Showing papers on "Ettringite published in 1998"

Ettringite and oxyanion-substituted ettringites -- their characterization and applications in the fixation of heavy metals: a synthesis of the literature

Abstract: This report constitutes a synthesis of the literature relating to the formation, structure, analysis, and characterization of ettringite from a variety of sources including hydration of portland cement, calcium aluminates, expansive cements, rapid-set cements, hydration of fly ash and coal combustion by-products, and laboratory synthetic processes. The major focus of the review is on the formation and properties of oxyanion-substituted ettringites. Substituted ettringite structures generally contain oxyanions of amphoteric heavy metals such as chromium, boron, arsenic, and selenium. Such amphoteric metals, under the elevated pH conditions of hydrating portland cement, usually will form water-soluble oxyanion species such as chromate. However, it has been established that the ettringite structure can accommodate many such heavy metals oxyanions in place of the usual sulfate anion, and will thus exhibit a very high degree of insolubility at those pH values where heavy metal water-soluble oxyanions will readily be subjected to water leaching.

Cement-based materials

Abstract: Key advances toward a detailed understanding and modelling of the structure and properties of cement-based materials have been made in the past year. Significant topics that have been covered are the following: self-desiccation during curing; transport properties and modelling of transport processes; sulfate attack, including delayed ettringite formation; and the structure of calcium silicate hydrate.

A process for the treatment of effluent streams

Abstract: This invention relates to a process for removing sulphates and calcium from a water stream which comprises the steps of: combining the water stream and an amount of amorphous aluminium trihydroxide (Al(OH3)); allowing the formation of ettringite (3CaO.Al2O3CaSO4.31/32H2O) as a precipitate; and removing the precipitated ettringite from the water stream.

Characterization of the Calcium Silicate and Aluminate Phases in Anhydrous and Hydrated Portland Cements by 27Al and 29Si MAS NMR Spectroscopy

Abstract: 27Al and 29Si magic-angle spinning (MAS) NMR spectroscopies have proven to be valuable tools for characterization and quantitative analysis of the anhydrous and hydrated calcium silicate and aluminate phases in Portland cements. This paper describes the results obtained in the authors laboratory from the 27Al and 29Si MAS NMR studies of Portland cements during the past few years. A method for quantifying the calcium silicates (alite and belite) in anhydrous and hydrated Portland cements is presented. This employs deconvolution of 29Si MAS NMR spectra combined with the bulk SiO2 content determined from elemental analysis. Direct experimental proof for Al guest-ion substitution in alite and belite and in their hydration products is obtained employing 27A1 MAS NMR. 27A1 quadrupole coupling parameters and isotropic chemical shifts for synthetic samples of the aluminate phase (3CaO Al2O3) and its hydration products (ettringite and monosulphate) are accurately determined from 27Al MAS NMR spectra of the central and satellite transitions. These data are used in 27Al MAS NMR investigations of Portland cements, which provide information about the quantity of the aluminate phase in the anhydrous material, the formation of ettringite during hydration, and the conversion of this phase to monosulphate.

Using Natural Pozzolans to Improve the Sulfate Resistance of Cement Mortars

Abstract: Concrete in contact with a sulfate environment can severely degrade due to expansion accompanying the formation reaction products such as ettringite. The use of portland-pozzolan cement has been successful in mitigating this expansion. However, it is important to study the effectiveness of natural pozzolans to improve the resistance to attack by sulfates. This paper reports results of different portland-pozzolan cements containing different natural pozzolans and ASTM Types I, II, and V portland cements. The pozzolanic activity and composition of each pozzolan was evaluated. The susceptibility to sulfate attack was studied by measuring the expansion in mortar bars at different ages according to ASTM Method 1012 for 78 weeks. It was found that cements containing pozzolans with high activity or low alumina content improved the sulfate resistance. Also, the pozzolan content in the cement was found to be important.

Scientific background to thaumasite formation in concrete

Abstract: The thaumasite form of sulfate attack is described scientifically in relation to the more commonly experienced sulfate attack manifested by gypsum and ettringite.

Stabilization of Sulfate-Contaminated Crushed Concrete Base with Type V Cement and Fly Ash

Abstract: The sulfate-contaminated crushed concrete base of a parking lot began to experience severe heaving because of the formation of secondary, expansive minerals (ettringite and thaumasite). Tests were performed in the laboratory to study the possibility of limiting the mobility of soluble sulfate ions by adding either a sulfate-resistant (Type V) cement or a combination of Type V cement and fly ash. The use of cement did not only prove practical because of the high moisture content of the base, but a mixture of 10 percent Type V cement and 9 percent Class C fly ash by weight yielded promising results for drying and encapsulating the free sulfate ions and providing a reasonably strong base.

Determination of Optimal Composition of Stabilized Phosphogypsum Composites for Saltwater Application.

Abstract: Phosphogypsum (CaSO^ lHzO, PG), a solid by-product of phosphoric acid production, has been classified as a “Technologically Enhanced Natural Radioactive Material” (TENR), because it contains radionuclides (eg., radium ^) and some trace toxic metals in concentrations, which may pose a potential hazard to human health and the environment. The current regulated disposal method for PG is on-site stockpiling, which has created a serious environmental management problem. An appealing solution to the problem is the use of stabilized PG for aquatic enhancement activities. This solution can eliminate the airborne vector of transmission for radon^ and therefore may provide a safe alternative to the current stockpiling practices. The determination of low cement content (<10%) stabilized PG composites has been investigated. Varying combinations of PGrcement, PGrfly ashrlime and PGrfly ashrcement were fabricated for laboratory and field experiments. Field saltwater submergence studies and response surface with process variable analysis shows that only the PGrfly ashrcement composites are able to survive in the Gulf coast saltwater environment when cement content is less than 10%. Scanning electron microscopy (SEM) shows that ettringite formation is potentially responsible for degradation of PG stabilized composites. SEM and microprobe analysis showed that conditions necessary for stabilized PG composites to survive in the saltwater environment arer (1) the stabilized PG composites should have a strong sulfate resistant surface and (2) the local pH environments on the stabilized PG composites should be above 11. This higher local pH environment will result in the formation o f calcium carbonates, which

Pcc pavement deterioration and expansive mineral growth

Abstract: In order to evaluate the importance of newly-formed minerals in the premature deterioration of Iowa highway concrete, a two-phase study was undertaken. In the first phase, the authors performed petrographic and SEM/EDAX analyses to determine chemical and mineralogical changes in the aggregate and cement paste of samples taken from Iowa concrete highways that showed premature deterioration. In the second phase, they experimentally simulated environmental changes occurring in highway concrete after different deicer chemicals were applied, in order to evaluate the role of deicers in premature deterioration. In highways exhibiting premature concrete deterioration, ettringite completely fills many small voids and lines the walls of larger voids. Microscopic ettringite is also commonly disseminated throughout the paste of many samples. Severe cracking of cement paste is usually associated with ettringite locations, and strongly suggests that ettringite contributed to deterioration. Pyrite is present in coarse/fine aggregates in several concretes. Sulfate ions released by its oxidation contribute to ettringite formation. In poorly performing concretes containing reactive dolomite aggregate, brucite resulting from partial dedolomitization of the aggregate, was most common. No cracking was observed to be spatially associated with brucite, but most brucite crystals are microscopic in size and widely disseminated in the cement paste of less durable concretes. Expansion stresses associated with its growth at many microlocations may be relieved by cracking at weaker sites in the concrete. In the experimental phase of the study the authors found that each deicer salt can cause characteristic concrete deterioration by altering dedolomitization rims at the coarse-aggregate paste interface, by altering cement paste, and/or by forming new expansive minerals in the paste. Magnesium in deicer solutions produces the most severe paste deterioration by forming non-cementitious magnesium silicate hydrate and brucite. Chloride in deicer solutions promotes decalcification of paste and alters ettringite to chloroaluminate. Acetate seems to accentuate Mg-induced deterioration. Magnesium chloride, calcium magnesium acetate and magnesium acetate were the most deleterious.

Influence of the sulfate level in the clinker phase on the performance of superplasticized concretes

Abstract: Portland cement contains sulfur compounds from the clinker phase and from added calcium sulfate (e.g., gypsum) which acts as a set regulator. The purpose of this investigation was to study the influence of the sulfate content in the clinker phase on the performance of superplasticized concrete mixtures in terms of initial slump level at a given water-cement ratio (0.45), slump-loss rate, and compressive strength at early and later ages. Two batches (A and B) of clinker from the same kiln source were studied, the main difference being the content of sulfate (SO3) in the clinker (0.72% and 1.40%, respectively). Different percentages of natural gypsum, as wet regulator, were interground in a laboratory mill to manufacture portland cements: A1, A2, A3 from clinker A, and B1, B2, B3 from clinker B. Three levels of total sulfate content in terms of SO3 were set: 3.0% in portland cements A1 and B1; 3.5% in portland cements A2 and B2; 4.0% in portland cements A3 and B3. At a given sulfate content in portland cement, the lower the clinker sulfate content, the more effective is the slump increase of the concrete caused by the superplasticizer addition. Moreover, the lower is the clinker sulfate content, the lower is the slump-loss rate of the superplasticized concrete mixture. Finally, at a given water-cement ratio, there is a reduction in the compressive strength at early ages (

Arsenate Interactions with CaO: Formation of Johnbaumite

Abstract: In hyperalkaline en~cironments such as concretes and alkaline wastes, the aqueous geochemistry of arsenate should be controlled by reactions of arsenate ions with alkaline phases such as portlandite, calcite and ettringite. This should result in the formation of surface complexes, and/or solid solutions of metalarsenate precipitates. At present, insufficient thermodynamic data is available to facilitate prediction of Ca-arsenate precipitates. In addition, little spectroscopic data is available for As(V) interactions with Ca mineral phases or for Ca-arsentate precipitates. The present study examined the sorption of arsenate by CaO. Examination of reaction products with direct physical methods (XRD, SEM, FTIR, Raman and XAS) indicated the formation of johnbaumite (Cas(AsO4)3(OH)).

Method for manufacturing high-early flyash cement concrete

Abstract: An early-and high-strength concrete of powdered coal ash and cement is prepared from ground karstenite (or burnt plaster), cement, powdered coal ash, broken stone, sand, water and water-reducing agent through proportional mixing and normal-pressure steam curing which can promote the reaction of C3A and C4AF in cement with active Al2O3 in powdered coal ash and karstenite to form a great deal of fibrous ettringite to increase early and post strength.

Additive for improving quality of mortar and concrete

Abstract: PROBLEM TO BE SOLVED: To improve both the workability and the durability while suppressing an alkali aggregate reaction by combining a calcium type zeolite with gypsum. SOLUTION: This additive is obtained by adding gypsum in an amount so as to provide the amount of the added calcium zeolite within the range of <10wt.% based on the sum of the calcium type zeolite and a cement within the range of <=20.0wt.% ratio of the amount of the added calcium type zeolite based on a binder (cement and an admixture) to the calcium type synthetic zeolite having <=7.0wt.% content of an alkali metallic component expressed in terms of anhydrides as Na2 O equiv. and 0.5-4.5μm average grain diameter. Thereby, ettringite is produced by a reaction of alumina in the zeolite with the gypsum to relax the shrinkage of a mortar or a concrete. As a result, cracking hardly occurs. Early deterioration due to an alkali aggregate reaction or salt damage, etc., is hardly caused.

Inorganic construction material

Abstract: PROBLEM TO BE SOLVED: To provide an inorganic building material that has excellent strength, dimension stability and durability. SOLUTION: The raw materials including 30-80 pts.wt. of alunite powder, 10-50 pts.wt. of lime powder, 1-20 pts.wt. of gypsum powder are mixed with water, molded in a prescribed shape and solidified by aging whereby the objective inorganic building material including ettringite and monosulfate. In a preferred embodiment, the alunite powder contains 10-50 pts.wt. of alunite and 30-80 pts.wt. of quartz. In addition, this inorganic material may include reinforcing fibers and/or light weight materials. COPYRIGHT: (C)1999,JPO

Synthetic ettringite, calcium silicate compact containing the ettringite and calcium silicate compact

Abstract: PROBLEM TO BE SOLVED: To provide a method for producing a synthetic ettringite effective as a bulk density reducing material capable of being lightened without decreasing production efficiency while securing the practically necessary strength and dimensional stability of a calcium silicate compact to be used as a building material and/or a whitening material, a method for producing synthetic ettringite and a calcium silicate compact contg. synthetic ettringite and a calcium silicate compact. SOLUTION: An aq. suspension of calcium hydroxide is added to an aluminum sulfate soln. in 5.8 to 6.2:1 molar ratio of calcium hydroxide to aluminum sulfate and at an addition rate of 0.6mol of calcium hydroxide to one mol of aluminum sulfate per minute while keeping the aluminum sulfate soln. and the suspension of calcium hydroxide at 50-90 deg.C to produce synthetic ettringite. The synthetic ettringite obtained by this method has 50-100μm fiber length or 13.0ml/sec straining rate.