Showing papers on "Ettringite published in 1995"

Highly Active Absorbent for SO2 Removal Prepared from Coal Fly Ash

Abstract: The absorbent for SO{sub 2} from the flue gas of a coal-fired electric power station was prepared from calcium oxide, calcium sulfate, and coal fly ash and examined for the relation between the desulfurization activity and the structure. The activity is closely related to the progress of the hydration reaction taking place during preparation procedures. The activity increased with the hydration time and reached a maximum activity in 12 h. The hydration resulted in the formations of ettringite and calcium silicate. By elevation of the temperature for drying the hydration products, the activity markedly increased up to 400 C, which was caused by the removal of water covering the calcium component in the ettringite. For the efficient removal of SO{sub 2}, the existence of NO in the flue gas is required. NO{sub x} plays a catalytic role for oxidation of SO{sub 2} to SO{sub 3} which reacts with CaO to form CaSO{sub 4} as a final product.

Investigation of prestressed concrete railway tie distress

Abstract: Based on petrographic, SEM, and DTA studies, damage to the ties investigated was due to delayed ettringite formation that caused microcracks radiating from localized areas of ettringite development, expansion of the paste or mortar system relative to the coarse aggregates, and macrocracks due to the internal expansion relative to the exterior surface region. Based on information available about the cement used for the majority of the ties as well as studies, the DEF was caused by the presence of large amounts of sulfate in the cement (probably in relation to the C3A content) and by the very slow solubility of much of that sulfate due to its presence in the clinker phase of the cement.

Phase development in cement in relation to the secondary ettringite problem

Abstract: Phase relationships and mass balances in the quinary system CaO—Al2O3—CaSO4—Na2O—H2O have been calculated at different temperatures and the results are applied to explain the formation of secondary...

Reduction of sulfate swell in expansive clay subgrades in the dallas district. interim report

Abstract: The addition of hydrated lime to clay soils is one of the most common methods of soil stabilization. However, when sulfates are present in the soil, the calcium in the lime reacts with the sulfates to form ettringite, an expandable mineral. This expansion causes a considerable amount of economical as well as structural problems. Sulfate related heave has been experienced along IH 45 and FM 1382. In this research, a field test method was developed to locate sulfate bearing soils. A permittivity probe was used to measure the electrical conductivity of the in situ soil. The electrical conductivity was then related to sulfate content in soils. Expansion tests were performed to determine the amount of expansion that occurs when lime is added to soils with different sulfate contents. A model that relates the amount of expansion of clay soils to electrical conductivity was proposed. The use of low calcium fly-ashes was investigated and proposed as an alternative form of stabilizer for sulfate bearing soils.

Reduction of sulfate swell in expansive clay subgrades in the Dallas district

Abstract: The addition of hydrated lime to clay soils is one of the most common methods of soil stabilization. However, when sulfates are present in the soil, the calcium in the lime reacts with the sulfates to form ettringite, an expandable mineral. This expansion causes a considerable amount of economical as well as structural problems. Sulfate related heave has been experienced along IH 45 and FM 1382. In this research, a field test method was developed to locate sulfate bearing soils. A permittivity probe was used to measure the electrical conductivity of the in situ soil. The electrical conductivity was then related to sulfate content in soils. Expansion tests were performed to determine the amount of expansion that occurs when lime is added to soils with different sulfate contents. A model that relates the amount of expansion of clay soils to electrical conductivity was proposed. The use of low calcium fly-ashes was investigated and proposed as an alternative form of stabilizer for sulfate bearing soils.

Ettringite formation in lime-remediated mine tailings. I: Thermodynamic modeling

Abstract: The optimal geochemical parameters required to form ettringite in a lime-remediated site have been investigated with the use of thermodynamic modelling. The speciation of sulphate and 10 metals analyzed in samples ofa mine tailing from northern Quebec was calculated with a computer model. The model also includes calcium, which would result from the addition of lime to the system. Two speciation calculations were done. The first one is a hypothetical speciation before the onset of precipitation and was used to devise the stability fields of minerals susceptible of forming in high pHs. The stability fields were determined by building activity ratio diagrams with sulphate, aluminum and calcium species concentrations for pHs ranging from 4 to 14. The investigation was carried out for a 25°C solution with an ionic strength less than 0.12. Thermodynamics predict that an optimal stability field for ettringite in the mine tailing samples containing 10 -3 mol/L of SO 4 -2 , 10 -2 mol/L of Al(OH) 4 - requires at least 10 -2 mol/L of Ca +2 and a pH larger than 11.6. The sulphate contents of the mine tailing samples, however, limit the precipitation of ettringite and monosulphate could be an end-precipitate. The second speciation is in equilibrium with precipitates and provides information on the speciation of the residual metals. If ettringite could be precipitated as a stable form, it could serve as a sorptive agent for residual metals, and due to its binding properties, consolidate tailings.

The formation, stability and microstructure of calcium sulpoaluminate hydrates present in hydrated cement pastes, usingin situ synchrotron energy-dispersive diffraction

Abstract: Synchrotron radiation-energy dispersive diffraction has been used for the first time to study the formation and stability of the calcium sulphoaluminate hydrates in hydrated Portland cement pastes. By using this technique it has also been possible to investigate microstructural and compositional characteristics of the ettringite (AFt) phase. The longer term slow development of the monosulphate (AFm) phase has also been monitored, although the characterized content is quite low. Differences were detected between the microstructural characteristics of the AFt phase formed in the high ferrite sulphate-resisting-type cement pastes, as compared with the equivalent phase formed from the ordinary Portland cements. These differences were especially significant at later hydration times and have been ascribed to compositional differences between the ettringite (AFt) formed from the two different types of cement.

New Techniques for Monitoring Cement Hydration Under Simulated Well Conditions

Abstract: Fourier transform infrared spectroscopy and synchrotron X-ray powder diffraction methods are described for studying cement hydration chemistry at temperatures up to 200 C, covering the normal temperature range of wellbore cementing. The methods provide complementary information on the transformation of silicate, ferrite and sulfate minerals. The thermal decomposition of the cement mineral ettringite is shown to occur at 114 C in a sealed system in contact with water. The FTIR spectrum of a well cement slurry hydrating at 150 C and 2,000 psi is analyzed. The anomalous thickening time behavior of certain cements around 75--100 C is discussed in the light of new data on the hydration of a Class G cement at 65 and 95 C, with and without retarder.

An ettringite cement based on a C-A-S glass and gypsum

Abstract: Inorganic binders conslstmg of a CaO—Al2O3—SiO2 glass and gypsum were prepared. Such cements set and harden producing ettringite as the main product of hydration. This proc;ess is not associated with an expansion of the paste. A simultaneous addition of calcium hydroxide adversely affects the strength obtained.

Calorimetric studies of special cements

Abstract: The kinetics and even the mechanism of cement reaction with water can be successfully investigated by use of microcalorimetry. In this study this method was applied to follow the hydration of the new family of portland cements containing C12A7* and C11A7·CaF2 addition as well as special cement with C3A replacement by calcium sulphoaluminate. It has been found that C11A7·CaF2 acted as hydration retarder. The heat evolution curves for C12A7 containing samples without CaF2 are very similar to those for the reference portland cement samples. XRD and SEM studies confirm the results described above, relating to the retardation of alite hydration. The process is positively modified by the addition of anhydrite. In the presence of calcium sulphoaluminate (4CaO·3Al2O3·SO3) the hydration at early stage occurs with the rapid formation of large amount of the ettringite phase. The calcium fluoride acts as a set retarder. The full compatibility of calorimetry with SEM and XRD results should be underlined.

Production of large-sized lightweight foamed concrete panel

Abstract: PURPOSE:To prevent cracking during steam curing at and under a high tap. and high pressure by adding gypsum dihydrate to a slurry so as to contain a specific rate of calcium sulfate with respect to a total solid content, thereby producing a large-sized lightweight concrete panel. CONSTITUTION:The gypsum dihydrate is added to the slurry mixture composed of a siliceous raw material, cement, calcareous raw material contg. slaked lime and/or quicklime as main raw materials and water in such a manner that the slurry contains the calcium sulfate at an internal ratio of 1 to 5wt.% of the total solid content in the process for producing the large-sized lightweight concrete panel by casting the slurry into a form arranged with reinforcing bars, curing the slurry to solidify half and subjecting the slurry to the steam curing at and under the high temp. and high pressure. Acicular ettringite crystals are formed by a hydration reaction of the cement and calcium sulfate which are the calcareous material in the half solidified and cured body. The expansion by the deficiency of the ettringite in the half solidified and cured body is suppressed and the cracking is caused if the amt. of the ettringite to be formed is below 6wt.%. The crag by decomposition and shrinkage arising from the increased formation of the ettringite arises in the half solidified and cured body when the at. exceeds 10wt.%.

Composition for fire-resistant coating

Abstract: PURPOSE:To obtain a fire-resistant coating composition further more excellent in the fire-resistant performance than those of conventional compositions. CONSTITUTION:The fire-resistant coating composition fundamentally contains ettringite as a main component. The fire-resistant coating composition contains 100 pts.wt. of the ettringite and 5-500 pts.wt. of an inorganic compound particles releasing an non-flammable gas at 100-1000 deg.C. The fire-resistant coating composition contains 100 pts.wt. of the ettringite and 0.01-50 pts.wt. of titanium oxide particles. Or the fire-resistant coating composition contains 100 pts.wt. of the ettringite, 5-500 pts.wt. of the inorganic compound particles and 0.01-50 pts.wt. of titanium oxide particles.

Treatment of incineration ash of combustible waste

Abstract: PURPOSE:To open the reutilizing way or the incineration ahs of combustible waste at a relatively low cost and to prevent the elution of heavy metals to the utmost especially with respect to the incineration ash containing harmful heavy metals. CONSTITUTION:Moisture is added to the incineration ash of combustible waste composed of org. matter or a mixture of org. and inorg. matters and an additive containing at least calcium oxide and aluminum oxide is further added thereto to be mixed with the moistened incineration ash under stirring to form ettringite by hydration reaction. This ettringite is granulated into a sand grain shape.