Showing papers on "Ettringite published in 2001"

Structure and Performance of Cements

Abstract: 1. Cement Manufacture 2. Composition of Cement Phases 3. The Hydration of Portland Cement 4. Calcium Aluminate Cements 5. Properties of Concrete with Mineral and Chemical Admixtures 6. Special Cements 7. Developments with Oilwell Cements 8. Gypsum in Cements 9. Alkali-Silica Reaction in Concrete 10. Delayed Ettringite Formation 11. Chloride-Corrosion in Cementitious Systems 12. Blastfurnace Cements 13. Properties and Applications of Natural Pozzolanas 14. Pulverised Fuel Ash as a Cement Extender 15. Metakaolin as a Pollolanic Addition to Concrete 16. Condensed Silica Fume as a Cement Extender 17. Cement-Based Composite Micro-Structures 18. X-Ray Powder Diffraction Analysis of Cements 19. Electron Microscopy of Cements 20. Electrical Monitoring Methods in Cement Science 21. Nuclear Magnetic Resonance Spectroscopy and Magnetic Resonance Imaging Studies of Cements and Cement-Based Materials 22. The Use of Synchroton Sources in the Study of Cement Materials

Structural changes during thermal dehydration of ettringite

Abstract: Changes in crystal structure of ettringite on dehydration in normal atmosphere have been re-examined by means of thermogravimetry, X-ray diffraction, and 27Al nuclear magnetic resonance spectroscopy. Synthetic ettringite crystals were heated at various temperatures up to 200°C for durations of up to 7 h. The ettringite structure maintains some long-range order until the coordination number of calcium changes to 5 by dehydration of twelve water molecules from channels and columns with heat treatment at 70°C. When the coordination number of calcium decreases to 5, the short-range order of the structure is disrupted and ettringite becomes X-ray-amorophous. Subsequently, the rest of the water molecules in the columns and bridging OH groups in calcium polyhedra are removed, essentially destroying the framework of the columns. This is accompanied by changes in the coordination number of aluminium from 6 to 4.

Ettringite in hydration of portland cement concrete and its occurrence in mature concretes

Abstract: In the last 10 years, expansion and cracking in concretes exposed to moist environments have received much scrutiny, and especially the part played by ettringite formation in such processes. It has frequently been assumed that the formation of ettringite cracks or other cavities readily observable by optical petrography or scanning electron microscopy is responsible for the observed distress. It has also been claimed that the late formation of ettringite in concrete not subjected to an elevated temperature can cause distress. This paper examines the validity of these assumptions, in light of what is known about chemical and microstructural aspects of ettringite formation in concrete. It also considers claims that damage from delayed ettringite formation can occur in concretes that have not been subjected to an elevated temperature.

Ettringite Formation and Sulfate Attack on Concrete

Abstract: The paper presents a critical review of the relationship between ettringite formation and sulfate attack. Ettringite formation is associated with expansion. However, any ettringite-related expansion is not necessarily related to sulfate attack. Early ettringite formation (EEF) which occurs immediately (within hours) in a plastic fresh mixture does not produce any damaging expansion and is associated with the regulation of setting time of portland cement paste. Expansion after the hardening of cement paste can be advantageously used for development of chemical prestress in expansive cements. Delayed ettringite formation (DEF) occurs at late ages and the related heterogeneous expansion in a very rigid hardened concrete can produce cracking and spalling. Two different types of DEF are examined depending on the sulfate source: DEF caused by external sulfate attack (ESA) or internal sulfate attack (ISA). ESA, related to the interaction of environmental sulfate with the cement matrix, can be precluded by the use of impermeable concrete. On the other hand, ISA occurs in a sulfate-free environment due to the interaction of internal sulfate (from cement or gypsum contaminated aggregate) with calcium-aluminate hydrates of the cement paste. Two different mechanisms of DEF caused by ISA are examined. The first one is based on the thermal decomposition of ettringite in high-temperature cured concrete elements and the subsequent re-formation of ettringite at ambient temperature in a saturated atmosphere. According to the second mechanism ISA is based on a chain of three essential events (microcracking, late sulfate release, and exposure to water) and DEF could occur even at room temperature.

Chromium(III)-Ettringite Formation and its Thermal Stability

Abstract: Attempts have been made to replace aluminium(III) by chromium(III) in the ettringite structure because of practical importance of a waste treatment technology. The optimum conditions of Ca6[Cr(OH)6]2(SO4)3⋅26H2O formation and its thermal stability are reported.

Mechanisms of set acceleration of Portland cement through CAC addition

Abstract: The use of calcium aluminate cement as an efficient way of accelerating the setting time of Portland cements has been known for decades. In the absence of other additives, the poor strength development of such mixtures limits their use. However, additives can enable improved strength development for certain applications. This paper discusses the hydration mechanisms occurring in these systems. The precipitation and growth of ettringite are discussed as related to simple calcium aluminate cement/calcium sulphate mixtures, followed by discussion of the early hydration of pure Portland cements and the mechanism whereby the C 3 A reaction is controlled by calcium sulphate additions to avoid rapid setting. Studies of OPC/CAC systems indicate that the CAC additions destabilise the balance of the reaction of calcium sulphate with C 3 A. In these binary systems, set may be brought about by ettringite formation even when only low amounts of this phase are present. The reaction of CAC with different forms of calcium sulphate (gypsum, hemihydrate or anhydrite) can be used to understand why different Portland Cements require different levels of CAC addition.

Hydration Characteristics of Portland Cement after Heat Curing: I, Degree of Hydration of the Anhydrous Cement Phases

Abstract: The degree of hydration of the four major anhydrous cement phases in three U.K. portland cement mortars has been observed during the period of water storage at room temperature after an initial short-term heat cure. Such a heat cure at 85° or 100°C for 12 h generally accelerated the initial hydration of the four major anhydrous minerals in portland cement. Subsequent retardation of the degree of hydration of the alite, tricalcium aluminate, and ferrite phases was observed when these heat-cured mortars were stored at ambient temperature. General similarity but some differences in hydration behavior were observed between the three cements. The hydration of belite in the heat-cured mortars during storage at room temperature produced porous inner products that favored deposition of ettringite and reduced the risk of expansive ettringite formation. The substantial retardation in hydration of the aluminate-bearing phases, especially the ferrite phase, during the storage at room temperature raised the overall SO3/Al2O3 ratio of the cement hydrates formed, bringing about a potential for ettringite formation and hence the risk of expansion through delayed ettringite formation.

Reduction of concrete deterioration by ettringite using crystal growth inhibition techniques

Abstract: Many researchers have concluded that secondary or delayed ettringite is responsible for serious premature deterioration of concrete highways. In some poorly performing Iowa concretes, ettringite is the most common secondary mineral but its role in premature deterioration is uncertain since some researchers still maintain that secondary ettringite does not itself cause deterioration. The current research project was designed to determine experimentally if it is possible to reduce secondary ettringite formation in concrete by treating the concrete with commercial crystallization inhibitor chemicals. The hypothesis is such that if the amount of ettringite is reduced, there will also be a concomitant reduction of concrete expansion and cracking. If both ettringite formation and deterioration are simultaneously reduced, then the case for ettringite induced expansion/cracking is strengthened. The experiment used four commercial inhibitors - two phosphonates, a polyacrylic acid, and a phosphate ester. Concrete blocks were subjected to continuous immersion, wet/dry and freeze/thaw cycling in sodium sulfate solutions and in sulfate solutions containing an inhibitor. The two phosphonate inhibitors, Dequest 2060 and Dequest 2010, manufactured by Monsanto Co., were effective in reducing ettringite nucleation and growth in concrete. Two other inhibitors, Good-rite K752 and Wayhib S were somewhat effective, but less so than the two phosphonates. Rapid experiments with solution growth inhibition of ettringite without the presence of concrete phases were used to explore the mechanisms of inhibition of this mineral. Reduction of new ettringite formation in concrete blocks also reduced expansion and cracking of the blocks. This relationship clearly links concrete expansion with this mineral - a conclusion that some research workers have disputed despite theoretical arguments for such a relationship and despite numerous observations of ettringite mineralization in prematurely deteriorated concrete highways. Secondary ettringite nucleation and growth must cause concrete expansion because the only known effect of the inhibitor chemicals is to reduce crystal nucleation and growth, and the inhibitors cannot in any other way be responsible for the reduction in expansion. The mechanism of operation of the inhibitors on ettringite reduction is not entirely clear but the solution growth experiments show that they prevent crystallization of a soluble ettringite precursor gel. The present study shows that ettringite growth alone is not responsible for expansion cracking because the experiments showed that most expansion occurs under wet/dry cycling, less under freeze/thaw cycling, and least under continuous soaking conditions. It was concluded from the different amounts of damage that water absorption by newly-formed, minute ettringite crystals is responsible for part of the observed expansion under wet/dry conditions, and that reduction of freeze resistance by ettringite filling of air-entrainment voids is also important in freeze/thaw environments.

Hydration Characteristics of Portland Cement after Heat Curing: II, Evolution of Crystalline Aluminate‐Bearing Hydrates

Abstract: The development of crystalline aluminate-bearing hydrates in portland cement mortars during water storage at room temperature for periods of up to 1 year after an initial heat cure for 12 h has been observed by quantitative X-ray diffraction analysis and backscattered electron imaging. Ettringite was present in the mortars immediately after a short-term cure at 20° and 60°C, calcium carboaluminate (C4ACH11) at 60°C, monosulfate at 85°C, and hydrogarnet at 85°C and above. Ettringite started to form after an induction period ranging from several days to several months after the initial heat cure at 85/100°C, and developed substantially during the period of expansion of the mortar associated with delayed ettringite formation (DEF). Ettringite growth was also observed in the nonexpansive cement mortars. Development of the ettringite bands occurred exclusively in the expansive mortars. Although monosulfate observed in the mortars that had been heat cured at 85°C sometimes increased in amount on initial storage at room temperature, it appeared to vary little in amount for up to 1 year. The amount of hydrogarnet in the heat-cured cement product did not change significantly during storage at room temperature for more than 1 year. DEF expansion of the heat-cured mortars was attributed to ettringite band formation, which started to form at the surface of the cement product and gradually developed inwards.

The Effect of Ettringite Formation on Expansion Properties of Compacted Spray Dryer Ash

Abstract: The results of a series of laboratory tests performed to correlate swelling with ettringite formation are presented in this paper. A spray dryer ash was compacted according to ASTM Standard procedures (ASTM D698) and allowed free access to water over an extended period of time. Volume change was recorded while X-ray diffraction and scanning electron microscopy were used to measure changes in the mineralogical composition of the ash. After several days, the formation of ettringite like minerals was apparent. Swelling, however was minimal.

Identification of Composite Cement Hydration Products by Means of X-Ray Diffraction

Abstract: In this paper the effect of limestone, fly ash, slag and natural pozzolana on the cement hydration products is studied. Four composite cements containing limestone, natural pozzolana from the Milos Island, slag and fly ash have been produced by intergrinding clinker (85%), the above main constituent (15%) and gypsum. The grinding process was designed in order to produce cements of the same 28d compressive strength. The hydrated products, formed after 1–28 days, were studied by means of X-ray diffraction. Unhydrated calcium silicate compounds of clinker and hydration products such as C*H, C*S*H and ettringite are clearly observed. Although there is not significant differentiation among samples hydrated for the same period of time, modifications of calcium aluminate hydrates as well as sulfoaluminate hydrates, are indicated by the XRD patterns. In samples of limestone cement, monocarboaluminate is formed in the first 24 hours and is still present after 28 days.

Stucco mortar and binder containing additives

Abstract: of EP0856495A plastering mortar, which contains aggregate and binder based on lump gypsum or its mixtures with floor plaster gypsum in the ratio 1:0 to 1:1, as well as optional light aggregates and/or retarder, includes, as aggregate, ettringite in a gypsum:ettringite wt. ratio of 75:25 to 40:60. Preferably, the light aggregate is expanded perlite or vermiculite and the retarder is tartaric acid containing up to 2 wt.% calcium hydroxide.

Influence of Portland cement composition on pozzolanic reactivity of metakaolin

Abstract: Reactions of metakaolin with lime as well as in presence of gypsum and additionally C3A were studied. Also the mixture of metakaolin with two industrial cements was examined. The results show that in the mixture with lime hydrogehlenite and C-S-H were formed but, in the presence of gypsum, firstly ettringite crystallises but thenafter the typical sequence of reaction took place with ettringite decrease and monosulphate formation. Additionally, gypsum activates the pozzolanic reactivity of metakaolin. The type of clinker and, especially, the potassium content has a pronounced influence on the strength development of blended cement.

Resume of ettringite formation and stabilization and its influences to materials

Abstract: ? The reseach results are resumed that ettringite formation and stabilization and influences tomaterials has been done. 〔