The hydration of monocalcium aluminate at different temperatures
01 Mar 1988-Cement and Concrete Research (Pergamon)-Vol. 18, Iss: 2, pp 311-320
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.
TL;DR: In this article, the development from conventional high cement materials, through low cement and ultra-low cement castables to the present materials which may be entirely free of CAC is discussed.
Abstract: Castable refractories containing calcium aluminate cement (CAC) are used ubiquitously in a range of furnace lining applications in the iron and steel, cement, glass, ceramic, and petrochemical industries. This review outlines their development from conventional high cement materials, through low cement and ultra-low cement castables to the present materials which may be entirely free of CAC. Castables are defined in terms of both CaO content and installation procedure. Production routes, compositions, and microstructural evolution on hydration, setting, dehydration, and firing are described for pure CACs and castable refractories. The development of the low cement systems is discussed in terms of particle packing, dispersion, and rheology highlighting the influence of colloidal matrix additions of silica and alumina. Recent developments including cement free, self-flowing, shotcreting, and basic castables are described and the potential for carbon-containing systems evaluated.
01 Jan 2003
TL;DR: In terms of the length of time it has been produced, the volume produced and the breadth of applications, calcium aluminate cements are by far the most important class of non-Portland cements.
Abstract: In terms of the length of time it has been produced, the volume produced and the breadth of applications, calcium aluminate cements are by far the most important class of non-Portland cements. Calcium aluminate cements are a relatively large family with a range of compositions which varies much more widely than Portland cement. Today the two major markets for calcium aluminate cement are in castable refractories and in dry mix mortars for special construction applications, which together account for around 80% of consumption. The use in technical concretes, for example, sewer linings, rapid repair, etc. is rather small. Calcium aluminate cements are known for their rapid strength gain, especially at low temperatures, superior durability across several categories and high temperature resistance. Their ability to consume water rapidly during hydration makes them a preferred component in building chemistry applications as this contributes to construction expediency. CACs are highly versatile materials that can be used as the full binding material, or as is more common, a component of a blended system where the contribution is based on the final desired properties.
TL;DR: In this article, the solubility of AH 3, CAH 10, C 2 AH 7.5 and C 3 AH 6 was determined experimentally at 7 to 40°C and up to 570 days.
Abstract: The solubility of AH 3 , CAH 10 , C 2 AH 7.5 , and C 3 AH 6 was determined experimentally at 7 to 40 °C and up to 570 days. During the reaction of CA, at 20 °C and above initially C 2 AH 7.5 formed which was unstable in the long-term. The solubility products calculated indicate that the solubilities of CAH 10 , C 2 AH 7.5 and C 4 AH 19 increase with temperature while the solubility of C 3 AH 6 decreases. Thus at temperatures above 20 °C, C 3 AH 6 is stable, while at lower temperature also CAH 10 and C 4 AH 19 are stable, depending on the C/A ratio. At early hydration times, CAH 10 can be stable initially at 30 °C and above, as the formation of amorphous AH 3 stabilises CAH 10 with respect to C 3 AH 6 + 2AH 3 . With time, as the solubility AH 3 decreases due to the formation of microcrystalline AH 3 , CAH 10 becomes unstable at 20 °C and above.
TL;DR: In this article, the growth kinetics of calcium aluminate crystallites from a polymeric precursor were studied during calcination as a function of temperature, using transmission electron microscopy (TEM).
Abstract: The Pechini process was used to produce high-purity, monocalcium aluminate (CaAl 2 O 4 ) powders at temperatures as low as 900 o C. Absorption spectrometry and BET measurements revealed particles with sizes ranging from submicrometer to 100 μm, with specific surface areas as high as 10 m 2 /g. Auger electron spectrometry (AES) was used to study the progressive elimination of surface carbon from the organic burnout as a function of temperature. The growth kinetics of calcium aluminate crystallites from a polymeric precursor were studied during calcination as a function of temperature, using transmission electron microscopy (TEM)
TL;DR: In this paper, the results of a study of the hydration of calcium monoaluminate and Lafarge Secar 71 (CA + CA2) using 27Al MAS and 1H-27 Al CPMAS NMR spectroscopy were presented.
Abstract: This paper presents the results of a study of the hydration of calcium monoaluminate (CA) and Lafarge Secar 71 (CA + CA2) using 27Al MAS and 1H-27 Al CPMAS NMR spectroscopy. CPMAS, high H0 magnetic field strengths (11.7 T), and high MAS spinning speeds (13–14 kHz) provide significant new information about the Al structural environments present during hydration and about the mechanisms of hydration. Important results include the following: (1) Hydrated Al(4) and Al(6) environments are present during the incubation period. This observation is consistent with previously published hydration models. (2) The overall conversion of Al(4) in unhydrated phases to Al(6) in hydrated phases and the crystalline phases detected by XRD are similar to the results of previous studies. (3) There are previously undetected hydrated reaction intermediates present during the main reaction stage. At temperatures above 25°C this phase contains Al(4), and at 4°C it contains Al(4) and possibly Al(5). The incubation period observed for this reaction may be related to the nucleation of this phase.
TL;DR: In this paper, the strength of high alumina cement (HAC) concrete is measured as a percentage of the one-day strength, which depends upon the rate, R, at which the conversion reaction takes place, and upon the water/cement ratio.
Abstract: Synopsis High alumina cement (HAC) concrete suffers from a conversion reaction in which the metastable calcium aluminate hydrate compounds change to more stable compounds. This reaction takes place at a rate which depends upon a number offactors including temperature, water/cement ratio, stress and the presence of ‘releasable’ alkalis in the aggregate. As a result of the reaction, the concrete will lose strength and the strength will reach a minimum at various times depending upon conditions of storage; thereafter there may be a small increase in strength. The strength at this minimum, S, expressed as a percentage of the one-day strength depends upon the rate, R, at which the conversion reaction takes place, and upon the water/cement ratio. An empirical relationship for laboratory-prepared specimens is S = (− 37·8 logeR + 39·5) − 100(w/c − 0·4). However, this cannot be used for unknown concretes until a reliable method of determining the original water/cement ratio of HAC concrete is available. Rapidly co...
TL;DR: In this paper, a setting time parameter has been determined from the heat evolution versus time curve measured by means of a thermocouple embedded in the cement paste as well as by the Gillmore needle.
Abstract: A setting time parameter has been determined from the heat evolution versus time curve measured by means of a thermocouple embedded in the cement paste as well as by the Gillmore needle. The setting time increases with increasing temperature, until it reaches a maximum between 26 and 30°C, for four out of the five refractory calcium aluminate cements studied. This anomalous retardation is not, therefore, restricted to ciment fondu, as reported elsewhere. The rate of consumption of CA during the first 24 hours of hydration as determined by X-ray diffraction also indicated a retardation at 30°C compared with 20°C for Secar 51 and Secar 71. The cause of the anomaly cannot be the presence of C 2 S, which is virtually absent in most of the cements studied. An alternative explanation must be found, which should also explain why Secar 80 behaves differently from the other cements investigated.
TL;DR: In this article, the authors studied the hydration of CaAl2O4 (CA) by calorimetry, analysis of the liquid phase, measurement of the combined water, and electron microscopy.
Abstract: Hydration of CaAl2O4 (CA) was studied by calorimetry, analysis of the liquid phase, measurement of the combined water, and electron microscopy. During the induction period, the solution remains almost unchanged and is equilibrated temporarily with both superficially intrusion-hydrated CA particles and Al(OH)3 gel formed by dissociation of Al(OH)4– ions, the solubility of the Al(OH)3 gel being 10–4.24 molkg–1 at 25°C, while the intrusion-hydrated layer on the CA particles grows following a nearly linear law to reach a critical thickness (∼3 nm at 10° to 20°C, or 12 nm at 30°C). At this point destruction of the layer occurs, nuclei of hydrous compounds are generated, and the induction period terminates. Subsequent reaction proceeds in accordance with the rate equation of Schiller based on the dissolution-crystallization mechanism.
TL;DR: In this article, the authors examined the conversion reaction in high alumina cement/concrete samples by scanning electron microscopy and differential thermal analysis and found that the conversion process is strongly influenced by the original water/cement ratio, both in facility with which it occurs and in the ultimate scale and distribution of the conversion products.
Abstract: The conversion reaction in high alumina cement/concrete samples is examined by scanning electron microscopy and differential thermal analysis. The transformation, which involves the dissolution of platy/acicular crystals of the metastable hydrates CAH 10 and C 2 AH 8 and the crystallisation of the lower hydrate C 3 AH 6 (in the form of faceted icositetrahedra), is seen to be strongly influenced by the original water/cement ratio, both in facility with which it occurs and in the ultimate scale and distribution of the conversion products. In commercial concrete samples it is illustrated how inevitable inhomogeneities in the water content of the cement can give rise to a very uneven and localised pattern of conversion. The observations indicate that conversion must occur by a “through solution” mechanism and the conclusion is that although the process is thermodynamically favoured by increasing temperatures of hydration, kinetically it is governed by the availability of free (liquid) water within the cement microstructure.
TL;DR: The more favourable experimental conditions of cooling and grinding were defined for calcium aluminates obtained by solid-solid reaction with C/A ratio in the range 0.96 -1.04 as mentioned in this paper.
Abstract: The more favourable experimental conditions of cooling and grinding was defined for calcium aluminates obtained by solid-solid reaction with C/A ratio in the range 0.96 – 1.04. The composition of the samples was related to the apparition of different phases : “substoichiometric” solid including CA and CA 2 , with a calcium deficiency “super-stoichiometric” samples including C 12 A 7 with an excess of calcium as well as mixtures of sub and super-stoichiometric solids could be obtained. “Super-stoichiometric” samples presented ionized anionic vacancies. During hydration, “super-stoichiometric” aluminate led to the formation of a first hydrate. The hydration kinetics depended on the ratio C 12 A 7 /CA 2 of the samples.