Showing papers on "Ettringite published in 1991"

Synthesis, Crystal Chemistry and Stability of Ettringite, A Material with Potential Applications in Hazardous Waste Immobilization

Abstract: Recent results on synthesis, crystal chemistry, structure and stability of ettringite* , with specific reference to use of this material in hazardous waste immobilization, are presented. Ettringites with selenate, chromate, borate, sulfite and carbonate replacing sulfate have been synthesized and characterized by XRD for phase purity and structure. In X-ray powder diffractograms, selenate and chromate etringites have the normal trigonal P31c ettringite structure, whereas the borate, sulfite and carbonate ettringites can be indexed on a hexagonal unit cell with a halved c parameter. In excess of 95% of the Se and B in solutions spiked with various concentrations of these elements has been removed by precipitation of ettringite. Ettringites appear to be unstable in solutions outside of the approximate pH range 11.0-12.5, and there is a potential for some oxyanion exchange between ettringites and oxyanion-containing solutions. Buffering of pH, and dissolved Ca, Al and sulfate activities by other components of ettringite-based hazardous waste forms, combined with low-reactivity C-S-H matrices, could largely mitigate these concerns.

The early hydration of limestone-filled cements

Abstract: The early hydration of limestone-filled cements has been studied as part of a programme of work examining the effects of limestone fillers on cement and concrete properties. Thermogravimetric and X-ray diffraction techniques were used to identify and determine the quantity of hydrates and calcium carbonate present at a variety of ages. In addition, electron microscopy techniques have been used to study the microstructure of the limestone- filled cements. 5% and 25% additions of limestone enhance the formation of calcium hydroxide (CH) at early ages because they provide nucleation sites for its growth. Larger amounts of ettringite are observed at early ages with increasing level of limestone addition. No monosulphate phase is observed. It has previously been reported that there is a reaction between the tricalcium monocarboaluminate (monocarbonate). Thermogravimetric analysis confirms a reduction with time in the amount of limestone present and X-ray diffraction shows a corresponding increase in the amount of monocarbonate present. Electron micrographs show that 25% limestone additions influence both the size and distribution of regions of CH. In OPC pastes small regions of CH are evenly distributed throughout the paste, while in limestone-filled pastes larger regions of CH are unevenly distributed throughout the paste. Additions of limestone fillers enhance the formation of hydration rims of calcium silicate hydrate surrounding tricalcium silicate (C3S) particles because they increase the rate of hydration of C3S. (A) For the covering abstract of the conference see IRRD 853281.

Improved characteristics in sulfate soils treated with barium compounds before lime stabilization

Abstract: Some sulfate-bearing soils stabilized with calcium hydroxide (lime) have developed heave over periods of time. This heave is thought to result from reactions of soluble sulfates, calcium hydroxide, and free aluminum in the soil or groundwater, or both, to form ettringite (3CaO.Al2O3.3CaSO4.32H2O), a highly water-expansive mineral. Laboratory testing, using the California bearing ratio (CBR) method, has indicated increased bearing strength values and decreased swell when barium hydroxide or barium chloride was added to sulfate-rich soils before lime application. A California soil containing sodium sulfate had increased strength values when either barium compound was used with lime as compared with specimens with lime only. A barium hydroxide treatment followed by lime application to a Texas soil containing sodium sulfate was successful, showing increased CBR values and a decrease in percent swell. Potential volume change tests were conducted on a Colorado soil and the California and Texas soils using lime only and lime added to soils treated with barium hydroxide or barium chloride. The barium hydroxide plus lime treatment showed a marked decrease in swell pressure when compared with lime-only treatment. The mix of barium chloride plus lime decreased in swell pressure, but not as significantly as the mix of barium hydroxide plus lime. The presence of ettringite in the treated soils was determined using scanning electron microscopy. Ettringite formation was not detected in the California or Colorado soils for either combination of barium hydroxide or barium chloride plus lime. The Texas soil contained an abundance of ettringite in the mix of barium chloride plus lime, and it was present, but sparse, in the mix of barium hydroxide plus lime.

Hydration Reactions and Etrringite Formation in Selected Cementitious Coal Conversion By-Products

Abstract: Cementitious reactions and ettringite formation, which occur when water is added to high-calcium fly ash, to many dry flue gas desulfurization solids, and to two of the residues from the emerging “clean coal” technologies, fluidized bed combustion and limestone injection multiphase burner, have been studied. The parameters varied included water to solid ratio and curing time. Crystalline reactants and products were monitored by X-ray powder diffraction. The amount of ettringite, the principal crystalline cementitious reaction product, was determined after three months of curing. Results are discussed in terms of available Ca, Al and S in each by-product and w/s ratio.

The use of gypsum mortar in historic buildings.

Abstract: Gypsum has been used as a mortar since the time of the Pyramids. It has several advantages over traditional lime including lower fuel requirements for manufacture, greater strength and more rapid setting time. Its major disadvantages is its relatively high solubility compared to lime mortar. Structures using gypsum mortar are found extensively throughout the arid Middle East, but are also found in wetter climates including France and Germany. Cement grout used in the restoration of gypsum mortar structures has caused major damage because of the reaction between the gypsum and the cementitious material to form ettringite.

The Development of Microstructure in Calcium Sulphoaluminate Expansive Cement

Abstract: Using backscattered electron images, it is shown that an expansive clinker is polymineralic in nature, with individual particles containing the constituents (lime, anhydrite and anhydrous calcium sulphoaluminate) to form ettringite on hydration. The hydration reaction with time of a simple paste of a mixture of this expansive clinker with Ordinary Portland Cement (OPC) has been monitored using backscattered electron images, and the formation of ettringite and a change in the morphology of the calcium hydroxide formed from the added free lime from the expansive clinker is observed. Thermal analysis studies on pastes of the pure expansive clinker have confirmed this apparent change in morphology.

The effect of combined additions of silica fume, calcium sulphoaluminate, and gypsum on the hydration and strength development of Ca2SiO4

Abstract: The strength development and hydration of β- and γ-C2S mixes in the presence of silica fume, calcium sulphoaluminate and gypsum is strongly accelerated due to ettringite and C—S—H formation. Calcium sulphoaluminate contributes to strength only when conditions are modified to maximize the formation of ettringite. Therefore an addition of free CaO is needed in samples with low β-C2S content. Strength in samples without C4A3S is lower because it depends on C—S—H formation. γ-C2S hydration is neglibible compared with β-C2S hydration, remaining unreacted even after more than one year in water.

Permeability properties of fly ash from furnace-sorbent-injection process

Abstract: The paper discusses tests of the applicability of furnace sorbent injection (FSI) waste solids for use as synthetic waste landfill liners by measuring the mechanical strength and permeability of moisture-cured samples. FSI waste solids were received from the EPA-sponsored demonstration of limestone injection multistage burner (LIMB) technology at Ohio Edison's Edgewater power plant. The demonstration coal was about 3% sulfur, and the sorbent used was calcium hydroxide--Ca(OH){sub 2}--modified with lignosulfonate. Samples of FSI waste were moisture-cured and analyzed for formation of ettringite, which--in cementitious systems--can cause disruption of structure if formed after the cement paste has hardened. No ettringite was detected and mechanical strength, as measured by unconfined compressive strength, increased with longer moisture curing time. Permeability of moisture-cured samples decreased with increasing moisture curing time. The increase of mechanical strength and accompanying decrease of permeability are believed to be due to the effect of the formation of bonds between the fly ash and Ca(OH){sub 2} with subsequent filling of intraparticle voids, evidenced by increasing specific surface area of FSI waste samples cured for long periods of time. Examination of morphology by SEM confirmed structure development.

Carbonation of expansive concrete and change of hydration products. in: durability of concrete. second international conference. august 4-9, 1991, montreal, canada. volume ii

Abstract: Microstructure and mechanical properties of concrete with expansive additive are reported compared with ordinary concrete. Samples of long-term concrete (22 years) were collected from an actual building built in 1967 with calcium sulfoaluminate (CSA) used as expansive additive. Hydration products were separated from these samples by using heavy media and analyzed by means of DSC, XRD and FT-IR. The morphology of the mortar portion was observed by SEM. No differences were detected on the carbonation depth and the compressive strength between CSA concrete and ordinary concrete. Qualitative analysis shows that following carbonation of concretes, C-S-H was changed to silica gel or to C-S-H with low Ca/Si ratio and decomposed to CaCO (sub 3), whereas ettringite (AFt), presents in CSA concrete, was decomposed to CaCO (sub 3), Al (OH) (sub 3) gel and gypsum. Quantitatively, hydration products in carbonated CSA concrete are larger than in carbonated ordinary concrete. Therefore, decomposition rate of AFt by carbonation is slower than that of C-S-H.

Estimating Geochemical Behavior of Cocnretes to be Placed at Yucca Mountain

Abstract: The interactions of J-13 water with cementitious materials were examined at 25°C using the geochemical code EQ3NR/EQ6. Results of the modeling suggested the following conclusions: (1) Solution chemistry for calcium-rich concretes was similar to that for silica-rich concretes. (2) Solution pH was strongly buffered by J-13 water at neutral values for small amounts of added concrete. For larger amounts of concrete the pH was dominated by the concrete and typically had values near 12. (3) If ettringite or excess portlandite are present, their decomposition opens the concrete structure. Excess silica reacts with hydrogarnet to tighten the concrete structure. (4) The permeability changes for the limited conditions discussed here are small.

Hydraulic binder based on a mixture of calcium sulphate and of a cement composition containing calcium aluminates

Abstract: Water-resistant hydraulic binder based on a mixture of calcium sulphate and of a cement composition containing calcium aluminates. The said binder contains, in effective quantity, additives capable, when it sets, of inhibiting the formation of hydrated calcium aluminates at the expense of the crystallisation of ettringite in tabular form.

Humidity regulating agent and production thereof

Abstract: PURPOSE:To obtain a humidity regulating agent for keeping an atmosphere at a desired humidity by using an ettringite-based body dehydrated by heating and sepiolite as principal components. CONSTITUTION:10-90wt.% sepiolite is mixed with 90-10wt.% ettringite-based body dehydrated by heating and the mixture is dehydrated by heating and allowed to stand in an atmosphere at a constant humidity for a certain period of time. By this treatment, the ettringite is prevented from absorbing moisture at a low humidity and the sepiolite is so moistured that moisture is released at a lower humidity. Ettringite dehydrated by heating and sepiolite may be used and mixed or sealed in the same hermetically sealed vessel. When the humidity rises remarkably, the moisture is absorbed by the moisture absorbing ability of the ettringite. When the humidity changes to such a degree that the change is due to a change of temp. close to ordinary temp., the humidity is kept constant by the moisture absorbing and releasing property of the sepiolite.

Method for improving the characteristics of sulfate bearing soils

Abstract: A method is shown for reducing the swelling action of sulfates in clay bearing soils while increasing the bearing strength values of the soils by treating soils having high sulfate content with a barium containing compound in an amount effective to react with the sulfate present in the soils, thereby forming less soluble reaction products and decreasing the tendency of the soil to form ettringite. The soil is further stabilized by the application of lime.