Thermal analysis of hydrated calcium aluminates
01 Sep 1996-Journal of Thermal Analysis and Calorimetry (Kluwer Academic Publishers)-Vol. 47, Iss: 3, pp 765-774
TL;DR: Differential scanning calorimeter has been used to study the dehydration characteristics of hydrated calcium aluminates such as CA, CA2 and C12A7 where C and A stand for CaO and Al2O3 respectively.
Abstract: Differential scanning calorimeter (DSC) has been used to study the dehydration characteristics of hydrated calcium aluminates such as CA, CA2 and C12A7 where C and A stand for CaO and Al2O3 respectively. Dehydration of CAH10 and C2AH8 (whereH=H2O) occur ∼ at 160–180°C and 200–280°C respectively. These two phases are unstable and ultimately get transformed to AH3 and C3AH6. Dehydration of AH3 and C3AH6 occur between 290 and 350°C and overlap at lower scanning rate. The activation energy for dehydration of the stable AH3 and C2AH6 phases has been found to be 107.16 and 35.58 kJ mol−1 respectively. The compressive strength of the hydrated calcium aluminates has been determined. The result shows that in the case of CA, almost 90% of ultimate strength has been attained in 1 day whereas in CA2, ultimate strength has been attained in 14 days and in C12A7 in 1 day. DSC results have been correlated with the rate of strength developments.
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TL;DR: In this article, the hydration stages and phase transformation mechanism of the Sr2+-doped calcium zirconium aluminate cement at room temperature were studied. And the authors found that the partial isovalent substitution of Ca2+ for Sr2+, such as Ca7−xSrxZrAl6O18, reduced the hydraulic reactivity.
Abstract: This paper studies hydration stages and phase transformation mechanism of the Sr2+-doped calcium zirconium aluminate cement at room temperature. Features of different hydration stages of this cement paste are identified by X-ray diffraction, scanning electron microscopy, differential thermal analysis, thermogravimetric analysis, evolved gas analysis and heat evolution test. It was found that the partial isovalent substitution of Ca2+ for Sr2+ such as Ca7−xSrxZrAl6O18, with x = 0.3, 0.6, 1.0, reduced the hydraulic reactivity of Ca7ZrAl6O18 phase. The course of hydration of mixed oxides of the type 6CaO·SrO·3Al2O3·ZrO2 documented using microcalorimetry was supported by investigations of the solid hydration products. Research showed that the hexagonal hydrates were stable at early and middle curing ages, an indication that including Sr in solid solution could effectively inhibit the conversion from both C4AH13–19 and C2AH8 to C3AH6 and AH3. Calcium monocarboaluminate $${\text{C}}_{4} {\text{A}\bar{\text{C}}\text{H}}_{11}$$
phases were also found in this system due to the carbonation process of C4AH13 phase. Substitution of Ca2+ ions by Sr2+ ions in hexagonal calcium aluminate hydrates causes structural disorder and then contributes to the broadening of lines in the powder X-ray diffraction patterns of layered structures of metastable hydrates. At later curing ages, the formation of two hydrogarnet phases, one Sr-rich (C,Sr)3AH6 and the other Ca-rich (C,Sr)3AH6, was proved.
11 citations
TL;DR: In this article, the influence of precursors (CA, C7A3Z, C12A7, C6SrA3z, C 7A2FZ and commercial calcium aluminate cement 'Gorkal 70') on the structure, microstructure and thermal stability characterizations of C3AH6 through hydration was investigated.
Abstract: In this paper, the influence of precursors (CA, C7A3Z, C12A7, C6SrA3Z, C7A2FZ and commercial calcium aluminate cement ‘Gorkal 70’) on the structure, microstructure and thermal stability characterizations of C3AH6 through hydration was investigated. The materials were characterized by X-ray diffraction, differential thermal analysis–thermogravimetric analysis–evolved gas analysis and scanning electron microscopy combined with energy-dispersive spectroscopy 24 h and 72 h after starting the hydration process. Results of investigation confirmed the influence of precursor on shape and grain size of C3AH6. The CaO/Al2O3 mass ratio of precursors before the hydration process affects the size of C3AH6 crystals: the higher the CaO/Al2O3 value, the larger the size of the crystals of C3AH6. Moreover, the presence of Sr and Fe affects the formation of stable C3AH6 crystals.
10 citations
TL;DR: The conditions for the sorption and desorption of the guests are decisive for the properties of the inclusion compounds as mentioned in this paper, and they are surveyed for many types of inclusion compounds.
Abstract: The guests present in inclusion compounds participate in bonding with the hosts. The conditions for the sorption and desorption of the guests are decisive for the properties of the inclusion compounds. They are surveyed for many types of inclusion compounds.
10 citations
TL;DR: In this paper, the main hydrates in ordinary Portland cement paste (C-S-H, portlandite, aluminates and sulfo-aluminates) were modelled separately and then combined to form the model of the cement paste.
Abstract: The aim of this study is to establish a dehydration model that is able to calculate the water release of cement paste at elevated temperature. The main hydrates in ordinary Portland cement paste (C–S–H, portlandite, aluminates and sulfo-aluminates) were modelled separately and then combined to form the model of the cement paste. In this way, the model is able to predict the dehydration of cement pastes with different compositions without re-fitting. For each hydrate,
the dehydration law takes account not only of the maximum quantity of water that can be released at each temperature but also of the kinetics to reach that equilibrium. The model is thus able to reproduce the paste dehydration for any heating rate. Thermogravimetric tests were carried out with different heating rates to validate the model in comparison to experimental results from the literature. The results demonstrated the capability of the model in different situations.
8 citations
TL;DR: In this article, the design, synthesis and hydraulic activity of novel Sr2+-doped Ca7ZrAl6O18 cement in respect of potential applications as: matrix used in nuclear waste immobilisation, components of X-rays and gamma radiation shielding concretes or utilized in refractories technology.
Abstract: This paper describes the design, synthesis and hydraulic activity of novel Sr2+-doped Ca7ZrAl6O18 cement in respect of potential applications as: matrix used in nuclear waste immobilisation, components of X-rays and gamma radiation shielding concretes or utilized in refractories technology. The obtained material was characterized by X-ray diffraction (XRD), scanning electron microscopy combined with energy-dispersive spectrometry (SEM/EDS) and heat flow calorimetry techniques. It was well established that the addition of Sr2+ cations to form solid solution with Ca7ZrAl6O18 can alter the hydraulic behaviour of this phase. A microcalorimetric study (heat flow calorimetry technique) of Sr2+-doped Ca7ZrAl6O18 cement hardening was combined with the phase characterisation by XRD and SEM/EDS. The thermal stability and mechanism dehydration formed hydrates were assessed by simultaneous thermal analysis i.e. thermogravimetry (TG) and differential thermal analysis (DTA) at different hydration times. The detailed scheme of thermal decomposition of the hydrated Sr2+-doped Ca7ZrAl6O18 cement has been clarified based on evolved-gas-analysis data with a quadrupole mass spectrometer (QMS). Two hydrated phases C4AH19 and C2AH8 persisted at the early stage of hydration, whilst recrystallisation to Ca-rich C3AH6 was inhibited. Hydration of Sr2+-doped Ca7ZrAl6O18 resulted in the formation of two hydrogarnet phases, one Sr-rich and the other Ca-rich.
6 citations
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10,842 citations
TL;DR: The chemistry of hydration of monocalcium aluminate, CA, has been studied at several temperatures using conduction calorimetry, X-ray diffraction and other techniques as discussed by the authors.
Abstract: The chemistry of hydration of monocalcium aluminate, CA, has been studied at several temperatures using conduction calorimetry, X-ray diffraction and other techniques. At 4 °C, hydration to the decahydrate CAH 10 occurs about 15 hours after mixing; this hydration time increases with increasing temperature up to 30 °C, and C 2 AH 8 appears as a hydration product. At 40 °C rapid hydration to C 2 AH 8 is followed over a period of weeks by the ‘conversion’ reaction producing C 3 AH 6 . The reaction of CA to form crystalline hydrates was monitored by X-ray diffraction analysis; the results indicate that hydration also produces significant amounts of noncrystalline material. The enthalpies of the reactions involved in hydration and ‘conversion’ were measured by conduction calorimetry.
63 citations
TL;DR: In this article, the authors studied the hydration behavior of Secar 71 refractory aluminous cement using conduction calorimetry, X-ray diffraction and thermal analysis techniques.
Abstract: The hydration behaviour of Secar 71 refractory aluminous cement has been studied over a range of temperature using conduction calorimetry, X-ray diffraction and thermal analysis techniques. The first reaction is the hydration of monocalcium aluminate, CA, producing a sharp calorimeter peak within 12 hours of mixing. Following this reaction the other major component, calcium dialuminate (CA 2 ), undergoes slow hydration over a period of weeks. The enthalpies of these two reactions have been measured. The initial hydration products of both components are CAH 10 , or C 2 AH 8 with alumina gel. These can subsequently convert to C 3 AH 6 ; after one month significant conversion is observed even at room temperature. A method of estimating the degree of conversion based on X-ray measurement of C 3 AH 6 is proposed.
46 citations
TL;DR: Differential scanning calorimetry (dsc) was used to investigate the freezing behaviour of hardened cement paste (hcp) subjected to nacl and cacl2 solutions containing up to 5.4 mol cl/l as mentioned in this paper.
Abstract: Differential scanning calorimetry (dsc) was used to investigate the freezing behaviour (-175 deg c to 20 deg C) of hardened cement paste (hcp) subjected to nacl and cacl2 solutions containing up to 5.4 mol cl/l. Thermograms were also recorded for samples dried to relative humidities (rh) between 61% and 96%. The low-temperature phase transition at -38 deg c exhibited the freezing behaviour of an aqueous solution in gel pores of approximately equal to 4 nm radius. Chloride ions strongly reduce the coupling forces between gel particles and structure the gel pore water. On desorption critical behaviour of the low-temperature transition energy was observed at 0.52 mol cl/l which is the concentration most damaging to hcp. Above 1.3 mol cl/l freezing of supercooled bulk water in macropores was observed at more than one temperature. (Author/TRRL)
31 citations
TL;DR: In this paper, the hydration process of a high alumina cement was investigated through ultrasonic wave propagation techniques, and a correlation between wave amplitude and velocity and breaking strength of the cement was demonstrated.
Abstract: The hydration process of a high alumina cement was investigated through ultrasonic wave propagation techniques A correlation between wave amplitude and velocity and breaking strength of the cement is demonstrated The wave behaviour through hydrating cements was found to be a function of the cement-to-water ratio between values of 2 and 4 Changes in the temperature of the hydrating samples were found to exhibit a similar dependence
6 citations