Showing papers on "Calcium oxide published in 1996"

Sulfate-Bearing Soils: Problems with Calcium-Based Stabilizers

Abstract: Several roads, airfield pavements, and parking lots in Texas and other states in the western United States have suffered severe pavement damage due to expansive minerals formed from the reactions of calcium-based materials used to stabilize sulfate-bearing soils. Remediation costs for projects that suffer sulfate-induced heave damage are very high, because often the entire pavement may have to be removed and reconstructed. Observations from several projects are described to illustrate the phenomenon of sulfate-induced heave and the current methods to predict the problem. Two recent projects described include one using cement as a replacement for lime and a second using a double application of lime. The discussion also includes limitations of the present methods for determining the so-called soluble sulfate levels in soils. The practice of a double application of lime and several other alternative methods and their limitations are discussed. Although research has clearly identified the expansive minerals as being calcium bearing, no published investigations of non-calcium-based stabilizers that could effectively stabilize sulfate-bearing soils were found.

Reaction products formed in lime-stabilized marine clays

Abstract: The use of lime to improve the behavior of marine clays has been in practice in several parts of the world over a long period. In this paper, the beneficial changes occurred in a marine clay due to the diffusion of lime from lime column and lime injected points have been studied using XRD and SEM techniques. The new compounds formed have been identified using the XRD technique. The diffusion of lime has been established through the measurements of lime content and pH from the samples taken at different locations and at various time periods. The transformation of the soil particles due to lime attack on the clay minerals present in a marine clay has been studied and discussed.

Control of calcium hexaluminate grain morphology in in-situ toughened ceramic composites

Abstract: The influence of processing conditions on the morphology of calcium hexaluminate (CA6) grains in Al2O3: 30 vol% CaO·6Al2O3 (CA6) ceramic composites was investigated. Specimens were prepared by in-situ reaction sintering using precursor powders of alumina, and either calcium carbonate or calcium oxide. In some samples, 1 vol% anorthite glass was added as a sintering aid. X-ray diffraction was used to study the phase development in the as-calcined and sintered states. The resultant microstructures were characterized using both scanning electron microscopy (SEM), and imaging secondary ion mass spectrometry (SIMS). It was found that the CA6 grains developed a platelike morphology when CaCO3 was used as the starting calcium-rich powder. In contrast, samples prepared using CaO resulted in equiaxed CA6 grains. This result was observed to be independent of the anorthite glass addition. The findings are rationalized in terms of distinct CA6 reaction mechanisms, resulting from differences in the reactivity of the powders during the early stages of calcining.

Engineering Behavior of Lime-Treated Louisiana Subgrade Soil

Abstract: Lime stabilization is often used to treat subgrade soils when they are soft and cohesive in nature. A study was conducted to investigate the engineering behavior, including the resilient and strength behaviors, of a lime-treated subgrade soil. The lime treatment procedure was adapted from the specifications of the Louisiana Department of Transportation and Development. Silty clay, a soil often found in Louisiana subgrades, is used as a base soil. A summary of various engineering properties of a lime-treated soil from resilient modulus, unconfined compression strength, and California bearing ratio (CBR) tests conducted at five moisture content and dry density levels is provided. Tests were also performed on the raw soil without lime treatment, and these results were compared with those of tests with the lime-treated soil. The comparisons indicate that the present lime treatment method results in an increase in strength and resilient modulus properties and a decrease in plasticity characteristics and plastic strains. A regression model with three constants was used to analyze the resilient modulus test results. The model constants are presented as functions of soil properties. Resilient modulus correlations that use either CBR or unconfined compression strength, moisture content, dry density, degree of compaction, and stresses as dependent attributes are developed.

Modification of clay soils using lime

Abstract: The paper examines the changes brought about in different clay soils by the addition of relatively small quantities of lime which can produce considerable benefits both in the long term and during construction. The different effects that occur with different clay types are highlighted and the quantities of lime required to achieve these effects in each case are discussed. These include the hydration of quicklime causing drying of clay soils on site and the modification of a clay soil to a material that is more workable and stronger. Details are given of changes in plasticity of British and Canadian clays, London Clay and three Jurassic Clays. Methods of construction are considered in the light of the recommendations made from laboratory test data. For the covering abstract see IRRD 887532.

Method of manufacturing high purity calcium carbonate

Abstract: Flue gases exhausted from a lime kiln (10) containing lime kiln dust are treated to produce a high purity calcium carbonate precipitate This is accomplished by passing the flue gases from the lime kiln (10) through a kiln dust separator (12) so that lime kiln dust containing calcium oxide is removed from the remaining flue gas (15) The lime kiln dust is mixed with water so that an aqueous suspension of the lime kiln dust is formed Quicklime or calcium oxide in the lime kiln dust is thus hydrated to form calcium hydroxide The solids within the suspension are then allowed to settle in settling tank (14) so that a clear, saturated solution of calcium hydroxide having a pH of about 124 is formed This solution is then contacted with carbon dioxide gas in reaction tank (16) so that the calcium hydroxide reacts to form a high purity calcium carbonate precipitate This high purity calcium carbonate precipitate is then allowed to settle from supernatant water of the solution in thickener tank (20) The high purity calcium carbonate is separated from the supernatant water, while the supernatant water is recycled through conduit (24) and used as the water mixed with the lime kiln dust to form the aqueous suspension

Phase Diagram Approach to the Fluxing Effect of Additions of CaCO3 on Australian Coal Ashes

Abstract: This paper presents a phase diagram approach to predict the melting temperatures of coal ash/flux mixtures and the viscosity versus temperature characteristics of the molten slags. This is illustrated by calcium oxide fluxing studies of three Australian bituminous coal ashes covering a range of silica to alumina ratios. Reasonable agreement was obtained between the observed melting temperatures of the calcium oxide/coal ash mixtures and predictions based on equilibrium phase diagrams for the ternary system silica, alumina, and calcium oxide or for the quaternary system silica, alumina, calcium oxide, and ferrous oxide. The approach is supported by energy dispersive scanning electron microscopy (SEM), SEM microprobe, and X-ray diffraction (XRD) studies of the slags. Slag viscosity measurements were made with a rotational viscometer over the range 1200−1600 °C. The measured viscosities were compared with predicted values using a model based on experimental results for the ternary system silica, alumina, and...

Method for increasing the rate of compressive strength gain in hardenable mixtures containing fly ash

Abstract: The present invention relates to concrete, mortar and other hardenable mixtures comprising cement and fly ash for use in construction. The invention provides a method for increasing the rate of strength gain of a hardenable mixture containing fly ash by exposing the fly ash to an aqueous slurry of calcium oxide (lime) prior to its incorporation into the hardenable mixture. The invention further relates to such hardenable mixtures, e.g., concrete and mortar, that contain fly ash prereacted with calcium oxide. In particular, the fly ash is added to a slurry of calcium oxide in water, prior to incorporating the fly ash in a hardenable mixture. The hardenable mixture may be concrete or mortar. In a specific embodiment, mortar containing fly ash treated by exposure to an aqueous lime slurry are prepared and tested for compressive strength at early time points.

Removal of Sulfur Dioxide from Flue Gas by the Absorbent Prepared from Coal Ash: Effects of Nitrogen Oxide and Water Vapor

Abstract: Removal of SO2 from flue gas by the absorbent prepared from coal fly ash, calcium oxide, and calcium sulfate was studied under different reaction conditions to elucidate the effects of the reaction temperature, water vapor pressure, and coexistence of NO in a flue gas. The SO2 removal activity increased with an increase in NO concentration up to 500 ppm at 130 °C. The SO2 removal activity increased as the water vapor pressure increased until a monolayer coverage with water molecule is achieved. As the adsorbed water exceeded the monolayer coverage, the SO2 removal activity suddenly decreased and calcium sulfite became the main product instead of calcium sulfate. The NO removal activity increased with an increase in SO2 concentration up to 2000 ppm at 130 °C. The NO removal also increased with an increase in water vapor pressure. The NO removal markedly decreased as the adsorbed water exceeded the monolayer coverage.

Use of cement kiln dust in stabilizing clay soils

Abstract: This report contains current information on the use of cement kiln dust (CKD), both exclusively, and in combination with other stabilizing materials such as lime, portland cement, and fly ash in stabilizing clay soils. The data on the type and level of CKD additions and the resulting engineering properties of the modified clay soils such as unconfined compressive strengths, Proctor moisture-density relationships, California bearing ratios (CBR), liquid- and plastic-limits, swelling, shrinkage, and permeability characteristics are documented. The informational gaps are identified and needed additional work is recommended.

Thermal Decomposition of NF3 with Various Oxides

Abstract: The thermal decomposition of NF3 by various oxides was investigated using gas chromatography, BET surface areas, and X-ray powder diffraction. The decomposition of NF3 using alumina can occur at temperatures as low as 100 °C, with steady complete conversion for almost 4 h at 400 °C. The reaction is shown to be stoichiometric. Using Na-doped alumina, conversion of NF3 dropped off faster than conversion with the undoped alumina. Similarly, calcium oxide did not react with the NF3 as completely or last as long as the alumina did; even at 400 °C the conversion of NF3 was only briefly greater than 90%. HX zeolite was also reacted with NF3, with higher and more consistent conversion than occurred with calcium oxide and Na-doped alumina, but did not perform as well as the alumina. Over 98% conversion was maintained at 400 °C for more than 1 h. In all cases, nitrogen oxides and the corresponding metal fluorides were the only major products from the reactions. The XRD patterns confirm the presence of metal fluorid...

Regeneration of Sulfided Calcium-Based Sorbents by a Cyclic Process

Abstract: A unique process for regenerating calcium-based sorbents which are in the form of calcium sulfide was demonstrated by employing thermogravimetric analysis (TGA). The process converts calcium sulfide to calcium oxide by subjecting particles of the material to repeated cycles of oxidation and reduction at temperatures between 950 and 1100 °C. During each cycle a small portion of material is first converted to calcium sulfate by oxidation and then to calcium oxide by reduction. Air can be used for oxidation and any of the following gases can be used for reduction: 30 mol % CO, 5 mol % CH4, or 2 mol % C3H8. Repeated sulfidation and regeneration of typical calcium-based sorbents seems to enhance the reactivity of the materials. However, incorporation of iron oxide or flyash containing an appreciable concentration of iron oxide in the sorbent has a deleterious effect.

Hydraulic cement with accelerated high strength development

Abstract: The invention consists in a method of manufacturing cements based on calcium sulfoaluminate, consisting in the format ion of a klinker through the roasting of raw materials to temperatures of more than 1200°C in such a manner that the klinker thus produced has a high content of calcium sulfoaluminate, tricalcium silicate and dicalcium silicate and, during the milling thereof or prior to the milling thereof adding to the klinker thus produced with the above recited basic components, optimal amounts of calcium oxide or hydroxide and calcium sulfate, in order to activate said klinker. Upon said milling, a temperature of about 100 to 125°C is to be observed and a time enough to convert gypsum to hemihydrate, thus making that the calcium sulfate present in the thus obtained product be in a condition to react quickly during tyre hydration of said cement; thus giving place to the formation of etringite; for which the presence of a pH above 12 is needed. Of course, it is also claimed as a novelty the activated cement, resulting from the calcination and milling methods of this invention.

The role of lime migration in lime pile stabilization of slopes

Abstract: Lime piles, which essentially consist of holes in the ground filled with lime, have been successfully used worldwide for the in situ treatment of failing clay slopes. The literature on the technique does not, however, permit a full explanation as to why lime piles work or how to design them. It is widely reported that migration of the lime from the piles into the surrounding clay provides the major stabilizing mechanism. This paper aims to review the literature on lime migration from piles and to report the results of recent research at Loughborough University to determine migration mechanisms from compacted quicklime piles. Potential applications are then discussed in the light of the findings. Hydroxyl (OH - ) ion migration by diffusion processes in intact clay is shown to be limited by high reactivity to approximately 20-30 mm. Nevertheless hydraulic transport via dehydration cracking or other discontinuities in the clay mass could result in migration extending for far greater distances. Calcium (Ca 2+ ) ion migration, would be expected to occur over distances greater than 1 m in ten years. Thus while full clay stabilization might only occur to a limited degree, modification of the clay is likely to be widespread. Mineralogical changes indicate a strength gain equivalent to that of lime mixed with clay at the initial lime consumption (Eades & Grim 1966) value plus 2% and cured for 28 days.

Deep stabilisation using lime

Abstract: The paper outlines the principles behind the improvement to soils in the form of lime piles, lime columns and lime slurry pressure injection. Lime columns are used primarily as an alternative to stone columns for stabilising soft soils, but have been used for slope stability. Lime piles have similar applications but pressure injection is mainly used in slope stability. Potential UK applications of the techniques are discussed. A review is given of research, using both laboratory studies and field studies, carried out at Loughborough University into lime-clay mixing, and more particularly, the use of lime piles for slope stabilisation. The main findings of the research are utilised to provide indicators of how the other deep stabilising techniques might be investigated and introduced into UK practice. For the covering abstract see IRRD 887532.

Use of Lime in Recycling Asphalt

Abstract: Two Strategic Highway Research Program asphalts were aged in a pressure oxygen vessel (POV) with and without admixture of CaO and Ca(OH)2 at several concentrations These same asphalts were then aged by low-temperature air blowing, and the resulting materials were softened by mixing with three recycling agents obtained by supercritical extraction of asphalts These rejuvenated asphalts were mixed with varying amounts of CaO and aged in the POV Oxidation rates and hardening were measured at various temperatures, and the resulting kinetic parameters were used to estimate hardening at road conditions The hardening rate was always reduced by lime addition The oxidation rate was sometimes reduced The recycling agents alone reduced the hardening rates relative to those of the original asphalts, but the effect was further enhanced by CaO additions

Catalyst for converting methane into higher hydrocarbon products

Abstract: Methane is converted into higher hydrocarbon products by oxidative coupling, by bringing a gaseous mixture containing methane and oxygen into contact with a solid catalyst formed from calcium oxide, magnesium oxide, a lanthanide oxide plus possibly lithium oxide, wherein: the lithium content is between 0 and 0.20 wt %, the calcium/magnesium atomic ratio is between 0.08 and 0.7, and the (calcium+magnesium)/lanthanide atomic ratio is between 0.8/1 and 8/1, and preferably between 2/1 and 4/1.

Treatment of solid containing material derived from effluent or waste from wastepaper treating process

Abstract: A method of treating solid-containing material contained in or produced from an effluent or waste from a process for the treating of waste paper. The solid-containing material includes constituents of the waste paper and organic material including cellulose fibers and inorganic particulate material including calcium carbonate and other inorganic particulate material included in the waste paper. The solid-containing material is subjected to a heat treatment that incinerates the organic material present and forms from the inorganic particulate material present a calcined product including calcium oxide from thermal decomposition of calcium carbonate. The calcined product is formed into an aqueous suspension wherein calcium oxide is converted to calcium hydroxide. The calcined product in the suspension is comminuted and a reagent is added to the suspension to form a salt-forming anionic species to form an insoluble calcium salt precipitate mixed or aggregated with other inorganic particulate material derived from the waste paper constituents.

An introduction to lime stabilisation

Abstract: An outline is given of the history of the use of lime stabilisation which has been used extensively in the UK since the late 1980s. Quicklime has several advantages over hydrated lime in the UK. The effect of lime on clay and silt soils is explained and the mechanism of lime stabilisation is discussed. Consideration is given to the reaction of sulphate with cementitious materials causing heaving. The improvement and modification of clay soils with lime stabilisation is outlined. As the strength and cohesion of the soil is enhanced by stabilisation, it is possible to improve soils prone to movement by settlement or swelling so that they can be used in structural applications. Examples are given of the situations where stabilisation of soils can be made on economic and technical merit. Future opportunities are outlined. For the covering abstract see IRRD 887532.

Production of artificial aggregate

Abstract: PROBLEM TO BE SOLVED: To reduce the residue of heavy metals such as lead by grinding a mixture of ash, bentonite, a composition adjusting material and a specific reducing agent, adding water to the mixture, molding, drying and baking. SOLUTION: Ash of municipal refuse is mixed with bentonite as a binder, a composition adjusting material composed of a mineral containing a silica such as silica sand, pottery stone, feldspar, kaolinite, kibushi-clay, etc., so as to give a mixture having a chemical composition after the baking comprising 20-80wt.% of silica and 10-35wt.% of calcium oxide. The mixture is mixed with 2-9wt.% calculated as the amount of carbon of coal or coke as a reducing agent, granulated into <=15μm average particle diameter, blended with water and molded to give a molding product. The molding product is dried, thrown into a rotary kiln revolving at 0.2-0.8 radian/second revolving speed and baked at 1,000 to 1,250 deg.C for 30 to 120 minutes.

Process for treatment of exhaust gas

Abstract: A process for removing environmental contaminants from an exhaust gas by a semidry method through contact with an absorbent slurry containing alkaline compounds in a reactor, comprising the steps of adding a material containing calcium sulfate and/or calcium sulfite, a material containing aluminum oxide, and a material containing silicon dioxide to a material capable of supplying calcium oxide, mixing the mixture with water, curing the whole mixture in hot water, and then introducing and dispersing the resulting absorbent slurry in the reactor for contact with the environmental contaminants. In a modification the absorbent slurry is obtained by adding a material capable of supplying aluminum oxide, silicon dioxide, and calcium sulfate and/or calcium sulfite to a material capable of supplying calcium oxide, firing the mixture at 750-950 DEG C, mixing the firing product with water, and curing the mixture in hot water. To the absorbent slurry obtained in either way is added a soluble compound that elevates the boiling point of water.

Micronized alkaline earth metal carbonate

Abstract: The invention concerns a micronized alkaline earth metal carbonate prepared with or without the use of an agent preventing crystal growth, and a process for its preparation and use. The prepared micronized calcium carbonate, barium carbonate with a BET surface area ranging from 3 to 30 m2/g or strontium carbonate with a BET surface area ranging from 3 to 50 m2/g is suitable in particular for preparing capacitors, thermistors and other oxide ceramic electrical components containing calcium oxide, barium oxide or strontium oxide, and high-temperature superconductors. Ammonium salts or alkylammonium salts of carboxylic acids having a total of between 3 and 12 carbon atoms and at least two COOH groups or at least two OH groups, e.g. salts of maleic acid or nitric acid, are used as agents preventing crystal growth.

Production of lightweight foamed concrete

Abstract: PROBLEM TO BE SOLVED: To largely reduce the main raw materials and save the resources without retardation of the process steps in the production of lightweight foamed concrete by adding calcite to the fuel coal and using the ash of the combusted calcite-mixed fuel coal instead of a part or the whole of siliceous materials, calcareous materials and the gypsum for the foamed concrete. SOLUTION: In this invention, the coal ash occurring from the fluidized bed type coal fired boiler is utilized and the fired coal ash contains 20-40 wt.% of siliceous materials, for example, SiO2, 15-45 wt.% of calcium oxide (quick lime), 5-10 wt.% of gypsum, 5-20 wt.% of alumina and the like. The objective lightweight foamed concrete is produced by adjusting the Ca/Si ratio of the coal fired ash to 0.3-0.9, when necessary by adding silica rock or sand, then kneading the adjusted ash, a proper amount of water and an aluminum powder and aging the resultant slurry with steam. The quick lime and the gypsum in the fired ash generates heat, when they come into contact with water, to contributes to the hardening reaction of the slurry and the quick lime reacts with SiO2 to form tobermorite that is the main mineral in the lightweight concrete.

Process and apparatus for preparing calcium carbonate

Abstract: The present invention concerns a process and an apparatus for preparing calcium carbonate The process comprises reacting a calcium oxide raw material, which contains at least some hydrated calcium oxide, with carbonate ions in an aqueous medium for preparing precipitated calcium carbonate The calcium carbonate is separated from the suspension and subjected to further treatment for preparation of the desired end product The invention is characterized in that at least 80 % of the particles of the hydrated calcium oxide have a particle size of less than 3 νm and the reaction is allowed to proceed without mixing the reaction mixture When a pigment suitable for paper coating, having a particle size of 02 to 04 νm, is to be prepared, the carbonation reaction is allowed to continue for 20 to 25 minutes before it is interrupted in an impact mixer (12)