3 composite membrane

Poly(vinylidene fluoride)‐polyacrylonitrile‐based membranes containing Fe₂O₃ nanoparticles were prepared by electrospinning technique and characterized by HR‐SEM, FTIR, and XRD analysis. The effect of electrolyte in the electrospun nanofibers on electrolyte uptake, ionic conductivity and porosity were studied. The electrospun membranes containing Fe₂O₃ showed an enhanced ionic conductivity than that of without Fe₂O₃. Among the prepared membranes, the membrane with 7 wt % Fe₂O₃ has the highest liquid electrolyte uptake of 562% and ionic conductivity of 6.81 × 10^?2 S cm^?1. The photovoltaic performance for open circuit voltage (V_oc), Short‐circuit current density (J_sc), Fill factor (FF), and η of the DSSC fabricated with 7 wt % Fe₂O₃ are 0.77 V, 10.4 mA/cm², 0.62 and 4.9%, respectively. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41107.     

A study of the iron phase and its hydration behaviour in high alumina cement

The Mössbauer spectroscopic investigations of dry sample of Indian high alumina cement (HAC) show that Fe₂O₃ occurs in the CF, CA and C₂F_1?xA_x phases. The presence of CF component in this cement is being reported for the first time and its unambiguous detection has been made possible by the selectivity of Mössbauer spectroscopy. The spectra of hydrated HAC have been discussed in terms of hydration of the above mentioned compounds. The X-ray diffraction studies of dry as well as hydrated HAC are also included.     

The role of alumina on performance of alkali-activated slag paste exposed to 50 °C

The strength and microstructural evolution of two alkali-activated slags, with distinct alumina content, exposed to 50 °C have been investigated. These two slags are ground-granulated blast furnace slag (containing 13% (wt.) alumina) and phosphorous slag (containing 3% (wt.) alumina). They were hydrated in the presence of a combination of sodium hydroxide and sodium silicate solution at different ratios. The microstructure of the resultant slag pastes was assessed by X-ray diffraction, differential thermogravimetric analysis, and scanning electron microscopy. The results obtained from these techniques reveal the presence of hexagonal hydrates: CAH₁₀ and C₄AH₁₃ in all alkali-activated ground-granulated blast-furnace slag pastes (AAGBS). These hydrates are not observed in pastes formed by alkali-activated ground phosphorous slag (AAGPS). Upon exposure to 50 °C, the aforementioned hydration products of AAGBS pastes convert to C₃AH₆, leading to a rapid deterioration in the strength of the paste. In contrast, no strength loss was detected in AAGPS pastes following exposure to 50 °C.     

Properties of type K expensive cement of pure components II. Proposed mechanism of ettringite formation and expansion in unrestrained paste of pure expansive component

A model is proposed to account for the behaviour of unrestrained paste hydrated pure K expansive component, based on growth of a porous ettringite layer around the C₄A₃S? grains, resulting in an expanding sphere. At the critical degree of hydration, α_cr, first contact is formed between the expanding spheres and they start pushing against each other, causing expansion. The model accounts quantitatively for the course of expansion and variation in porosity upon hydration and the variations of these parameters with temperature, as well as for the pore size distribution.     

Analysis of the surface of tricalcium silicate during the induction period by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy allows the analysis of surface layers with a thickness of a few nanometers. The method is sensitive to the chemical environment of the atoms since the binding energy of the electrons depends on the chemical bonds to neighboring atoms. It has been applied to the hydration of tricalcium silicate (Ca₃SiO₅, C₃S) by analyzing a sample after 30 min of hydration. Also two references have been investigated namely anhydrous C₃S and intermediate phase in order to enable a quantitative evaluation of the experimental data. In the hydrated C₃S sample, the analyzed volume (0.2 mm² surface by 13 nm depth) contained approximately 44 wt.% of C₃S and 56 wt.% of intermediate phase whereas CSH was not detected. Scanning Electron Microscopy data and geometric considerations indicate that the intermediate phase forms a thin layer having a thickness of approximately 2 nm and covers the complete surface instead of forming isolated clusters.     

Alkali activation of a novel calcium‐silicate hydraulic binder with CaO/SiO2 = 1.1

A quasi‐amorphous low‐calcium‐silicate hydraulic binder, with an overall CaO/SiO₂ (C/S) molar ratio of 1.1, was produced. This cementitious material was then hydrated with aqueous solutions containing 3 wt% alkalis (either NaOH, Na₂CO₃ or Na₂SiO₃). The evolution of the hydration processes of the samples were monitored by compressive strength testing, XRD, FTIR, ²⁹Si and ²⁷Al MAS NMR, isothermal calorimetry and TGA. It was found that the nearly exclusive hydration product formed was a C‐S‐H phase with a semi‐crystalline structure. More importantly, the paste prepared with the Na₂SiO₃ solution developed compressive strength values similar to those of ordinary portland cements (OPC) with faster early age kinetics. In addition, the isothermal calorimetry results indicated that these new hydraulic binders present much lower heat of hydration values compared with a traditional OPC. The results presented here open the possibility of producing cement with a compressive strength comparable to that of OPC but with lower CO₂ emissions during the production process and with lower hydration heat related problems during the production of concrete structures.     

Crystal structure refinement and hydration behaviour of 3CaO·SiO2 solid solutions with MgO,Al2O3 and Fe2O3

In this paper, analytical evidence on crystal structure and hydration behaviour of 3CaO·SiO₂ (C₃S) solid solutions with MgO, Al₂O₃ and Fe₂O₃ is presented. Samples were prepared using an innovative sol–gel process as precursor. The Rietveld refinements of XRDs show significant changes in the crystal structures for C₃S solid solutions with MgO and Al₂O₃, but only small changes for Fe₂O₃. Low concentrations of MgO do not change the hydration of C₃S, but 1.4 wt.% increases the reactivity after some days. With Al₂O₃ the initial and long-term hydration is activated, but the main reaction of C₃S decreases. Fe₂O₃ retards the hydration for several days, the long-term reaction is not affected or even activated. Altogether changes in hydration activity are more dominated by the type of oxide than by the height of changes in lattice parameters. The results can help to interpret the reactivity of different Portland cements and improve the quality control of the cement production.     

Hydration of Tricalcium Silicate Followed by 29Si NMR with Cross-Polarization

The use of cross-polarization (CP) NMR in conjunction with magic angle sample spinning (MASS) to examine the hydration reaction of tricalcium silicate (C₃S) is described. In particular the very early stages of the reaction both with and without admixtures has been studied as well as the hydration in a ball mill. The combination of CP and non-CP ²⁹Si NMR permits the distinction between silicate units associated with protons, i.e., in hydrated material, and those in anhydrous material. It has been found that in paste hydration there is steady formation of a small amount of hydrated monomeric silicate units during the induction period. In ball mill hydration the formation of the crystalline calcium silicate hydrate, afwillite, which contains only hydrated monomeric silicate species, can be monitored. These results are interpreted in terms of possible mechanisms for C₃S hydration.     

Influence of Fe2O3 on the hydration kinetics of tricalcium aluminate

The influence of Fe₂O₃ on the hydration kinetics of tricalcium aluminate (C₃A) was studied in order to clarify the mechanism of improving hydration resistance of CaO by in-situ synthesized tricalcium aluminate. The Krstulovic-Dabic model was used to investigate the hydration processes of Fe₂O₃-C₃A solid solution and calculate the corresponding kinetic parameters. The hydration products were analyzed by the X-ray diffraction and scanning electron microscope. The results indicated that the Krstulovic-Dabic model simulated the hydration processes of Fe₂O₃-C₃A solid solution at different stages effectively. The hydraulic activity of Fe₂O₃-C₃A solid solution decreased with the addition of Fe₂O₃. Reasonable amount of Fe₂O₃ addition reduced the hydration rate in the initial stage of Fe₂O₃-C₃A solid solution hydration, while the hydration rate of Fe₂O₃-C₃A solid solution was increased with excessive amount Fe₂O₃. The hydration process was controlled by multiple mechanisms due to an incomplete layer of hydration products was formed on the surface.     

Infrared and Mössbauer study of two Indian cements

The infrared and Mössbauer spectroscopic studies have been carried out for dry and hydrated samples of Indian Ordinary Portland Cement and Portland Pozzolana Cement. The infrared wavebands have been assigned and discussed in the light of available literature. The Mössbauer data show that iron occurs as Fe³⁺ and occupies two sites with 6 and 4 coordinations and that hydration of cement renders the conformation around Fe atoms more symmetric. The possibility of formation of Fe(OH)₃ and its gel is also excluded on the basis of the values of Mössbauer parameters.     

Influence of Fe2O3 addition on the densification and oxygen ion conductivity of the GdSmZr2O7 ceramic

The influence of Fe₂O₃ addition as a sintering aid on the microstructure and electrical properties of the GdSmZr₂O₇ ceramic has been studied. The GdSmZr₂O₇ ceramic with 1 wt.% Fe₂O₃ is composed of a pyrochlore-type phase and a small amount of Gd_0.5Sm_0.5FeO₃. Fe₂O₃ is found to be an effective sintering aid for the GdSmZr₂O₇ ceramic, and a reduction in the sintering temperature of about 200 K is achieved. The total conductivity of the GdSmZr₂O₇ ceramic incorporated with or without 1 wt.% Fe₂O₃ obeys the Arrhenius relation. At 1173 K, the highest total conductivity of the GdSmZr₂O₇ ceramic with 1 wt.% Fe₂O₃ is about 20% higher than that of the GdSmZr₂O₇ ceramic. The GdSmZr₂O₇ ceramic with or without 1 wt.% Fe₂O₃ is an oxide-ion conductor in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels.     

Electrical conductivity of low-melting electrolytes for aluminium smelting

To determine the electrical conductivity of low-melting electrolytes (AlF₃-rich, e.g. NaF/AlF₃ molar ratio = 1.2), a tube-type cell was used, applying ac-techniques with a sine wave signal with small amplitude in the high frequency range.One melt tested contained 55 mol% NaF and 45 mol% AlF₃ with and without addition of 2 wt.% alumina. Another melt tested contained 55 mol% KF and 45 mol% AlF₃ with and without addition of 2 wt.% alumina. The electrical conductivity data in the molten system can be described by a simple equation of the Arrhenius type:

κ=A e^−B/T