Showing papers on "Ettringite published in 2003"

A state-of-the-art review on delayed ettringite attack on concrete

Abstract: The paper presents a critical review of the sulphate attack related to ettringite formation. This process is associated with expansion. However, not necessarily any ettringite-related expansion is related to the sulphate attack. For example, early ettringite formation 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 the development of chemical prestress in expansive cements. Delayed ettringite formation (DEF) occurs at late ages and the related heterogeneous expansion in a hardened concrete can produce cracking and spalling. Two different types of DEF are examined depending on the sulphate source: DEF caused by external sulphate attack or internal sulphate attack.

Removal of B, Cr, Mo, and Se from wastewater by incorporation into hydrocalumite and ettringite.

Abstract: Boron, chromium, molybdenum, and selenium often occur in high concentrations in fly ash leachates. During the leaching of fly ash in alkaline environments, hydrocalumite (Ca4Al2(OH)12(OH)2·6H2O) and ettringite (Ca6Al2(OH)12(SO4)3·26H2O) form as secondary precipitates. In this study, the removal of B, Cr, Mo, and Se oxyanions from high pH waters by incorporation into hydrocalumite and ettringite was examined. Experiments were performed by precipitating these minerals in solutions containing B, Cr, Mo, and Se oxyanions at conditions relevant to lime-leaching of fly ash as well as to fly ash containing concrete. The uptake of all four anions by hydrocalumite and ettringite was high. Anion uptake by hydrocalumite was larger than that by ettringite, and hydrocalumite was able to reduce anion concentrations to below drinking water standards. Ettringite showed an anion preference in the order of B(OH)4- > SeO42- > CrO42- > MoO42-. In contrast, borate was least preferred by hydrocalumite. Coordination, size, and ...

Modeling of damage in cement-based materials subjected to external sulfate attack. I: Formulation

Abstract: A chemomechanical mathematical model is presented to simulate the response of concrete exposed to external sulfate solutions. The model is based on the diffusion-reaction approach, and several mechanisms for the reaction of calcium aluminates with sulfates to form expansive ettringite are considered. Fick’s second law is assumed for diffusion of the sulfate ions. A second-order chemical reaction between reacting calcium aluminates and ingressing sulfates depletes the sulfate concentration. The products of the second-order reaction between the aluminates and sulfates are chosen among several competing mechanisms, and a rule-of-mixtures approach is used to relate the expansive nature of the products with the prescribed specific gravity of the compounds. It is furthermore assumed that the crystallization pressure of products of reaction results in a bulk expansion of the solid. The constitutive response of the matrix and the expansive stresses are calculated from the imposed volumetric strain. Microcracks are initiated when the strength of the matrix is reached, leading to changes in the diffusivity and a reduction in the elastic properties of the matrix. The variation of diffusivity is linked to the scalar damage parameter due to cracking of the matrix. Due to the changes in the diffusivity, the problem is treated as a moving boundary problem, and a methodology is proposed to adapt the solution of the 1D case to the 2D problem of a prismatic specimen. Theoretical expansion-time responses are obtained and compared with a variety of data available in the literature.

Modeling of Damage in Cement-Based Materials Subjected to External Sulfate Attack. II: Comparison with Experiments

Abstract: A study is presented to predict the degradation of cement-based materials due to external sulfate attack. Parameters of the model are chosen based on the mix design parameters, degree of hydration, and exposure conditions of concrete. A solution of the diffusion equation with a term for a second-order chemical reaction is proposed to determine the sulfate concentration and calcium aluminate profile as a function of time and space. The crystallization pressure of hydration products such as ettringite can lead to internal stresses. Using the volumetric information, the model predicts the generation of internal stresses, evolution of damage, reduction in stiffness, and thus expansion of a matrix phase. The theoretical expansion-time responses are obtained and compared with a variety of available data in the literature. The most important parameters are the w/c ratio, internal porosity, diffusivity of the cracked and uncracked material, and available calcium aluminates. The importance of controlling the pH of the test solution is clearly observed. Model simulations indicate a reasonable agreement with experimental expansion-time data available in the literature.

Sorption of selenite and selenate to cement minerals.

Abstract: The sorption of selenite and selenate to ettringite (3CaO x Al2O3 x 3CaSO4 x 32H2O), "monosulfate" (3CaO x Al2O3 x CaSO4 x 12H2O), and calcium silicate hydrate (C-S-H) was investigated in order to understand Se immobilization by cement in hazardous wastes. Sorption kinetics were fast with equilibrium between the minerals and Se species reached within 1 d. Selenite is suggested to sorb by surface reactions, and for ettringite, a sorption maximum of 0.03 mol kg(-1) was determined. Distribution ratios (Rd) for selenite were 0.18, 0.38, and 0.21 m3 kg(-1) for ettringite, monosulfate, and C-S-H, respectively. At high selenite additions, CaSeO3 was precipitated with a solubility product of Kso = 10(-7.27) (I = 0, 25 degrees C). Selenate sorbed only weakly to ettringite (Rd = 0.03), and no significant sorption to C-S-H was found. In contrast, sorption to monosulfate was strong (Rd = 2.06). With increasing selenate addition, XRD analyses revealed changes in the interlayer distance of monosulfate, in parallel with an increase of the ettringite fraction. Substitution of sulfate is suggested to be the relevant process. This indicates that selenate is sorbed more efficiently by monosulfate-rich cement, while the cement composition is of minor importance for selenite sorption.

Thaumasite––direct, woodfordite and other possible formation routes

Abstract: Two main formation routes for thaumasite exist below 15 °C. One is the direct route from C–S–H reacting with appropriate carbonate, sulfate, Ca2+ ions and excess water. The other route is the woodfordite route from ettringite reacting with C–S–H, carbonate, Ca2+ ions and excess water, in which thaumasite arises through the intermediate formation of the solid solution woodfordite. The woodfordite route for thaumasite formation appears to be relatively quicker (although still slow) than the direct route, presumably because with the former the ettringite already has the octahedral [M(OH)6] units that can facilitate the critical change from [Al(OH)6]3− to [Si(OH)6]2− groupings. Both routes are mutually dependent on each other. The presence of magnesium salts can modify the path to thaumasite formation. High pressure might be able to stabilise [Si(OH)6]2− groupings and allow thaumasite to become formed above 15 °C. This possibility is discussed.

An experimental study of combined acid and sulfate attack of concrete

Abstract: There is disagreement about the role of sulfuric acid in the thaumasite form of sulfate attack (TSA) of concrete. Some researchers suggest that thaumasite is formed only at pH above 10.5, whereas others report that the primary cause of deterioration in the affected M5 bridge foundations was sulfuric acid attack followed by neutral TSA. The aim of this work is to reconcile these conflicting views by undertaking parallel studies of concrete exposed to aggressive acid and sulfate solutions and concrete/clay interface work using weathered Lower Lias clay. Concrete specimens have been exposed to BRE Digest 363 sulfate class solutions and acidic and acidic-sulfate solutions at 4.5 ± 0.5 °C. Selected samples are being characterised at intervals up to 5 years. At this stage, results are reported for 5-month samples. Various binders including Portland cement, Portland–limestone cement, blastfurnace slag cement, pulverized-fuel ash cement and sulfate-resisting Portland cement at water/binder ratios (w/b) from 0.35 to 0.5 have been studied. Initial visual observations and X-ray diffraction analyses have identified thaumasite in some of the systems after 5 months immersion in solution. An overview of the ongoing parallel concrete/clay interaction work is also presented to contextualise the concrete work.

Swiss tunnel structures: concrete damage by formation of thaumasite

Abstract: The severity of concrete deterioration is related to the risk of failure of the construction and to the costs of repair. Severe conditions may also be related to interactions with water regarded as weakly aggressive, although these processes are slow and long periods of time are needed for heavy deterioration. We often encounter thaumasite in concrete exposed to ground water with high as well as with low sulfate contents. Thaumasite may form relatively thin layers near the concrete surface in contact with the water as well as larger continuous volumes of completely altered concrete. Based on the description of macro- and microscopic textures and the chemical alterations, a succession of corrosive reactions is postulated. It seems that natural ground water conditions in the Swiss Alps preferentially lead to a sulfate attack with the formation of thaumasite rather than the formation of ettringite. The consequences of these findings are discussed for underground constructions, properties of concrete used as well as for performance testing regarding sulfate attack.

Evolution of ettringite in presence of carbonate, and silicate ions

Abstract: The main objective of this work was to study the evolution of ettringite when in contact with a solution containing carbonate and silicate ions. Ettringite was synthesised and put in contact with three types of saturated solutions: (a) CaCO3 and silica gel saturated solution; (b) MgCO3 and silica gel saturated solution and (c) C–S–H gel plus portlandite and CaCO3 saturated solution. The temperature of the experiment was 4 ± 2 °C. At different ages the samples were studied by: FTIR, XRD, NMR, and SEM/EDX. Ettringite evolves with time, reacting with carbonates and silicates and decomposing to poorly crystallised products when it is in contact with solutions (a) and (b). Ettringite decomposition rate is much higher when it is in contact with saturated magnesium carbonate solution (b) than when in contact with the solution (a). After 90 days of exposure in solution (b) ettringite decomposition starts, followed by reorganization of the atoms giving place to a semi-stable poorly crystallised compound. However 130 days are necessary for ettringite decomposition in solution (a). Ettringite in contact with solution (c) evolves more slowly and it almost remains unchanged after 380 days of treatment. Ca(OH)2 formed in the C3S and C2S hydration retards ettringite decomposition.

Investigations on the influence of cement type on thaumasite formation

Abstract: A high sulfate resistance is required if cements are to be used in sulfate bearing waters and soils especially under conditions favouring thaumasite formation. A long period program of laboratory investigations was carried out on CEN cements to assess thaumasite and ettringite formation. The experimental concept involved mixing ground cement pastes with stoichiometrical components of gypsum, calcite and water. The specimens were stored at 6 °C whereby chemical worst case conditions for thaumasite formation were simulated. At time intervals XRD analysis was conducted. Apart from pure cements mixtures containing additives, pure C3S pastes with and without Al2O3 addition were investigated. The results confirm that thaumasite formation can be accelerated by Al2O3 bearing components in cements. However, thaumasite formation is also possible without active participation of Al3+. The assessment of sulfate resistance of cements only from the chemical point of view apparently gives results which are contrary to the field experience.

Thaumasite formation in concrete and mortars containing fly ash

Abstract: Due to recent reports on deterioration of concrete structures, the thaumasite form of sulfate attack has become a subject of study and close investigation This paper investigates the formation of thaumasite in concrete and mortars containing fly ash The results show that thaumasite formation can occur within 84 days of exposure to sulfate solutions High volumes of fly ash can limit or promote thaumasite formation depending on the type of cement used Thaumasite and ettringite were found among the deterioration products However, the thaumasite formation in the specimen prepared from sulfate resisting Portland cement was not accompanied by deterioration, except by 50% fly ash addition The mixtures of Portland limestone cement with 40% fly ash exhibited a very limited thaumasite formation while the mixtures with 50% had no thaumasite at all It is concluded that thaumasite can also be formed in mixtures incorporating fly ash

The occurrence of thaumasite as a product of seawater attack

Abstract: This paper describes the mechanism of the rapid deterioration caused by seawater attack on the bedding mortar of recently constructed harbour wall steps in South Wales. Within 2 years, the mortar had suffered severe cracking and spalling. The degraded mortars were examined under optical and electron microscopes and also using X-ray diffraction analysis. The reaction products formed included thaumasite, ettringite, brucite and hydrated magnesium silicate. A further end product of deterioration was the precipitation of large, non-interlocking crystals of calcite (termed popcorn calcite) in the areas of the mortar previously occupied by cement paste. Investigation of degradation of associated structural concrete from this site also showed the early stages of the thaumasite form of sulfate attack.

Raman scattering and X-ray diffraction study of the thermal decomposition of an ettringite-group crystal

Abstract: A Raman scattering and X-ray diffraction study of the thermal decomposition of a naturally occurring, ettringite-group crystal is presented. Raman spectra, recorded with increasing temperature, indicate that the thermal decomposition begins at ≈55 °C, accompanied by dehydration of water molecules from the mineral. This is in contrast to previous studies that reported higher temperature breakdown of ettringite. The dehydration is completed by 175 °C and this results in total collapse of the crystalline structure and the material becomes amorphous. The Raman scattering results are supported by X-ray diffraction results obtained at increasing temperatures.

Thaumasite formed by sulfate attack on mortar with limestone filler

Abstract: Early evidence of thaumasite formation in mortar with limestone filler exposed to sulfate containing tunnel water in Norway is reviewed. The problem is discussed in light of the new European cement standard allowing cements containing up to 35% limestone (e.g. CEM II/B-L) rendering them prone to detrimental sulfate attack. Experiments are performed where mortars with 20% limestone or quartz filler, respectively, are stored in 5% sodium sulfate solution saturated with gypsum at 5 °C. Length change, flexural strength and compressive strength are measured periodically for a year. The microstructure of the mortars is inspected by scanning electron microscopy and energy dispersive analyser of X-rays documenting the formation of sulfate containing species including ettringite and thaumasite.

Occurrences of thaumasite in laboratory and field concrete

Abstract: The mineral thaumasite (CaSiO 3 · CaCO 3 · CaSO 4 · 15H 2 O) has been observed by the authors in a number of concrete samples including (i) an 80 year-old aqueduct in Manitoba, (ii) a 33 year-old pavement in Ontario, (iii) test samples exposed to marine (tidal) conditions, and (iv) laboratory samples exposed to wet–dry cycles in sulphate solution. The source of carbonate differed for these cases being variously derived from de-dolomitization of dolostone aggregate, carbonate ions in the seawater, or atmospheric CO 2 through the process of carbonation. In some cases the thaumasite mineral was found in close association with the mineral ettringite (3CaO · Al 2 O 3 · 3CaSO 4 · 31H 2 O). The paper discusses the results of detailed analyses using optical and electron microscopy. Although the mineral thaumasite can be readily identified by these techniques, it is possible that the thaumasite form of sulphate attack is frequently misdiagnosed as conventional sulphate attack due to the similarities in crystal structure and appearance of the minerals thaumasite and ettringite.

Assessment of Self-Compacting Concrete Immersed in Acidic Solutions

Abstract: Ettringite and thaumasite can be found among the deterioration products of cementitious materials exposed to sulfate and hydrochloric attack. The results of a test program to investigate the acid resistance of self-compacting concrete (SCC) and conventional concrete (CC), immersed up to 18 weeks at 20°C in sulfuric and hydrochloric acid solutions, are described. The SCC was prepared with 47% carboniferous limestone powder, as a replacement for cement, and an ordinary portland cement. The CC was prepared with portland cement only. The water-binder ratios of the SCC and CC were 0.36 and 0.46, respectively. The parameter investigated was the time, in weeks, taken to cause 10% mass loss of fully immersed concrete specimens in a 1% solution of sulfuric acid and the same amount of loss in a 1% solution of hydrochloric acid. The investigation indicated that the SCC performed better than the CC in sulfuric solution but was slightly more vulnerable to hydrochloric acid attack compared to CC. The mode of attack bet...

Field experiences in concrete deterioration by thaumasite formation: possibilities and problems in thaumasite analysis

Abstract: In Germany, after testing two concretes deteriorated due to the formation of thaumasite, possibilities and problems in analysing thaumasite and ettringite-containing concrete samples will be discussed. The image of polished thin sections show, that thaumasite formation leads to microstructures similar to the microstructure of ettringite. It is recognisable in the microstructure that the formation of thaumasite happens as well via deterioration of CSH-phases as via the decomposition of ettringite. The identification of thaumasite using environmental scanning electron microscopy is possible without problems. Investigations using X-ray powder diffraction did show, that quantitative analysis of thaumasite and ettringite is possible by using the Rietveld method. The samples of deteriorated concrete (excluding coarse aggregate) contain 20–75% thaumasite.