Hydration Of Calcium Aluminate Cement Research Articles

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.

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.

Studies on the influence of different fly ashes and Portland cement on early hydration of calcium aluminate cement

The influence of three mineral additives, i.e. fly ashes from pulverized combustion and from fluidized combustion of hard coal as well as Portland cement, on early hydration (up to 28th day) of calcium aluminate cement was investigated. Cement pastes containing 0, 5 and 25 wt% of additives were studied by the use of calorimetry, thermal analysis and infrared spectroscopy methods. It was confirmed that hydration of calcium aluminate cement is closely dependent on the type of addition and its amount. The influence of additives of different properties on cement hydration was discussed basing on received results and other literature reports.

Effect of Sodium Silicate on Calcium Aluminate Cement Hydration in Highly Alkaline Media: A Microstructural Characterization

The present paper discusses calcium aluminate cement (CAC) hydration in four highly alkaline solutions (8M NaOH and three sodium silicate solutions with SiO2/Na2O moduli of 0.19, 0.69, and 1.17). The reference system was water-hydrated CAC. The hydrated materials were tested for 2- and 28-day compressive and flexural strength and the reaction products were characterized by XRD, fourier transform infrared, and NMR. In highly alkaline media, initial mechanical strength developed much more slowly than in CAC-water systems. Strength in alkali-activated CAC rose gradually with time, however, reaching values of approximately 80 MPa in 28-day specimens, depending on the SiO2/Na2O ratio in the solution used to hydrate the CAC. The chief conclusion drawn from this study was that hydration proceeds differently in alkaline and neutral media, with C3AH6 precipitating in the former. The addition of sodium silicate to the alkaline hydration solutions used to hydrate the CAC induced the precipitation of katoite [Ca2.93Al1.97 (Si0.64O2.56)(OH)9.44] and an alkaline aluminum silicate hydrate.

Kinetic modeling of calcium aluminate cement hydration

The hydration of iron-rich calcium aluminate cement (CAC) has been investigated by differential calorimeter and quantitative powder X-ray diffraction (QXRD). A simplified stoichiometric model of early age CAC hydration based on reaction schemes of the principal mineral monocalcium aluminate was employed. The CAC characteristic feature of retardation of nucleation and growth mechanism with temperature requires employing more than one kinetic mechanism to describe the resulting complex hydration kinetics. This paper proposes a single equation kinetic model of CAC hydration which comprises simultaneously three main mechanisms: nucleation and growth, chemical interaction and mass transfer. A gradual change between kinetic mechanisms was grasped with a reasonable inter-dependency of the kinetic parameters. The overall hydration kinetics was described relative to the amount of the both reactants, cement and free water.

Thermal properties of hydrating calcium aluminate cement pastes

As the hydration of calcium aluminate cements (CAC) is highly temperature dependent, yielding morphologically and structurally different hydration products that continuously alter material properties, a good knowledge of thermal properties at early stages of hydration is essential. Thermal diffusivity and thermal conductivity during CAC hydration was investigated by a transient method with a numerical approach and a transient hot wire method, respectively. For hydration at 15 °C (formation of mainly CAH 10), thermal diffusivity shows a linear decrease as a function of hydration degree, while for hydration at 30 °C there is a linear increase of thermal diffusivity. Converted materials exhibited the highest values of thermal diffusivities. The results on sealed converted material indicated that thermal conductivity increased with an increase in temperature (20–80 °C), while thermal diffusivities marginally decreased with temperature. The Hashin–Shtrikman boundary conditions and a simple law of mixtures were successfully applied for estimating thermal conductivity and heat capacity, respectively, of fresh cement pastes.

Calorimetric study of calcium aluminate cement blended with flue gas desulfurization gypsum

The use of by-product gypsum is an important alternative in concrete design. In present experiment, conduction calorimetry was applied to investigate the early hydration of calcium aluminate cement (CAC)/flue gas desulfurization (FGD) gypsum paste, supplemented with the determination of setting times and analysis of hydrates by X-ray diffraction (XRD). It was found that different profiles of heat evolution rate were presented depending on the CAC/FGD gypsum ratio. Two distinct exothermic peaks, associating with CAC hydration and ettringite formation respectively, appeared when the FGD gypsum content was less than 20%. Hydrate barrier mechanism was introduced to explain the difference in induction periods of the pastes with or without FGD gypsum. It is concluded that the blending of FGD gypsum accelerates the hydration of CAC for the quick formation of ettringite and generates greater hydration heat from per gram of pure CAC for the high exothermic effect of ettringite formation. The dissolution and diffusion of gypsum plays an important role of reacting controller during the hydrations of the pastes with FGD gypsum. The modified hydration process and mechanism in this case is well visualized by means of calorimetry.

Calorimetry in the studies of cement hydration

Calorimetry was applied to an investigation of the early hydration of Portland cement (PC)–calcium aluminate cement (CAC) pastes. The heat evolution measurements were related to the strength tests on small cylindrical samples and standard mortar bars. Different heat-evolution profiles were observed, depending on the calcium aluminate cement/Portland cement ratio. The significant modification of Portland cement heat evolution profile within a few hours after mixing with water was observed generally in pastes containing up to 25% CAC. On the other hand the CAC hydration acceleration effect was also obtained with the 10% and 20% addition of Portland cement. As one could expect the compressive and flexural strength development was more or less changed—reduced in the presence of larger amount of the second component in the mixture, presumably because of the internal cracks generated by expansive calcium sulfoaluminate formation.

Microstructure development in mixes of calcium aluminate cement with silica fume or fly ash

The most widely identified degradation process suffered by calcium aluminate cement (CAC) is the so-called conversion of hexagonal calcium aluminate hydrate to cubic form. This conversion is usually followed by an increase in porosity determined by the different densities of these hydrates and the subsequent loss of strength. Mixes of calcium aluminate cement (CAC) and silica fume (SF) or fly ash (FA) represent an interesting alternative for the stabilization of CAC hydrates, which might be attributed to a microstructure based mainly on aluminosilicates. This paper deals with the microstructure of cement pastes fabricated with mixtures CAC-SF and CAC-FA and its evolution over time. Thermal analysis (DTA/TG), X-ray diffraction (XRD) and mid-infrared spectroscopy (FTIR) have been used to assess the microstructure of these formulations.

Alkaline Activation of Blends of Metakaolin and Calcium Aluminate

The present paper reports the preliminary results obtained and analyzed in the framework of a joint Spanish–Japanese project on the behavior of blends of alkali‐activated metakaolin (MK) and calcium aluminate cement (CAC). In these experiments, the materials were activated with an 8M solution of NaOH, poured into molds, and subjected to brief thermal curing (2 or 20 h at 85°C). The hardened pastes were tested for mechanical strength and characterized for mineralogy and microstructure by a number of techniques (XRD, FTIR, BSEM, and MAS‐NMR). The results showed that under the above conditions, the Al and Ca in the CAC were taken up into the aluminosilicate formed as the main product of the alkali activation of MK. None of the CAC hydration compounds (CAH10; C2AH8; C3AH6; AH3) normally formed were detected in any of the cases studied.

Microstructural Evolution of Calcium Aluminate Cements Hydration with Silica Fume and Fly Ash Additions by Scanning Electron Microscopy, and Mid and Near‐Infrared Spectroscopy

Calcium aluminate cement (CAC) is less commonly used as ordinary Portland cements (OPCs) for structural concrete, is relatively expensive, but may have certain advantages when used for solidification of wastes; it introduces rapid strength gain and has a higher resistance to chemical attack than OPC. However, the most widely identified degradation process suffered by CAC is the so‐called conversion of hexagonal calcium aluminate hydrate to a cubic form. Mixes of CAC with silica fume (SF) or fly ash (FA) represent an interesting alternative for the stabilization of CAC hydrates, which might be attributed to a microstructure based mainly on aluminosilicates. This paper deals with the microstructure of cement pastes fabricated with binders of CAC–SF and CAC–FA, and their evolution over time. Mid infrared and near infrared spectroscopy have been used to assess the microstructure of these formulations. Microstructural characterization was completed by backscattering electron microscopy observation and microanalysis.

Deflocculated refractory concretes with a high cement content

Dependences are provided for the spreadability and mechanical characteristics of deflocculated concretes on the content of microsilica, deflocculant and water. The effect of microsilica on hydration of calcium aluminate cements is discussed.

Aditivos e sua influência no comportamento de secagem e resistência à tração de concretos refratários

A evolução tecnológica de concretos refratários tem tornado mais difícil a etapa de secagem devido a redução da sua permeabilidade e conseqüente aumento da dificuldade para que o vapor de água migre do interior do corpo para a superfície. A hidratação do cimento de aluminato de cálcio também causa decréscimo da permeabilidade dos concretos. Tal processo é significativamente afetado pela presença de aditivos utilizados para a dispersão do concreto. Neste contexto, este trabalho teve como objetivo relacionar o tipo de aditivo utilizado no processamento de concretos refratários com seu comportamento de secagem e resistência à tração. Foi verificado que os aditivos não atuam somente na dispersão das partículas da matriz e ligante, mas o efeito da sua atuação pode influenciar no tipo de hidrato formado. O aditivo CS se mostrou um eficiente dispersante promovendo a redução no consumo de água. Isso acarretou na formação preferencial dos hidratos C3AH6 disponibilizando mais alumina para a formação de AH3. Essa fase apresenta alta capacidade de preencher vazios interparticulas contribuindo para aumentar a resistência à tração do material. Por outro lado, o aditivo AC desfavoreceu a formação desses tipos de hidratos uma vez que atua como retardador da hidratação do cimento por meio do consumo de íons cálcio. Também a presença de um maior teor de água nos concretos preparados com este aditivo pode explicar o aumento da porosidade e decréscimo da resistência à tração do concreto.

Electrical conductivity and phase composition of calcium aluminate cement containing air-cooled and water-cooled slag at 20, 40 and 60 °C

Calcium aluminate cement (CAC) pastes containing Egyptian air-cooled slag (AS) or water-cooled slag (WS) were prepared using different amounts of slag, namely, 5, 10, 15, 20 and 25 mass%. The pastes were prepared with deionized water using the required water of standard consistency to produce normal workability. The variations of electrical conductivity with the hydration time were measured at 20, 40 and 60 °C. The results demonstrate that electrical conductivity is a useful technique to study the change in the phase composition at different temperatures during the setting and hardening of calcium aluminate cement as well as reflecting the role of AS and WS, preventing the conversion occurring during the CAC hydration.

Effect of calcium aluminate cement in water on hydration of aluminium powder

The addition of small amounts (0.25 wt-%) of calcium aluminate cement (CAC) to water greatly accelerates the hydration of Al powder. This behaviour is attributable to the increased basicity of the solution owing to the CAC leading to dissolution of the native Al2O3 film and early hydration products, as well as additional heat released by the accelerated hydration of Al and CAC. The significance of this for the use of Al powder in CAC bonded refractory castables is discussed.

Characterisation of early stage calcium aluminate cement hydration by combination of non-destructive techniques: acoustic emission and X-ray tomography

Abstract Techniques that monitor in real time the setting and hardening of calcium aluminate cements (CAC) are now of interest to the engineering community. This paper focuses on the association of two in situ techniques, acoustic emission (AE) and X-rays computed tomography (CT). These non-destructive techniques make it possible to follow the cement hydration in the early stage from a few minutes to a few hours after mixing. Results concerning a calcium aluminate cement paste, Secar71, are presented (water to cement weight ratio: 0.33; duration: 0–24 h). Both the acoustic emission (AE) signals and X-rays absorption values were recorded and analysed. Information deduced from experiments clearly show that a close correlation exists between the variations of the X-ray absorption values and the AE activity during the early stage of hydration. Based on the experiments, the combined analysis (quantitative and qualitative) of both AE and CT data make it possible to propose a relationship between the experimental characteristics recorded during the tests and the different mechanisms taking place during the early hydration of a cement paste. A chronology of cement setting is also proposed.

Acoustic emission characterisation of calcium aluminate cement hydration at an early stage

Abstract In this paper, a new application of the acoustic emission (AE) technique is proposed. This non-destructive and in situ method, widely used in damage characterisation, permits us to follow the cement hydration at the early age from a few minutes to a few hours after mixing. The acoustic emission (AE) signals concerning a pure aluminous cement paste, (water to cement weight ratio=0.4; temperature of measurement: 20 °C) were recorded and analysed. A specific treatment of data deduced from a fracture process identification procedure was applied. Different filters were set on the AE signal duration based on the characteristic of amplitude distribution. From the value of AE signal amplitude, correlation with physical and chemical changes were found.. Based on the experiments, the relationship between the AE characteristics recorded during the tests and the different mechanisms, which take place during setting, was emphasised. The results showed that the new application of the AE technique can be considered as a valuable tool to study the hydration of a cement at young age.

Application of X-ray computed tomography to characterise the early hydration of calcium aluminate cement

Techniques that monitor in situ the setting and hardening of calcium aluminate cements (CACs) are of interest to the engineering community. This paper focuses on one non-destructive technique, X-ray computed tomography (CT). The early hydration of the cement can be followed from a few minutes to a few hours after mixing. This technique is based on both X-ray absorption measurements and qualitative observations in chosen axial cross-sections. Results concerning an aluminous cement, Secar 71, are presented (water-to-cement weight ratio: 0.33; duration: 0–24 h). Information deduced from these measurements clearly show that a strong variation of X-ray absorption values occurs during the early age of hydration. Both quantitative and qualitative analyses of CT data allow a chronology of cement setting to be proposed.

Application of ultrasonic testing to describe the hydration of calcium aluminate cement at the early age

Nondestructive and in situ characterisation techniques, such as ultrasonic measurements, permit to follow cement hydration at the early age from a few minutes to a few hours after mixing. The technique reported in this paper is based on measurements in reflection modes. Results concerning an aluminous cement, Secar71, are presented (water-to-cement weight ratio (W/C): 0.3 and 0.4; temperature of measurement: 20°C; duration: 0–24 h). Information deduced from ultrasonic measurements (longitudinal wave velocity, reflection coefficient) associated with other data obtained from X-ray diffraction (XRD), differential thermal analysis (DTA) and thermogravimetric (TG) measurements enable to propose a qualitative description of hydration's chronology. The sensitivity of these ultrasonic parameters to hydrates formation and structuring is underlined.

The effect of additions of alkaline-earth metal chlorides on the setting behavior of a refractory calcium aluminate cement

The literature concerned with the effect of alkaline and alkaline-earth metal chlorides on the hydration of calcium aluminate cements is confused. For example, some authors have suggested that magnesium chloride acts as a retarder, while others stated that it acts as an accelerator. Accordingly a thorough investigation has been carried out into the setting behaviour and early hydration of Secar 71, mixed with a range of concentrations of MgCl 2, CaCl 2, SrCl 2, and BaCl 2. At 12 °C all chlorides brought about retardation of the setting time of the cement, which increased with increasing concentration of the admixture. At 20 °C, at low concentrations they brought about acceleration of the set, but acted as retarders at higher concentrations. At 28 °C and 36 °C, retardation was observed and its extent increased with increasing concentration of the admixture.