Showing papers on "Ettringite published in 1994"
TL;DR: In this paper, the formation and expansion of ettringite crystals are discussed on the basis of principles of physico-chemistry, and it is suggested that eittringite forms by a through-solution mechanism and that expansive force results from crystallization pressure as well as swelling pressure converted from the reduction of chemical potentials.
156 citations
TL;DR: In this paper, the authors determined the solubility product of ettringite under equilibrium conditions and examined the data used in predictive thermodynamic models, and calculated activity coefficients using specific ion interaction approach.
101 citations
TL;DR: In this paper, the distribution curves for ettringite, gypsum, and portlandite phases were obtained by using layer by layer XRD analysis and interpreted in terms of material damage due to sulfate attack.
92 citations
TL;DR: In this article, the authors studied the kinetic law of ettringite decomposition by CO2, providing the theoretical basis to retard the ettinite carbonation, and established the carbonation reaction reaction kinetic equation.
65 citations
TL;DR: In this article, the influence of C3A addition on sulphate desorption was investigated in hydrating systems containing C-S-H gel and gypsum with respect to delayed ettringite formation.
53 citations
TL;DR: In this article, a sulfate-resisting Portland cement and pastes made from it were studied by X-ray microanalysis of polished sections in the scanning electron microscope, Xray diffraction and thermal analysis.
53 citations
TL;DR: In this paper, a study intended to develop a blend of gypsum and Portland cement, which would possess the advantages of both materials, such as early strength and enhanced workability, but would be free of the deleterious effect of ettringite.
Abstract: The present work describes a study intended to develop a blend of gypsum and Portland cement that would possess the advantages of gypsum (high early strength and enhanced workability) and Portland cement (improved durability in moist conditions), but would be free of the deleterious effect of ettringite, which is formed when gypsum and Portland cement interact. This was achieved by preparing a blend of 75% gypsum with a 25% mixture of Portland cement and silica fume. An optimum silica fume—Portland cement ratio of 0·25-0.66 provided a system with strength levels twice as high as those of pure gypsum. The system also exhibited a high wet/dry strength ratio of ∼60 % after 200 days of water immersion. This improvement was explained by the reduction in ettringite formation and the development of a microstructure in which gypsum crystals were engulfed by CSH.
44 citations
TL;DR: The interaction between airborne sulfur compounds and mortars has been investigated in this paper, where simulated experiments were carried out in a flow chamber where temperature, relative humidity and SO2 concentration were controlled.
Abstract: The interaction between airborne sulfur compounds and mortars has been investigated. Simulation experiments were carried out in a flow chamber where temperature, relative humidity and SO2 concentration were controlled. Samples of lime, pozzolan and cement mortars were exposed for 90 days in air with 3 ppm of SO2 concentration, at 25°C temperature and 100% relative humidity. Following exposure, the mortar samples were studied by x-ray diffraction, infrared spectroscopy and high pressure liquid chromatography. The data obtained from bulk and surface analyses clearly show that the materials reacted with sulfur dioxide to form calcium sulfite hemi-hydrate and calcium sulfate dihydrate. The quantity of sulfite and sulfate measured depends on the physical and chemical properties of the three mortars, but is independent of their CaCO3 content. Formation of ettringite and thaumasite did not occur in the 90 day period taken into account in the present work.
37 citations
01 Jan 1994
TL;DR: In this paper, chemical and microstructural aspects of delayed ettringite formation are reviewed, and it is shown that the rate and ultimate extent of expansion are influenced by factors of three types: chemistry, paste microstructure, which determines the stresses produced by its formation.
Abstract: Abstract Delayed ettringite formation (DEF) can damage concrete that has experienced a temperature above about 70°C. Claims that slow release of sulfate from the clinker can have a similar effect in concrete not thus heated are unsupported. Chemical and microstructural aspects of DEF are reviewed. Expansion results from formation of ettringite crystals of submicrometre size in the paste, the larger crystals readily observed in cracks and voids being recrystallisation products. The rate and ultimate extent of expansion are influenced by factors of three types: chemistry, which determines how much ettringite can be formed; paste microstructure, which determines the stresses produced by its formation; and concrete or mortar microstructure, which determines the response of the material to those stresses. Alkali present before the end of the heat treatment can increase expansion, but when present subsequently, it decreases expansion by inhibiting ettringite formation. Leaching therefore promotes expansion.
34 citations
TL;DR: In this paper, the effect of elevated temperature curing on mortars and the phenomenon of delayed ettringite formation (DEF) was investigated, and it was shown that the ultimate levels of ettricite reached do not correspond to the magnitude of expansion observed.
Abstract: This paper reports some preliminary results from a study of the effect of elevated temperature curing on mortars and the phenomenon of delayed ettringite formation (DEF). Mortars made from cements with sulphate levels of 3%, 4%, and 5% and with 5% sulphate and added alkali were cured at 20 and 90° C and subsequently stored in water. Expansion measurements showed a pessimum effect with increasing S03 content. Mortars which expanded showed a corresponding decrease in strength. X-ray diffraction (XRD) studies indicated that no ettringite is present after heat treatment but re-forms over time within the material. However, the ultimate levels of ettringite reached do not correspond to the magnitude of expansion observed. X-ray microanalysis shows that immediately after the heat treatment the aluminate species and most of the sulphate species are incorporated within the C-S-H gel. The concentrations of these species decrease during expansion, such that at the end of expansion the amounts remaining correspond to the presence of AFm phase mixed with C-S-H.
TL;DR: More than 200 individual particles within a portland scement paste were morphologically categorized by backscatter SEM and analyzed systematically by EDXA. Nearly all of the analyses fall either in a dense cluster around a mean C-S-H gel composition or along tie lines connecting this composition to theoretical compositions for calcium hydroxide, monosulfate, ettringite, and ferrite as mentioned in this paper.
Abstract: More than 200 individual particles within a portland scement paste were morphologically categorized by backscatter SEM and analyzed systematically by EDXA. Nearly all of the analyses fall either in a dense cluster around a mean C-S-H gel composition or along tie lines connecting this composition to theoretical compositions for calcium hydroxide, monosulfate, ettringite, and ferrite. “Phenograin” C-S-H gel (formed as a direct replacement product within large cement grains) contains small amounts of Al, Fe, and S, but otherwise shows almost negligible compositional variation. In contrast, smaller groundmass particles and shells of gapped phenograins show a slightly differentc S-H gel composition. Some of these, despite being morphologically homogeneous, show compositions falling along tie lines to the other recognized phases. This is attributed to extensive occurrence of intimate mixtures of C-S-H gel with the other components below the scale of SEM observation. Ionic substitutions appear to be minor except for the monosulfate and ettringite components in such mixtures. These show a variable but usually high deficiency in the sulfate content.
TL;DR: In this article, surface chemistry theories of crystal nucleation in pre-cracked material were examined and it was deduced that nucleation first occurs in cracks satisfying a minimum free energy condition.
TL;DR: In this article, the stability of Supersulphated Cement (SSC) was investigated at 95°C when subjected to relative humidities of 100, 53 and 11% of water vapour.
Abstract: The stability of Supersulphated Cement (SSC) is investigated at 95°C when subjected to relative humidities of 100, 53 and 11% of water vapour. Previously [1] investigations at 25, 50, 75°C under the same conditions of humidity reported the stability of ettringite, one of the initial hydration products. At 95°C, decomposition of ettringite, is found at all humidities and is rapid at 100% relative humidity. The hydration products of cement pastes at a water cement ratio of 0.27 were determined by thermogravimetry (TG) and X-ray diffraction (XRD). The formation of the hydragarnet, plazolite is recorded during the decomposition/dehydration process enhanced by possible carbonation. Rehydration studies on the products after storage for up to 9 months were carried out using distilled water and the samples tested for ettringite content. It is concluded that ettringite in SSC is inherently unstable at 95°C.
TL;DR: The influence of mineral admixtures on the properties of expansive cement mortars was studied in this article, where fly ash, condensed silica fume, and rice hull ash were used as partial replacements (15% by wt.) for ASTM Type K expensive cement in mortars with a water to cementitious materials ratio of 0.5.
TL;DR: In this paper, the authors examined carbonation reactions in portland cement grout examined in the laboratory suggest high attenuation of 14C in cementitious barriers for low and intermediate-level radioactive waste repositories.
TL;DR: In this paper, the influence of the Ca-chemistry of leachates from alkaline waste materials, such as MSWI bottom ash, on contaminant leaching is discussed.
Abstract: This study focuses on modelling the leaching of the major element calcium from fresh MSWI bottom ash leachates. It is emphasized that Ca-minerals in bottom ash mainly control leachate pH and hence, exert a strong influence on the leaching of contaminant metals and oxyanions from these waste materials. The geochemical speciation code MINTEQA2 was used to model the dissolved Ca concentration in leachates collected from carefully controlled batch laboratory experiments with fresh bottom ash covering a range of liquid/solid ratios and pH. It is shown that ettringite (Ca6Al2(SO4)3(OH)12·26H2O) and gypsum (CaSO4·2H2O) control Ca-leaching from fresh bottom ash at pH 10–12, and pH < 10, respectively. Ca-availability at low pH appears, therefore, to be controlled by the dissolution of gypsum. It is hypothesized that the coexistence of the minerals ettringite, gypsum, and gibbsite (Al(OH)3) controls the pH of fresh bottom ash at a value of approximately 10.2. The influence of the Ca-chemistry of leachates from alkaline waste materials, such as MSWI bottom ash, on contaminant leaching is discussed.
TL;DR: In this article, the authors studied the effect of pre-curing temperature on the strength of preformed building elements, and found that the most important effect on strength is due to the pre curing temperature, while the postcuring time has much less effect.
Abstract: Mixtures made from materials containing reactive oxides of calcium, aluminium, sulphur and silicon which are able, upon hydration, to generate calcium trisulphoaluminate and silicate hydrates, have been studied with a view to possible applications in the field of preformed building elements. The materials employed were natural gypsum, phosphogypsum, fly ash, blast furnace slag, Portland cement and hydrated lime. The samples were cured at 55, 70 and 85°C and 100% RH for 24 h, followed by further curing at ordinary temperatures and humidities (21°C, 67% RH) for up to 28 days. The cured samples were tested for compressive strength and shrinkage. It has been found that the most important effect on the strength is due to the pre-curing temperature, while the post-curing time has much less effect. In general, the optimum pre-curing temperature is 70°C. Satisfactory strength results were obtained even with systems containing up to 80% waste materials.
Patent•
25 Oct 1994TL;DR: In this article, a process for treatment of hazardous liquid waste comprising trace amounts of hazardous elements in solution as oxyanions by oxyanion fixation within ettringite and related minerals is described.
Abstract: A process for treatment of hazardous liquid waste comprising trace amounts of hazardous elements in solution as oxyanions by oxyanion fixation within ettringite and related minerals. In accordance with the disclosed process, reagents for forming ettringites are mixed with the waste stream resulting in the formation of oxyanion-substituted ettringite and related materials. The resulting ettringite and related minerals are separated by filtration from the liquid. Thereafter, the liquid, having an elevated pH, is neutralized by carbon dioxide sparging resulting in precipitation of excess reagents in the liquid. Thereafter, the precipitates are filtered, producing a clean liquid.
TL;DR: In this article, it was found that the preparation of an unpolished or polished sawn concrete surface by carbon deposition (for SEM work) allows alkali-silica gel reaction and DELAYED Ettringite formation (DEF) to be distinguished by the nuked eye.
Abstract: Alkali-silica gels or formations of late-developing Sample preparation and visual ettringite (delayed ettringite formation (DEF)) that fill examination sockets around aggregate particles may sometimes be observed by an optical microscope on unpolished or polished sawn concrete surfaces only as a ‘gel’ or as dark circles around the aggregate particles. They may thus be indistinguishable from each other. It has been found that the preparation of an unpolished or polished sawn concrete surface by carbon deposition (for SEM work) allows alkali-silica gel reaction and DEF to be distinguished by the nuked eye. However, X-ray elemental analysis recommended to confirm the visual observation.
TL;DR: In this paper, a CaO-Al2O3-SiO2 glass as Al3+ donor with CaSO4·2H2O, Ca(OH) and water was used to create hardened pastes.
Abstract: Cement pastes were prepared by mixing C3A, CA, C4A3S, C4AF or a CaO-Al2O3-SiO2 glass as Al3+ donor with CaSO4·2H2O, Ca(OH) and water. After curing in completely closed moulds to prevent expansion, the hardened pastes exhibited strength properties comparable with those of other inorganic cements. Ettringite (AFt phase) was formed as the main product of hydration.
TL;DR: In this article, two series of mixtures containing up to 90% waste materials have been tested to assess their potentialities for the manufacture of ettringite-based building materials.
Abstract: Two series of mixtures containing up to 90% waste materials have been tested to assess their potentialities for the manufacture of ettringite-based building materials. One series of mixtures is useful for the manufacture of preformed building elements by hydrothermal reaction between a chemical gypsum and reactive oxides or hydroxides of calcium and aluminium. Curing temperatures in the range 55–85°C have proved to give adequate mechanical properties. The other series of mixtures has been used to produce a clinker by firing at about 1200°C. The resulting binder contains calcium sulfoaluminate and sulfosilicate as well as anhydrite. Upon hydration it gives mechanical strengths well above those of ordinary portland cements. The wastes used in the present work are phosphogypsum, fly ash and blastfurnace slag.
TL;DR: In this paper, the mechanizm of soil stabilization by the formation of ettringite was investigated based on the results of a series of exerimental studies, and it has become clear that the mechanization depends on the physical and chemical activities in process of ETTringite formation in soil, which are mainly the decrease of water content caused by the fixation of pore water in soil as water of cristallization and the increase of the effective stress given to the soil particles by its formation.
Abstract: The mechanizm of soil stabilization by the formation of ettringite was investigated in this study. Based on the results of a series of exerimental studies, it has become clear that the mechanizm depends on the physical and chemical activities in process of ettringite formation in soil, which are mainly the decrease of water content caused by the fixation a large quantity of pore water in soil as water of cristallization and the increase of the effective stress given to the soil particles by its formation. The components necessary to form ettringite are widely available. Especially it makes economical to use some kinds of industrial by-products contained the active components for this reaction.
TL;DR: The phase composition and microstructure of samples of slag from a bench-scale (Florida Institute for Phosphate Research [FIPR]) and a pilot scale (campaign 5) run of the process were characterized as mentioned in this paper.
Abstract: A sulfur-recovery process from phosphogypsum produces a by-product slag that can potentially be used as aggregate in portland cement concrete. The phase composition and microstructure of samples of slag from a bench-scale (Florida Institute for Phosphate Research [FIPR]) and a pilot-scale (campaign 5) run of the process were characterized. Both slags have a frothy appearance. The FIPR slag consists of elongate, and often radiating crystals of gehlenite (Ca\d2Al(Si,Al)O\d7), enclosing larnite (β-Ca\d2SiO\d4). Some amounts of iron sulfide and magnetite are also present. Campaign 5 slag does not contain gehlenite since quartz is present. Its microstructure is also different, consisting of an intimate, heterogeneous mixture of calcium silicates and opaque phases. Residual gypsum is present in both slags but in greater amounts in campaign 5; the latter also contains some ettringite and native sulfur. The FIPR bench-scale run produced a completely burned residue while the pilot-scale run residue was underburned. However, mineralogically, both are suitable as aggregate in concrete as long as the amount of sulfate phases is not too high (< 4.5%).
01 Jan 1994
TL;DR: A quantity of expansive ettringite is formed in the presence of gypsum as discussed by the authors, which makes porosity of paste increase and strength decrease, and very high strength is obtained.
Abstract: microcrystal precipitates and very high strength is ob-tained.A quantity of expansive ettringite is formed in the pres-ence of gypsum.That makes porosity of paste increase and strength decrease.
TL;DR: In this article, by-products from two advanced coal technologies, Fluidized Bed Combustion (FBC) and Limestone Injection Multistage Burner (LIMB), were found to be cementitious when mixed with water and compacted.
Abstract: By-products from two advanced coal technologies, Fluidized Bed Combustion (FBC) and Limestone Injection Multistage Burner (LIMB), were found to be cementitious when mixed with water and compacted. However, exposure to natural conditions in test cells resulted in losses of strength and increases in permeability over a period of years. Changes in mineralogy and microstructure with time in recovered core samples have been characterized by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). Up to 80 wt% of the core materials had converted to crystalline hydrate phases by the end of two years. Ettringite, gypsum and portlandite were the initial hydration products. In the LIMB materials, formation of thaumasite in microfractures, channels and voids was detected after one year, and in the FBC material after two years. Thaumasite formation was accompanied by reductions in gypsum and portlandite; it did not appear to be forming at the expense of ettringite. EDS examination of many ettringite and thaumasite crystals showed that the former always contained some Si and the latter some Al, which is evidence for ettringite-thaumasite solid solution. Thaumasite formation accompanied marked losses in strength and increases in permeability.
Patent•
04 Oct 1994
TL;DR: In this paper, a method for producing a high-strength hardened cement material exhibiting a low shrinkage by using a high strength cement composition, especially a low-shrinkage and high strength Cement composition remarkably reduced in shrinkage factor during hardening process.
Abstract: PURPOSE:To provide a method for producing a high-strength hardened cement material exhibiting a low shrinkage by using a high-strength cement composition, especially a low-shrinkage and high-strength cement composition remarkably reduced in shrinkage factor during hardening process. CONSTITUTION:An alumina cement matter containing alumina cement, a superfine powder and a dispersant is kneaded with a hardening controlling agent and water. The resultant kneaded material is blended and kneaded with an ettringite product matter containing an ettringite product to form a hardened cement. The formed material is cured, thus producing the objective high-strength hardened cement material. This production method enables remarkable reduction of shrinkage during hardening process and production of a high-strength hardened cement material exhibiting >=300kgf/cm bending strength after drying at 110 deg.C for one day.