Showing papers on "Ettringite published in 1992"

The effect of secondary ettringite formation on the durability of concrete: a literature analysis

Abstract: The report comprises a review and analysis of the available literature pertaining to the causes, effects and prevention of secondary (delayed) ettringite in concrete. Over 300 publications have been examined. Case studies of damage in concrete possibly caused by secondary ettringite formation are examined first. Fundamental research on secondary ettringite formation, its chemistry, and deposition mechanisms is then reviewed. Key investigations on the topic are analyzed in detail. Next, the potential importance of (a) method of heat-curing and (b) the chemistry of cement is outlined. In the final chapter, a rapid test for evaluation of potential secondary ettringite susceptibility (the "Duggan" test) is evaluated. The analysis indicates that there appears to be a potential for a secondary ettringite formation problem in North America; it is highly probable that secondary ettringite formation can lead to significant deterioration of heat-treated concrete. However, it is unlikely that secondary ettringite formation is, or will be, the sole mechanism responsible for premature deterioration. The critical factors that determine extent of damage due to secondary ettringite formation are (a) duration of delay period before heating the concrete; (b) severity of the heating and/or cooling regime; and (c) the SO3/Al2O3 ratio of the cement. There is no evidence that non heat-treated concrete is susceptible to this phenomenon. Further research and improvements to the Duggan test may result in the development of a useful standard test method to assess the long-term dimensional stability and durability of concrete.

Time-resolved studies of the early hydration of cements using synchrotron energy-dispersive diffraction

Abstract: We outline the need to study the early hydration of cements by dynamic diffraction methods, and demonstrate how this can be done using a “hydration cell” designed specifically for the parallel beam geometry of synchrotron energy-dispersive diffraction. The method has been applied to three systems: a model component of portland cement, a high-alumina cement and an ettringite-based cement. The results show how this technique might be used to gather information on the kinetics of the chemical processes involved. In the case of ettringite, we can follow dynamically the crystallisation process on an unprecedented 20 second time scale.

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.

Relative importance of deleterious reactions in concrete: formation of AAR products and secondary ettringi te

Abstract: Aggregates, varying in alkali reactivity, were used for making mortar bars and concrete prisms, employing the same Portland cement Measurement of length changes of the specimens was carried out under various storage conditions and for different lengths of time Scanning electron microscopy of the specimens and energy-dispersive X-ray analysis of the products formed in them has shown that alkali-aggregate reaction (AAR) products develop first and much faster than ‘secondary’ ettringite (mat-form, filling cracks) associated with some deteriorated steam-cured concrete elements The expansion and cracking due to AAR was more rapid and extensive at higher temperature (80°C) than at lower temperature (40°C) Mat-forms of ettringite developed only after prolonged storage of the specimens in a fogroom, and the amount was larger in the specimens initially treated at 80°C, mainly because more AAR expansion and cracking occurred at the higher temperature, but also because the higher initial temperature caused large

Structure of Hardened Cement-Fly Ash Systems and Their Related Properties

Abstract: The influence of fly ash on the pore structure and microstructure of cement paste and mortar has been investigated. The methods applied include water vapour sorption isotherms, scanning electron microscopy, X-ray diffraction analysis and thermogravity/differential thermal analysis. The desorption isotherm test shows that, with low to medium cement replacement (F/(C+F) up to 0.30), fly ash does not significantly influence the pore structure as far as the ability to hold moisture is concerned. The same conclusion is applicable to the sorption isotherm, for which the specific surface area is a dominating factor. However, high volume fly ash addition results in a coarser pore structure, which leads to more moisture loss per paste at high relative humidity conditions, e.g. at 85% RH. Correspondingly, the sorption isotherm reveals a lower specific surface area value for the high volume fly ash mortar. The microstructural analysis shows that more calcium hydroxide per cement is produced in the paste with fly ash, which in turn reacts with fly ash to form calcium silicate hydrates and calcium aluminate hydrates similar to those produced by the cement hydration. However, the amount of calcium hydroxide per cement in the paste is lower only for the high volume fly ash specimens at late ages, i.e. later than 28 days. The analysis of the amount of cement at each age shows that the hydration of cement is approximately the same for the paste with and without fly ash. In conjunction with this, the hydration products in the paste with fly ash are usually a compound of calcium-aluminate-silicate with a low calcium oxide to silicon-alumina ratio. Since fly ash hydration is a slow process, the effect of calcium sulphate hydrate (as an activator) on the hydration of fly ash was studied. It has been shown that an appropriate addition of gypsum (CaSO4.2H2O) enhances the decomposition and reaction of fly ash. The reaction products include ettringite ({Ca6[Al(OH)6]2.24H2O}[(SO4)3.1.5H2O]), as well as calcium aluminate hydrate, e.g. C3AH6. It was observed that X-ray energy dispersive analysis of the ettringite-like phase reveals, more often than not, the inclusion of Si in the crystal lattice. The pore-filling effect of fly ash has been shown to be about the same as that of cement. However, this effect depends totally on the reactive phase of fly ash, which is about 50% for the fly ash studied in this work. The other factors influencing the pore structure, such as carbonation and ageing of the hydration products, were also tested. It is shown that the former results in coarser pore structure, and the latter affects some change in the C-S-H gel structure. In addition to the experimental work, the author has theoretically treated the effect of fly has, the desorption and adsorption isotherms, and the relation between shrinkage and the desorption isotherm.

Alkali silica reaction processes: the conversion of cement alkalies to alkali hydroxide. durability of concrete. g.m. idorn international symposium, 1990 annual aci convention, toronto, ontario, canada

Abstract: Among the important stages of the overall ASR process as it occurs in concrete is the conversion of alkali sulfate dissolved from the cement to alkali hydroxide. The results of laboratory studies are presented to provide an understanding of this process and of its effects. This conversion process depends on continued formation of ettringite, and does not take place until the cement gypsum is exhausted. Provision of a large excess of gypsum or interference with early ettringite production by high temperature exposure may postpone or prevent it, thus reducing the OH (-) ion concentration and the possibility of ASR reaction.

Radioactive waste processing container

Abstract: PURPOSE:To obtain a radioactive waste processing container enhanced in safety by constituting a container formed using cement so that calcium hydroxide is not substancially contained in the cement layer thereof. CONSTITUTION:For example, the main body A of this processing container is formed by integrally lining a steel drum 1 with a polymer impregnated con crete layer 2. Calcium hydroxide formed in Portland cement becomes a composi tion forming ettringite by the reaction with calcium sulfoaluminate and gypsum. In this case, calcium hydroxide generated form Portland cement is entirely consumed by calcium sulfoaluminate clinker to become absent in the system. As a result, a radioactive waste processing container hard to generate damage such as a crack or release and rich in durability is obtained.

Solidifying material for processing radioactive wste

Abstract: PURPOSE:To enhance durability and safety by preparing a solidifying material using cement so that calcium hydroxide is not substantially present therein CONSTITUTION:A solidifying material is composed of a composition forming ettringite by the reaction of calcium hydroxide formed by the hydration of tricalcium silicate and discalcium silicate in Portland cement with calcium sulfoaluminate and gypsum (CaSO4) As a result, the radioactive waste solidifying material forming solidified matter hard to generated a crack or release and rich in durability is obtained and, therefore, calcium hydroxide generated from Portland cement being a stimulating agent of slag is entirely consumed by calcium sulfoaluminate being the other one stimulating agent and no calcium hydroxide is present in the system

Structure for processing radioactive waste

Abstract: PURPOSE:To provide a structure hard to generate damage such s crack or release and rich in durability by forming the structure so that no calcium hydroxide is substantially contained. CONSTITUTION:A structure is composed of a composition forming ettringite by the reaction of calcium hydroxide formed by the hydration of tricalcium silicate and dicalcium silicte in Portland cement with calcium sulfoaluminate and gypsum. Calcium hydroxide generated from Portland cement being one stimulating agent of slag is entirely consumed by calcium sulfoaluminte being the other stimulating agent to become abse in the system. Therefore, the structure hard to generte damage such as a crack or release over a long period of time from the beginning and rich in durability is obtained.