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Showing papers on "Ettringite published in 1993"


Journal ArticleDOI
TL;DR: In this article, the authors show that the expansion that can be associated with delayed ettringite formation is driven by processes occurring within the paste and not by the formation of ettricite at aggregate interfaces.
Abstract: Cement pastes were cured at 80°C and examined by scanning electron microscopy, X-ray microanalysis and X-ray diffraction immediately or after storage in water for various periods at 20°C. Ettringite began to form a few days after the heat treatment, as very small crystals thinly dispersed throughout the paste in close admixture with C-S-H. It subsequently recrystallized in cavities typically 5-10 μm in size formed by dissolution of clinker grains or in other ways. There were no indications that this latter process disrupted the surrounding material. The results favour the view that the expansion that can be associated with delayed ettringite formation is driven by processes occurring within the paste and not by the formation of ettringite at aggregate interfaces.

111 citations


Journal ArticleDOI
TL;DR: Ettringite was formed during the hydration of C3A with gypsum at temperatures between 25 and 80°C as mentioned in this paper, but the formation was retarded when hydration was carried out in KOH solutions whose concentrat...
Abstract: Ettringite was formed during the hydration of C3A with gypsum at temperatures between 25 and 80°C. Ettringite formation was retarded when hydration was carried out in KOH solutions whose concentrat...

102 citations


Journal ArticleDOI
TL;DR: In this article, the chemical, morphological and microstructural changes which occurred during moist curing and soaking have been determined using thermal analysis, X-ray powder diffraction analysis, and scanning and transmission electron microscopy combined with EDAX.
Abstract: The results of detailed analyses of the samples discussed in part I are reported. The chemical, morphological and microstructural changes which occurred during moist curing and soaking have been determined using thermal analysis, X-ray powder diffraction analysis, and scanning and transmission electron microscopy combined with EDAX. The analytical results together with the physical observations have shown that the period of volume instability and swelling coincides with the period of gypsum consumption and ettringite formation. However, swelling is not caused by growth of crystalline ettringite but is the result of water absorption. An osmotic mechanism is proposed in which concentration gradients are generated within a surface colloidal layer on the clay particles. Ettringite rods nucleate and grow within this layer and C-A-S-H gel is produced in the areas between the rods. This results in concentration gradients at the clay particle surfaces which provide the driving force for osmosis. The implications of these findings are discussed in relation to lime- stabilization of soils.

75 citations



Journal ArticleDOI
TL;DR: The simultaneous presence of alkali and ettringite in concrete has been reported by several authors as mentioned in this paper, however, the proposed interpretations of this phenomenon, and its relationship to concret...
Abstract: Simultaneous presence of alkali—silica gel and ettringite in concrete has been reported by several authors. However, the proposed interpretations of this phenomenon, and its relationship to concret...

67 citations


Patent
10 Sep 1993
TL;DR: In this article, a fly ash may be activated with strong alkali to yield a novel, dense, quick-setting cement having unexpected properties, including high amounts of the plate-like crystalline phase and a dense matrix, due to higher reactivity of the glassy phase in fly ash.
Abstract: Class C fly ash may be activated with strong alkali to yield a novel, dense, quick-setting cement having unexpected properties. The setting time of the cement decreased as the pH of the activating solution increased. The microstructure and phase assemblage of the novel cement depended on the pH. Ettringite was absent beyond pH 14.30 (2.0N); a hexagonal plate-like crystalline form (stratlingite (gehlenite hydrate, C 2 ASH 8 ) and other compounds) became more abundant at higher pH. At higher pH the microstructure was characterized by high amounts of the plate-like crystalline phase and a dense matrix, due to higher reactivity of the glassy phase in fly ash. The novel cement will have uses in a number of areas, including fixation of hazardous wastes such as radioactive wastes, applications where rapid setting is desired, and formation of concrete in hot environments where ordinary Portland cements may tend to crack due to their heat of hydration.

55 citations


Journal ArticleDOI
TL;DR: In this article, the authors compared hydration reactions of C3A and C4AF with calcium sulfate hemihydrate and gypsum and the kinetics of the reactions compared.
Abstract: Hydration reactions of C3A and C4AF with calcium sulfate hemihydrate and gypsum were investigated and the kinetics of the reactions compared. The rates of C3A and C4AF hydration, as determined by heat evolution, vary depending on whether the sulfate-containing reactant is gypsum or calcium sulfate hemihydrate. The following sequence of reactions involving C4AF occurs when hemihydrate is the reactant: gypsum formation during the first hour, ettringite formation between 20 and 36 hours, and the conversion of ettringite to monosulfate over a period of about 12 hours. Monosulfate formation initiates prior to the complete consumption of gypsum. The onset of this conversion occurs at a shorter hydration time when hemihydrate is a reactant and the total amount of heat evolved is lower. The hydration reactions in saturated calcium hydroxide solution occur more slowly than those in water. Based on heat liberation, C4AF reacts at a much higher rate than C3A. Ettringite formation occurs during the first 8 to 9 days of C3A hydration. Once the gypsum is consumed, ettringite converts to monosulfate during two additional days. Compared to gypsum, hemihydrate decreases the rates of hydration of both C3A and C4AF. The effects on the hydration characteristics of C4AF are significant. The hydration of C3A with gypsum in water, in saturated Ca(OH)2 solution, and in 0.3M NaOH solution were compared. Heat evolution is the lowest for hydration in 0.3M NaOH. The onset of monosulfate formation occurs prior to the complete reaction between gypsum and C3A in the NaOH solution.

48 citations


Journal ArticleDOI
TL;DR: In this article, the formation of ettringite was investigated by isothermal calorimetry over the range of temperature from 50 to 70°C using three sources of alumina: tricalcium aluminate, high alumina cement and calcium aluminate sulfate.

33 citations


Journal ArticleDOI
TL;DR: In this article, secondary ettringite formation within concrete mixtures seems to be limited by density of concrete structure the properties of which are influenced by W/C ratio and heat treatment maximum temperatures.

27 citations


Journal ArticleDOI
TL;DR: In this paper, the early hydration of minepack mixtures forming ettringite is studied, where the growth can be perceived over steps as small as 20 seconds, as well as shifts in diffraction peak positions which are interpreted in terms of a time-dependent composition of the ettric phase.

21 citations


Journal ArticleDOI
TL;DR: In this paper, the kinetics of hydration processes of tricalcium aluminate and monocalcium alumininate in the presence of calcium sulphates and calcium hydroxide were studied at room temperature by differential calorimetry and X-ray diffraction analysis.

Journal ArticleDOI
TL;DR: In this paper, the second part of a two-part paper on the hydration characteristics of Type K expansive cement pastes and the effect of silica fume addition is presented.


Journal ArticleDOI
TL;DR: In this paper, the initial hydration of quick cements was investigated with the help of X-ray diffraction and scanning electron microscopy as well as via the determination of the chemically bound water.

ReportDOI
01 Apr 1993
TL;DR: In the case of NACL, this problem has been solved by saturating the mixing water with sodium chloride as mentioned in this paper, which results in teracalcium aluminate dichloride-10 hydrate (Chloroaluminate).
Abstract: : Throughout the 1980's expansive behavior was one of the principal design criteria for grouts and concretes to be used in underground nuclear-waste repository sealing. The purpose of this requirement was to make a material that would fit tightly into the host-rock cavity that it is inducing the formation of tetracalcium aluminate trisulfate-32-hydrate (Ettringite) from the reaction between the calcium aluminates or anhydrous calcium sulfoaluminates in the cementitious materials and the calcium sulfates added to the system. This is known to cause expansive behavior under certain conditions (Poole and Walkeley 1992). Another design criterion has been that mixtures cannot cause dissolution of the host rock. this is not normally a problem with most host rocks, but can be a substantial problem when placing concrete in rock composed of water-soluble minerals. In the case of rocks composed of halite (NACL), this problem has been solved by saturating the mixing water with sodium chloride. As a result, teracalcium aluminate dichloride-10 hydrate (Chloroaluminate) forms from the calcium aluminates in the cementious materials. This phase is mildly expansive. So, in hydrated systems, at least two forms of hydrated calcium aluminates may coexists, or one may form preferentially, or one may replace the other, depending on the chemical environment.