Showing papers on "Ettringite published in 1996"

Delayed ettringite formation — Processes and problems

Abstract: The scope of the delayed ettringite formation (DEF) problem is reviewed. Based on the experience of the writer and others, DEF is to be expected in both steam cured and non-steam cured concrete. The microstructural details appear identical. The common association with alkalisilica reactions is explored, and explained in terms of both prior cracking providing space for ettringite and in terms of the reduction in alkali hydroxide concentration in the pore solution attendant on ASR. The controversy concerning basic DEF mechanism between proponents of ‘homogenous paste expansion’ and ‘crystal pressure’ is explored. In the writer's opinion the microstructural evidence is not compatible with homogeneous expansion, but is a distinct consequence of crystal pressure developed by ettringite. The thermodynamic analysis that limits expansive pressure in terms of the degree of supersaturation that can be supported may be applicable to ettringite in air voids, but is inapplicable to ettringite in cracks. Fracture mechanics considerations predict local stress concentrations at the crack tips that are many times those that can be generated in open spaces. An apparent example of such effects in opening up space between mica lamellae in steam cured concrete by ettringite deposition is provided.

Initial hydration reactions and mechanisms of delayed ettringite formation in Portland cements

Abstract: The aluminate hydration reactions in Portland cements were followed by X-ray diffraction. Using a computerized focusing Huber-Guinier diffractometer, the patterns were obtained from the pastes directly, without preliminary drying or grinding. It is shown that the widely accepted theories on the hydration of C3A in the presence of calcium sulfate cannot be applied generally to commercial Portland cements. Only in cases of pastes nearly free of CO2 will the ettlingite formed in the course of the induction period transform to monosulfate. In cement pastes containing more than about 0.5% CO2 the conversion is prevented and monosulfate is replaced by hemicarbonate or monocarbonate. The quantity of CO2 sufficient to replace monosulfate by carbonated AFm phases depends on the ratio of SO3 to C3A and the amount of available Al2O3. The results clearly show that with cements or concretes containing monosulfate the risk of delayed ettringite formation as a result of carbonation reactions should be taken into account. The interlayer sulfate groups in the crystal structure of monosulfate can be replaced by carbonate. This exchange results in increasing sulfate concentration in the pore solution and thus leads to delayed ettringite formation.

Thermal decomposition of ettringite Ca6[Al(OH)6]2(SO4)3·26H2O

Abstract: The thermal decomposition of ettringite in the presence of water and impurity gypsum has been observed by energy-dispersive synchrotron X-ray diffraction and FTIR spectroscopy. The decomposition occurs rapidly at 114 °C to produce a monosulfate, with 0.96 nm interlayer spacing, and bassanite. The thermal expansion of ettringite from 70 to 110 °C is 42 × 10–6 K–1 in the a direction and 22 × 10–6 K–1 in the c direction. The gypsum acts as a transient intermediate in the decomposition. The mechanism of this rapid decomposition is discussed.

Delayed ettringite formation : A concern for precast concrete ?

Abstract: Internal sulfate attack on precast concrete products has been investigated by the author. The cause for these failures has been identified as delayed ettringite attack, a process whereby the normal formation of ettringite in plastic portland cement concrete is either delayed or is formed and then decomposed, only to destructively reform in the hardened concrete after months or years of exposure to water. Reasons for the apparent newness of the phenomenon, and methods for preventing it, are given on the basis of investigations of incidences during the past several years.

The role of sulfate mineralogy and cure temperature in delayed ettringite formation

Abstract: Warm-cured cement pastes tend to become expansive during service at normal ambient temperatures. Evidence that delayed ettringite formation is responsible is critically reviewed and it is concluded that ettringite is partially or completely destabilised during thermal cure. These features are corroborated and quantified by thermodynamic calculations. Reformation of ettringite at ambient temperature has potential for expansion but other associated reactions could also contribute. The relevant chemical and mineralogical balances are reviewed. Some tentative suggestions are given to avoid or minimise delayed ettringite formation but it is emphasised that further focused research is required to quantify the causes of expansion and formulate suitable protocols to avoid dimensional instability.

On the delayed expansion of heat cured Portland cement pastes and concretes

Abstract: Under high temperature heat curing conditions the C 3 A phase of Portland cement hydrates to yield monosulfate. The latter phase converts to ettringite during subsequent curing at ambient temperature, but only after an induction period of many months. This reaction may be associated with an expansion, but only with cements sufficiently high in both C 3 A and SO 3 . The amount of ettringite needed to cause expansion is distinctly lower than that needed to have this effect in cements with excessive SO 3 contents.

Dielectric behaviour of hardened cement paste systems

Abstract: The hydration of Portland cement involves initial dissolution of CaO, CaSO4.1H20 or CaSO4-2H20, alkaline contents and aluminate phases etc. This is followed by the reaction of C3S and C2S with free water and formation of C-S-H, calcium hydroxide (CH), ettringite and other minor compounds, which bridge the individual particles and fill up the capillary pores. These reactions promote formation of a rigid microstructure and strength development. The stability of C-S-H depends on the variation of environmental conditions as evidenced by changes in morphology, interfacial regions, pore structure and intrinsic strength. These characteristics can be studied by means of impedance or conductance measurements. Electrical methods are considered as a nondestructive, rapid way to determine the physical properties, such as strength, porosity and permeability. Electrical conductivity measurement (ECM) [14] and a.c. impedance spectroscopy (ACIS) [5-15] have been used in cement and concrete hydration investigations. Information on hydration behaviour and microstructural development was obtained. These studies indicated that certain parameters obtained from ECM and ACIS are very sensitive to microstructural characteristics of cement paste and concrete. Dielectric constant measurement (DCM) is one of the most popular methods of characterizing solid materials because it is easier to apply than chemical analysis techniques. The electrical, as well as physical and structural, characteristics of hydrated cement systems can be evaluated. There is a paucity of data [16-20] reported on the dielectric properties of cement and concrete, especially at RF frequencies. In this study, the dielectric properties of hardened cement paste at various water/cement ratios were investigated over the frequency range from 1 MHz to 1 GHz. This preliminary work revealed that the dielectric constant value of hydrated cement paste varies indirectly with original water/cement ratio. Type 10 Portland cement was used. The chemical composition (wt%) was as follows: SiO 2 19.83; CaO 61.21; Fe203 3.20; A1203 4.18; MgO 4.09; SO3 3.93; Na20 0.45; K20 0.82. Fresh cement was mixed at various water/cement ratios from 0.3 to 0.7 and cast in cylindrical moulds, 3 cm diameter × 6 cm. The samples were cured in 100% relative humidity environment for 24 h and subsequently immersed in saturated lime solution for 1 year. Cement paste samples were sliced to form 3 mm thick discs and

DEF: As a form of sulfate attack

Abstract: Delayed ettringite formation (DEF) is the term used to describe expansion of some heat cured portland cement concretes. Although meaningful progress has been made, the exact mechanism of the expansion itself is not entirely understood. The ettringite reformation is enabled, primarily, by thermal decomposition of ettringite formed prior to or during the cement paste setting as a result of excessive/improper heat curing. Observance in mature concrete of crystalline ettringite in air voids, cracks, and gaps around the aggregate is not in itself an evidence for DEF, but is the consequence of natural recrystallization of ettringite into available larger spaces. Occurance of DEF is dependent on materials, curing, and environmental conditions, and can easily be distinguished from ASR and other deterioration mechanisms by optical and electron-optical methods.

Durability of concrete with addition of limestone powder

Abstract: The paper describes the influence of powdered limestone and water-cement (w/c) ratio on the durability of concrete in 5% Na2SO4. Before immersion in sulphate solution the compressive strength, absorbability of concretes and phase composition of pastes were tested. Sulphate durability was assessed by means of a linear deformation of concrete and X-ray diffraction analysis of pastes with and without addition of limestone powder: The results show that a beneficial influence of powdered limestone on concrete durability was observed when w/c <0·60. Above this value, the powdered limestone has almost no essential effect on sulphate resistance. The addition of limestone powder to cement prevents the transformation of ettringite to monosulphate and hemisulphate. Monocarbonate and hemicarbonate were formed instead of those phases.

Microstructural and phase characteristics of phosphogypsum-cement mixtures

Abstract: The effects of the tricalcium aluminate (C 3 A) content of the stabilizing cement [Type II (1.3% C 3 A) cement and Type III cement (9.3% C 3 A)], curing time (seven to 90 days), proportion of PG (5–60%), grain size, and impurities in phosphogypsum (PG) (PG compared to gypsum) on the microstructural (by scanning electron microscopy) and phase properties of gypsum/PG and cement mixtures, a potential road-base material, were studied (by derivative thermogravimetry). Morphologically, two types of ettringite (well-defined crystals, >2 μm in lengths and ill-defined crystals, below 1 μm in length) were present in the cement and gypsum/PG mixtures. The impurities in PG retarded the initial hydration of cement in direct proportion to its amount in the mixtures. However, gypsum/PG increased the degree of hydration of cement in the mixtures in the long term. The reacted gypsum/PG produced a large amount of ettringite (within two months) compared to cement only. The amount of carbonation was uniformly low.

Reaction mechanism of the hydrothermally treated CaO-SiO2-Al2O3 and CaO-SiO2-Al2O3-CaSO4 systems

Abstract: Mixtures of CaO, amorphous SiO2 and kaolinite in the presence or absence of SO 4 2− ions (added as CaSO4·2H2O) were treated in suspension under hydrothermal conditions at temperatures of 80–200 °C. Kaolinite was added to replace 3 and 10% of the total weight of the dry mix with the overall CaO/SiO2 mole ratio being 0.83. The hydration products were investigated by XRD, IR spectroscopy and DTA techniques in order to elucidate their phase compositions. The results indicate that the presence of SO 4 2− ions leads to a marked increase in the reaction rate at all temperatures investigated. In the C-S-A system, the detected hydration products are calcium silicate hydrate which is then transformed into 1.13 nm aluminium-substituted tobermorite and α-C2SH by increasing the autoclaving temperature. In the C-S-A-C¯s system the hydrated products are calcium silicate hydrate, ettringite and monosulpho-alumirtate. On increasing the hydrothermal temperature they decompose, recrystallize and 1.13 nm aluminium-substituted tobermorite, α-C2SH and anhydrite II are formed. In both systems the excess Al2O3 appeared as a hydrogarnet phase, C3ASH4.

Effects of sulphate, calcium and aluminum ions upon the hydration of sulphoaluminate belite cement

Abstract: In the model sulphoaluminate belite cement, the process of hydration is governed by the diffusion and transport phenomena of the main ionic species. The sulphate components and combined sulphate and aluminium ions, exert an accelerating effect upon the kinetics of sulphoaluminate belite cement hydration. Aluminium and calcium ions delay the hydration by creating a retarding layer which can be considered a co-precipitate of aluminium and calcium hydroxides. This is revealed in the calorimetric curves by the duration of induction period and also by the intensities of the main peaks. The appearance of small additional peaks characterizes the formation of 'primary ettringite', due to the presence of sulphate ions in aqueous solution. The intensities of these peaks depend on the ion concentration too.

Hydration in the system C4A3¯S-C¯SH2-CH-H

Abstract: The system C4A3¯S-C¯SH2-CH-H was investigated at room temperature by means of conduction calorimetry to illustrate the role played by the size of the C4A3¯S particles (aggregates) and the balance ratio C¯SH2/CH. These two factors greatly affect the hydration kinetics in the system C4A3¯S-C¯SH2-CH-H and determine the hydrated phases formed. The amount of ettringite formed in the system depends on the C4A3¯S fineness, as determined by calorimetry. Ettringite and monosulphate are the main hydrated products, as identified by X-ray diffraction. The balance ratio C¯SH2/CH determines the mechanism of hydration and the nature of the calcium-aluminium sulphate hydrates.

Early Hydration of Modified Belite Cement Prepared by Adding Borax

Abstract: By adding borax into the raw mix, Modified Belite Cement clinker and cements were prepared. Clinkerization and hydration reactions were investigated in order to better understand. Borax-bearing MBC clinker sintered at 1300℃ for 1 hour showed excellent burnability. Borax stabilized α' and β-C₂S at room temperature. In the hydration of the cement prepared with the borax-bearing clinker, ettringite, monosulphate, C-S-H hydrates and CH were formed. The hydration of calcium sulphoaluminate was less reactive than the cement prepared with the controlled clinker at early hydration time. But, as hydration time elapsed, this cement showed more active hydraulicity and higher compressive strength development.

Reference Spectra for Environmentally Important Secondary Minerals: Ettringite (Ca6Al2(SO4)3(OH)12⋅26H2O) by XPS

Abstract: Ettringite, found in metamorphosed limestone and also as an important secondary mineral in concrete and certain combustion waste environments, plays an important role in concretion strength development, and in the control of pH and immobilization of trace elements in alkaline hazardous wastes. Consequently, the ability to identify ettringite via surface-sensitive spectroscopies is important for understanding cementitious mechanisms and the environmental behavior of wastes. We have used x-ray photoelectron spectroscopy (XPS) to study the binding energy of principal photoelectrons from an air-exposed external crystal surface of a well-formed needle-shaped ettringite sample from a bituminous limestone deposit in Asad, Israel. General survey and high resolution spectra were collected under standard operating conditions using a Perkin Elmer Physical Electronics 5200C spectrometer. Adventitious carbon was used as the energy referencing method. Charge-corrected binding energies were identified for the Ca 2p3/2, Ca 2p1/2, Al 2p, S 2p3/2, S 2p1/2, and O 1s photoelectrons. The mineral contains three oxygen species. Probable loss of waters of hydration resulted in the determination of binding energies for only the hydroxyl and sulfate oxygens. Nominal binding energies for all three species are also provided. These spectra are useful in using XPS to identify ettringite in leaching environments of combustion residues where the secondary mineral precipitated at ash particle surfaces.

Roller compacted base course construction using lime stabilized fly ash and flue gas desulfurization sludge-by-product

Abstract: Dewatered calcium sulfite and calcium sulfate sludges from unoxidized flue gas desulfurization (FGD) processes at coal fired power plants can be mixed with coal fly ash and lime to cause a cementitious chemical reaction used to construct a roller compacted base course (RCFGD) or an impermeable pond liner. The chemical reaction is described as time reacting with alumina from the fly ash which in turn reacts with the calcium sulfite and sulfate FGD waste to form calcium sulfo-aluminate compounds. Leachate data is similar to primary drinking water quality standards. Two field demonstrations of RCFGD and a proposed mix design procedure are described. Factors that affect strength gain and freeze-thaw durability such as optimum moisture content, fly ash to FGD ratio, and age of FGD are discussed. Better understanding is needed on how to predict long term strength performance and expansive potential given the nature of long term hydration forming ettringite compounds and the vulnerability to destructive freeze-thaw cycles.

Hydration and pH Behavior of the Cement Prepared from the Mixture of Haüyne-Containing Clinker, Anhydrite and Blastfurnace Slag

Abstract: Four types of cement were prepared by mixing hauyne-containing clinker, anhydrite and blastfurnace slag (HAS cement), and their hydration reaction and pH behavior were investigated. HAS cement pastes cured for 3-180 d contained ettringite, calcium silicate hydrate (C-S-H) and Al(OH)3 gel as hydrates, but neither Ca(OH)2 nor monosulfate hydrate. Measured pH values of the liquid phases coexisting with hardened HAS cement pastes were in the range of 11.3-12.3, and each of them agreed well with the pH value calculated from ionic concentrations in the liquid phase. Final hydrate of each cement paste was estimated from chemical composition of cement, and then a final pH of liquid phase was presumed from CaO/SiO2 ratio of estimated C-S-H. Measured pH showed similar tendency to the estimated final pH, but the former grew relatively higher than the latter especially in the range of lower pH. It was considered that coexisting alkali ions remarkably influenced liquid phase in that pH range. The compressive strengths of HAS cement mortars cured for 28 d were about 37-39N/mm2.