Hydration kinetics and microstructural development in the 3 CaO. Al2O3-CaSO4.2H2O-CaCO3-H2O system

Hydration and microstructural characteristics of mixtures containing C₃A and CaSO₄.2H₂O (0, 12.5, 25%) with or without addition of CaCO₃ (0, 12.5, 25%) are followed by X-ray diffraction, differential scanning calorimetry, differential thermogravimetry, scanning electron microscopy and conduction calorimetry. Depending on the length of hydration, products formed at different periods from 5 min to 3 days consisted of hexagonal calcium aluminate hydrate, cubic calcium aluminate hydrate, calcium monocarboaluminate hydrate, etrringite, calcium monosulfoaluminate hydrate and possibly a solid solution of hexagonal calcium aluminate hydrate with monocarbo-and sulfo-aluminate hydrates. Calcium carbonate retards or suppresses the formation of the cubic aluminate hydrate in the hydration of C₃A. It accelerates formation of ettringite and its conversion to the monosulfoaluminate phase when added to the C₃A+gypsum+H₂O mixture.

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Résumé On étudie l’hydratation et les caractéristiques de la microstructure de mélanges renfermant du C₃A et du CaSO₄.2H₂O (0, 12,5 et 25% avec ou sans addition de CaCO₃ (0, 12,5 et 25%) à l’aide de la diffraction X, de la calorimétrie par balayage différentiel, la thermogra-vimétrie différentielle, la microscopie électronique à balayage et la calorimétrie par conduction. Selon la durée de l’hydration, il se forme à différents intervalles allant de 5 minutes à 3 jours de l’hydrate d’aluminate de calcium hexagonal, de l’hydrate d’aluminate de calcium cubique, de l’hydrate de monocarboaluminate de calcium, de l’ettringite, de l’hydrate de monosulfoaluminate de calcium et parfois une solution solide d’hydrate d’aluminate de calcium hexagonal avec hydrate de monocarbo et sulfoaluminate. Le carbonate de calcium retarde ou empêche la formation d’hydrate d’aluminate cubique dans l’hydratation de C₃A. Il accélère la formation d’ettringite et sa conversion en phase monosulfoaluminate lorsqu’il s’ajoute au mélange C₃A+platre+H₂O.

     

Calcium aluminate cements in fly ash/calcium aluminate blend phosphate cement systems: Their role in inhibiting carbonation and acid corrosion at a low hydrothermal temperature of 90°C

Study was focused upon formulating sodium polyphosphate-modified fly ash/calcium aluminate blend (SFCB) geothermal well cements with advanced anti-carbonation and anti-acid corrosive properties. At a low hydrothermal temperature of 90°C, to improve these properties, we investigated the effectiveness of various calcium aluminate cement (CAC) reactants in minimizing the rate of carbonation and in abating the attack of H₂SO₄ (pH 1.6). We found that the most effective CAC had two major phases, monocalcium aluminate (CA) and calcium bialuminate (CA₂), and a moderate CaO/Al₂O₃ ratio of 0.4. The reaction between sodium polyphosphate (NaP) and CA or CA₂ at room temperature led to the formation of amorphous dibasic calcium phosphate hydrate and anionic aluminum hydroxide caused by the decalcification of CA and CA₂. When SFCB cement made with this CAC was exposed to 4% NaHCO₃-laden water at 90°C, some carbonation of the cement occurred, forming calcite that was susceptible to the reaction with H₂SO₄. This reaction resulted in the deposition of gypsum gel scales as the acid corrosion product on the cement surfaces. The scale layer clinging to the cement protected it from further corrosion. Under such protection, the amorphous dibasic calcium phosphate hydrate crystal hydroxyapatite and anionic aluminum hydroxide crystal boehmite phase transitions were completed in acid solution. Meanwhile, the further chemical and hydration reactions of NaP with fly ash led to the formation of additional crystalline Na-P type zeolite phases. Thus, we propose that passivation of the surface of the cement by deposition of gypsum, following the formation of these reaction products, which are relatively inert to acid, are the acid corrosion-inhibiting mechanisms of the SFCB cements.     

Carbonation of ternary building cementing materials

The carbonation processes of ettringite and calcium aluminate hydrates phases developed by hydration of calcium aluminate cement, fly ash and calcium sulphate ternary mixtures have been studied. The hydrated samples were submitted to 4% of CO₂ in a carbonation chamber, and were analysed, previous carbonation and after 14 and 90 days of carbonation time, by infrared spectroscopy and X-ray diffraction; the developed morphology was performed with the 14 days carbonated samples. The results evidenced that ettringite reacts with CO₂ after 14 days of exposition time and evolves totally at 90 days; the developed hydrated phases C₃AH₆ in samples with major CAC content, also reacts with CO₂. Due to carbonation, calcium carbonate – mainly vaterite but also aragonite-, depending on the initial formulation, aluminium hydroxide and gypsum were detected.     

Carbonation of ternary building cementing materials

The carbonation processes of ettringite and calcium aluminate hydrates phases developed by hydration of calcium aluminate cement, fly ash and calcium sulphate ternary mixtures have been studied. The hydrated samples were submitted to 4% of CO₂ in a carbonation chamber, and were analysed, previous carbonation and after 14 and 90 days of carbonation time, by infrared spectroscopy and X-ray diffraction; the developed morphology was performed with the 14 days carbonated samples. The results evidenced that ettringite reacts with CO₂ after 14 days of exposition time and evolves totally at 90 days; the developed hydrated phases C₃AH₆ in samples with major CAC content, also reacts with CO₂. Due to carbonation, calcium carbonate – mainly vaterite but also aragonite-, depending on the initial formulation, aluminium hydroxide and gypsum were detected.     

Ultrasonic characterization of model mixtures of hydrated aluminous cement

In an aluminous cement, which mainly consists of CA, the stable phases arising during hydration are C₃AH₆ and AH₃. This communication presents a chemical route for the synthesis of C₃AH₆ which is based on hydration of C₃A. Mixtures of CA, C₃AH₆ and AH₃ are characterised by ultrasonic testing. The ultrasonic velocity obtained on these mixtures is lower than what is observed in hydrated aluminous cement of similar chemical composition. Interfaces are thought to play a significant role in the ultrasonic response of aluminous cements.     

Chemical activation of calcium aluminate cement composites cured at elevated temperature

The influence of sodium sulfate, as an activator, on the hydration of calcium aluminate cement (CAC)–fly ash (FA)–silica fume (SF) composites was investigated. Different mixes of CAC with 20% pozzolans (20% FA, 20% SF and 10% FA + 10% SF) were prepared and hydrated at 38 °C for up to 28 days. The hydration products were investigated by XRD, DSC and SEM. The results showed that sodium sulfate accelerated the hydration reactions of calcium aluminate cement as well as the reactions of FA and SF with CAH₁₀ and C₂AH₈ to form the strätlingite (C₂ASH₈). The later reactions prevent the strength loss by preventing the conversion of CAH₁₀ and C₂AH₈ to the cubic C₃AH₆ phase. The acceleration effect of Na₂SO₄ on the reactivity of fly ash was more pronounced than on the reactivity of silica fume with respect to reaction with CAH₁₀ and C₂AH₈ phases.     

Chemical activation of calcium aluminate cement composites cured at elevated temperature

The influence of sodium sulfate, as an activator, on the hydration of calcium aluminate cement (CAC)–fly ash (FA)–silica fume (SF) composites was investigated. Different mixes of CAC with 20% pozzolans (20% FA, 20% SF and 10% FA + 10% SF) were prepared and hydrated at 38 °C for up to 28 days. The hydration products were investigated by XRD, DSC and SEM. The results showed that sodium sulfate accelerated the hydration reactions of calcium aluminate cement as well as the reactions of FA and SF with CAH₁₀ and C₂AH₈ to form the strätlingite (C₂ASH₈). The later reactions prevent the strength loss by preventing the conversion of CAH₁₀ and C₂AH₈ to the cubic C₃AH₆ phase. The acceleration effect of Na₂SO₄ on the reactivity of fly ash was more pronounced than on the reactivity of silica fume with respect to reaction with CAH₁₀ and C₂AH₈ phases.     

Influence of slag substitution on some properties of sand-lime aerated concrete

The effect of substitution of sand with granulated slag on some properties of sand-lime aerated concrete was investigated. The compressive strength, the hydration kinetics and the nature of the hydration products were studied for samples autoclaved under 8 atm for different periods from 0.5 to 24 h. The results indicate that the substitution of slag leads to a marked increase of the compressive strength compared with that of the unsubstituted samples. The extent of hydration, as measured by the chemically combined water, free lime and free silica contents, is enhanced due to the slag substitution. The X-ray diffraction results show that the hydration products are mainly tobermorite-like phases, C₄AH₁₃,C₃AH₆ and CAH₁₀ in the slag-containing samples. Cement notations are used in the text, e.g. C=CaO, S=SiO₂, A=Al₂O₃ and H=H₂O.     

Analysis of cubic and orthorhombic C<Subscript>3</Subscript>A hydration in presence of gypsum and lime

Field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) have been used to study the microstructural changes and phase development that take place during the hydration of cubic (pure) and orthorhombic (Na-doped) tricalcium aluminate (C₃A) and gypsum in the absence and presence of lime. The results demonstrate that important differences occur in the hydration of each C₃A polymorph and gypsum when no lime is added; orthorhombic C₃A reacts faster with gypsum than the cubic phase, forming longer ettringite needles; however, the presence of lime slows down the formation of ettringite in the orthorhombic sample. Additional rheometric tests showed the possible effects on the setting time in these cementitious mixes.     

In-situ reaction of the very early hydration of C3A-gypsum-sucrose system by Micro-Raman spectroscopy

This paper studied in situ, by Micro-Raman spectroscopy, the very early hydration of C₃A in the presence and absence of sulphates and with sucrose as an additive. For C₃A hydration in the absence of gypsum, when carbonation is not avoided, carbonate-AFm phases are formed, but in the presence of gypsum, hydroxi-AFm are the main phases. Ettringite is the AFm stable phase developed initially at 70 min of hydration with gypsum and no monosulphate is formed. In the presence of sucrose, this salt, instead of sulphate, is adsorbed over the surface of the C₃A, avoiding its reaction with sulphates until sucrose desorption. Three hours are necessary to lead to ettringite formation. A nucleation poisoning/adsorption surface mechanism is proposed for added sucrose systems.     

Preparation of a very pure Fe2O3·xH2O gel: some structural and magnetic properties

A gel of iron (III) oxide hydrate was prepared using the oxidation of a FeC₂O₄· 2H₂O suspension with H₂O₂. Alkaline ions were not employed for the precipitation of the gel and the C₂O ₄ ^2– anions which are oxidized to CO₂ disappear rapidly from the solution. A very pure Fe₂O₃·xH₂O gel is thus obtained. X-ray diffraction and electron microscope studies showed the very little crystalline character of this gel and the different steps of its dehydration and crystallization with increasing temperature. Magnetic measurements at low temperature revealed the remarkable properties of this amorphous Fe₂O₃·xH₂O: superparamagnetism, blocking temperature, T _B, and thermo-remanent magnetization. These properties are characteristic of fine grains and mictomagnetic compounds.     

Effects of crystallinity and silica content on the hydration kinetics of 12CaO · 7Al2O3

The effects of 12CaO₇ · Al₂O₃ crystallinity on the kinetics of hydration reaction were studied by measuring the heat liberated using a conduction calorimeter. The crystallinity of the sample was modified by changing the cooling rate of the sample after synthesis reaction. In addition, the effects of silica addition to 12CaO₇ · Al₂O₃ glass on kinetics were also investigated. The results indicated that the glass underwent a faster initial kinetic hydration reaction compared with that of crystalline calcium aluminate. The addition of silica, on the other hand, decreased the reaction rates. The results are discussed in terms of the solvation rate of the calcium aluminate phases and the precipitation of the hydrate phases.     

Aluminium-27 solid state NMR spectroscopic studies of chloride binding in Portland cement and blends

Alumina-rich pozzolanic and latent hydraulic binders such as pulverised fuel ash, metakaolin, and ground granulated blast furnace slag, together with silica fume, are frequently added to Portland cement concrete to improve performance and to retard chloride ingress and thereby inhibit chloride-induced corrosion of the carbon steel reinforcement. ²⁷Al{¹H} MAS and CP/MAS NMR spectroscopies have been used to follow both the hydration processes of the cement blends and the interactions of chloride ion with the hydrated aluminium species. The spectra of the hydrated aluminate phases were interpretable on the basis that the AFt (Aluminate Ferrite tri-) phase ettringite, C ₆ A H ₃₂(3CaO·Al₂O₃·3CaSO₄·32H₂O, or C ₃ A·3CaSO₄·32H₂O), and the AFm (Aluminate Ferrite mono-) phases calcium mono-sulphoaluminate, C ₄ A H ₁₂ (3CaO·Al₂O₃·CaSO₄·12H₂O, or C ₃ A·CaSO₄·12H₂O), and the lamellar tetracalcium aluminate hydrate, C ₄ AH ₁₃ (3CaO·Al₂O₃·Ca(OH)₂·12H₂O, or C ₃ A·Ca(OH)₂·xH₂O) were present as the only hydrated species containing octahedrally-coordinated aluminium. In all cases, only the AFm phase Friedel's salt (3CaO·Al₂O₃·CaCl₂·10H₂O, or C ₃ A·CaCl₂·10H₂O) could be identified as the major chloroaluminate phase produced by the interactions of the cement pastes with chloride ion.     

Influence of calcium aluminate cement (CAC) on alkaline activation of red clay brick waste (RCBW)

In this paper, the effect of calcium aluminate cement (CAC) additions on the alkali activation of red clay brick waste (RCBW) was studied at room temperature and at 65 °C. RCBW was partially replaced with CAC (0–50 wt.%) and blends were activated with NaOH and sodium silicate solutions. The compressive strength evolution was tested on mortars and the nature of the reaction products was analysed by infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, microscopic studies and pH measurements. The results show that the use of CAC accelerates the activation process of RCBW so that 50 MPa were obtained in the blended mortars containing 40 wt.% CAC cured for 3 days at room temperature. CAC did not undergo normal hydration and only the C₃AH₆ phase was identified in the pastes blended with more than 30 wt.% CAC and cured at 65 °C, while the main reaction product was a cementitious gel containing Ca and Al from CAC.     

Microstructure and mechanical properties of accelerated sprayed concrete

Considering the different hydration processes of concrete without accelerator, sprayed concrete with low-alkali accelerator not only presents short setting times and high early-age mechanical properties but also yields different hydration products. This study presents an analysis of the mechanical properties of concrete with and without accelerator and sprayed concrete with three water–binder (w/b) ratios and four dosages of fly ash (FA) after different curing ages. It also examines the setting time, mineral composition, thermogravimetric–differential scanning calorimetry curves and microscopic images of cement pastes with different accelerator amounts. Furthermore, the setting time and microstructure of accelerated sprayed concrete with different w/b ratios and FA contents are examined. Results show that the retarded action of gypsum disappears in the accelerated cement–accelerator–water system. C₃A is quickly hydrated to form calcium aluminate hydrate (CAH) crystals, and a mesh structure is formed by ettringite, albite and CAH. A large amount of hydration heat improves the hydration rate of the cement clinker mineral and the resulting density, thereby improving mechanical properties at early curing ages. The setting times of the pastes increase with increasing w/b ratio and FA dosage. Thus, the hydration level, microstructure and morphology of the hydration products also change. Models of mechanical properties as functions of w/b, FA and curing age, as well as the relationship between compressive strength and splitting tensile strength, are established.     

A Multiphase Model for the Early Stages of the Hydration of Retarded Oilwell Cement

Cement is used in the oil industry to line oil wells. The major components of oilwell cement are tricalcium silicate (C₃S), dicalcium silicate (C₂S), tricalcium aluminate (C₃A) and calcium sulphate (CaSO₄). With the exception of C₂S, each of these plays an important role in the initial thickening of cement slurry. It is important to control the time that it takes for a slurry to thicken, and this is achieved by the addition of chemical retarders, which delay the onset of thickening. In this paper, the action of a retarder whose effects are firstly, to form a complex with calcium ions, and secondly, to inhibit the growth of ettringite crystals is investigated. Ettringite is a product of the hydration of C₃A and the subsequent reaction of the products with calcium sulphate. A modified version of a model for the hydration of C₃S previously investigated by Salhan, Billingham and King (J. Engng. Math. 45 (2003) 367), along with the chemical-kinetic scheme for the action of a retarder on ettringite proposed by Billingham and Coveney (J. Chem. Soc. Faraday Trans. 89 (1993) 3021) is used. The model distinguishes between liquid and solid phases, and treats water, which is significantly depleted by the formation of ettringite, as one of the chemical constituents. It is found that both of the chemical actions of the retarder contribute to slowing the initial reaction rate, and that the sudden crystallisation of ettringite as the effect of the retarder is overcome, investigated by Billingham and Coveney, occurs in successive layers around the surface of the cement grain A.C King: Professor King died on 14 January 2005     

New Ordered Structure of Amorphous Carbon Clusters Induced by Fullerene–Cubane Reactions

As a new category of solids, crystalline materials constructed with amorphous building blocks expand the structure categorization of solids, for which designing such new structures and understanding the corresponding formation mechanisms are fundamentally important. Unlike previous reports, new amorphous carbon clusters constructed ordered carbon phases are found here by compressing C₈H₈/C₆₀ cocrystals, in which the highly energetic cubane (C₈H₈) exhibits unusual roles as to the structure formation and transformations under pressure. The significant role of C₈H₈ is to stabilize the boundary interactions of the highly compressed or collapsed C₆₀ clusters which preserves their long‐range ordered arrangement up to 45 GPa. With increasing time at high pressure, the gradual random bonding between C₈H₈ and carbon clusters, due to “energy release” of highly compressed cubane, leads to the loss of the ability of C₈H₈ to stabilize the carbon cluster arrangement. Thus a transition from short‐range disorder to long‐range disorder (amorphization) occurs in the formed material. The spontaneous bonding reconstruction most likely results in a 3D network in the material, which can create ring cracks on diamond anvils.     

Vinylphosphonic acid-modified calcium aluminate and calcium silicate cements

Cementitious materials in terms of calcium phosphate cements (CPC) were prepared through the acid-base reaction between vinylphosphonic acid (VPA) and calcium aluminate cement (CAC) reactants or calcium silicate cement (CSC) reactants at 25 °C. Using CAC, two factors were responsible for the development of strength in the cements: one is the formation of an amorphous calcium-complexed vinylphosphonate (CCVP) salt phase as the reaction product, and the other was the high exothermic reaction energy. Because the formation of CCVP depletes the calcium in the CAC reactants, Al₂O₃·xH₂O gel was precipitated as a by-product. CCVP amorphous calcium pyrophosphate hydrate (CPPH) and Al₂O₃·xH₂O -AlOOH phase transitions occurred in the CPC body autoclaved at 100 °C. Increasing the temperature to 200 °C promoted the transformation of CPPH into crystalline hydroxyapatite (HOAp). In the VPA-CSC system, the strong alkalinity of CSC reactant with its high CaO content served in forming the CPPH reaction product which led to a quick setting of the CPC at 25 °C. Hydrothermal treatment at 100 °C resulted in the CPPH HOAp phase transition, which was completed at 300 °C for both the VPA-CAC and VPA-CSC systems, and also precipitated the silica gel as by-product. Although the porosity of the specimens was one of the important factors governing the improvement of strength, a moderately mixed phase of amorphous CPPH and crystalline HOAp as the matrix layers contributed significantly to strengthening of the CPC specimens.     

In situ chemical modification of C–S–H induced by CO<Subscript>2</Subscript> laser irradiation

Fire-induced compositional changes lead to strength loss and even failure in cement and concrete. Calcium silicate hydrate (C–S–H) gel, the main product of cement hydration, dehydrates at 25–200 °C, while temperatures of 850–900 °C alter its structure. A Raman spectroscopic study of the amorphous and crystalline phases forming after CO₂ laser radiation of cement mortar showed that C–S–H dehydration yielded tricalcium silicate at higher, and dicalcium silicate at lower, temperatures. Post-radiation variations were identified in the position of the band generated by Si–O bond stretching vibrations.     

Computation of X-ray powder diffractograms of cement components and its application to phase analysis and hydration performance of OPC cement

The importance of computed X-ray diffraction patterns of various polymorphs of alite (M₃, T₁, R{\bf \emph{M}_{3}, \emph{T}_{1}, \emph{R}}), belite (b\boldsymbol{\beta}, g\boldsymbol{\gamma}), aluminate (cubic, orthorhombic), aluminoferrite, gypsum and hemihydrate in the quantitative phase analysis of cement and its early stage hydration performance is highlighted in this work with three OPC samples. The analysis shows that the predominant silicate phases present in all the samples are M₃{\bf \emph{M}_{3}}-alite phase and b\boldsymbol{\beta}-belite phase, respectively. Both cubic and orthorhombic phases of C ₃ A, brownmillerite, gypsum and hemihydrates are present at different levels. Quantitative phase analysis of cement by Rietveld refinement method provides more accurate and comprehensive data of the phase composition compared to Bogue method. The comparative hydration performance of these samples was studied with w/c{\bf \emph{w/c}} ratio, 0·5 and the results are interpreted in the light of difference in phase compositions viz. b\boldsymbol{\beta}-C ₂ S/C ₃ S ratio, fraction of finer cement particles present in the samples and theoretical modeling of C ₃ S hydration.