Showing papers on "Ettringite published in 2004"

Dehydration and rehydration processes of cement paste exposed to high temperature environments

Abstract: Microstructural changes of an OPC cement paste after being exposed at various elevated temperatures and further rehydration have been evaluated using 29Si MAS-NMR. Thermogravimetry and XRD are also employed to complement the information. NMR studies of cement paste exposed to high temperatures demonstrate a progressive transformation of C-S-H gel that leads at 450°C, to a modified C-S-H gel. For temperatures above 200°C to a progressive formation of a new nesosilicate. At 750°C, the transformation of C-S-H is complete into the nesosilicate form with a C2S stoichiometry close to larnite, but less crystalline. Also is observed an increase of portlandite that takes place up to temperatures of 200°C. A progressive increase of calcite formation up to 450°C is noticed. The ettringite disappearance below 100°C is confirmed and the portlandite and calcite are converted to lime at 750°C. The initial anhydrous phases as larnite and brownmillerite remain unaltered during heating. Rehydration of the heated samples (450 and 750°C) shows recrystallization of calcite, portlandite and ettringite, and the C-S-H reformation from the new nesosilicate. The larnite and brownmillerite remain unaltered during rehydration. The developing of damaged due to the formation of microcracking is detected and improved because of rehydration phenomena.

Studies on Sulfate-Resistant Cement Stabilization Methods to Address Sulfate-Induced Soil Heave

Abstract: Performance of pavements has been affected by heave distress problems caused by sulfate rich soils treated with calcium-based stabilizers. A research study was conducted to address the effectiveness of sulfate resistant cement stabilizers Types I/II and V, for providing better treatment of sulfate rich soils. Experiments were designed and conducted on both control and cement treated sulfate soils to investigate compaction relationships, Atterberg limits, linear shrinkage and free swell strain potentials, unconfined compressive strength, and low strain shear moduli properties. This paper presents a comprehensive summary and analysis of these test results. Test results were statistically analyzed to study the potentials of sulfate resistant cement stabilization methods for significant enhancements to the strength and stiffness properties as well as reductions in swell and shrinkage strain potentials of natural sulfate rich soils. Mineralogical studies were used to verify research findings observed from the macro test results.

The paste hydration of brownmillerite with and without gypsum: A time resolved synchrotron diffraction study at 30, 70, 100 and 150 ??C

Abstract: Synchrotron energy dispersive diffraction has been used to monitor the mineral transformations which occur in the paste hydration of brownmillerite Ca2AlFeO5 in a closed hydrothermal environment at temperatures in the range 30 to 150 °C. In the absence of sulfate brownmillerite reacts rapidly to form a metastable hexagonal hydrate with a basal spacing of 1.07 nm. As temperature increases the metastable hydrate transforms to the stable hydrogarnet phase Ca3(Al,Fe)2(OH)12 with an Al/(Al + Fe) ratio of about 0.4. The lack of iron in the metastable hydrate suggests a through-solution mechanism while the iron content of the hydrogarnet indicates it must form on the surface of the brownmillerite, probably in contact with the Fe-rich residue remaining after dissolution. The addition of gypsum CaSO4·2H2O increases the rate of brownmillerite hydration. Ettringite Ca6Al2(SO4)3(OH)12·26 H2O is formed at ambient temperature and is replaced by calcium aluminium monosulfate-14 hydrate Ca4Al2O6(SO4)·14H2O at elevated temperatures (70, 100 and 150 °C). Transient increases in gypsum accompany this transformation and all three phases can co-exist at 70 °C. Monosulfate-14 is stable at 150 °C. Increases in brownmillerite peak intensities accompany the conversion of ettringite to monosulfate and are a result of sedimentation. This has implications for paste rheology and slurry design in oilwell cements at elevated temperatures.

Tricalcium aluminate hydration: Microstructural observations by in-situ electron microscopy

Abstract: Environmental scanning electron microscopy (ESEM) was used to study the microstructural changes that take place during the hydration of tricalcium aluminate (C3A) in the absence and presence of gypsum (CS¯H2; where A = Al2O3, C = CaO, H = H2O, S¯ = SO3). The ESEM proves to be a valuable tool in the observation of cement hydration and no specialised equipment other than the ESEM is required. The hydration process can be observed at any time without the need to halt the hydration process prior to specimen preparation. Subsequently, artefacts associated with specimen preparation, such as water loss and desiccation, are now avoided. In the absence of sulphate, amorphous gel, poorly crystalline hexagonal calcium aluminate hydrate (? C4AH19) and cubic calcium aluminate hydrate (C3AH6) are observed on the surface of C3A grains. When small amounts of sulphate (2% gypsum) are present the same phases are observed. If larger amounts of sulphate (25% gypsum) are added to the system amorphous gel products, crystalline ettringite (C6AS¯3H32) and monosulphate (C4AS¯H12) are observed. The crystalline products grow both within the amorphous gel and, where space allows, in interstices suggesting a through solution mechanism of transport.

Microstructural changes in concretes with sulfate exposure

Abstract: In prior papers the responses of concretes to 50,000 ppm MgSO4 exposure depending on cement type, w/cm and the presence of slag were described. The present paper completes this analysis by examining the effects of immersion of concretes produced using slag blended cements, in solutions containing 50,000 ppm of sodium sulfate. The spatial evolution of microstructure associated with carbonation and sulfate attack show differences which can be related to the nature of the cation associated with the sulfate, the cement type, and the w/cm ratio.

Tricalcium aluminate hydration: Microstructural observations by in-situ electron microscopy

Abstract: Environmental scanning electron microscopy (ESEM) was used to study the microstructural changes that take place during the hydration of tricalcium aluminate (C3A) in the absence and presence of gypsum (CS¯H2; where A = Al2O3, C = CaO, H = H2O, S¯ = SO3). The ESEM proves to be a valuable tool in the observation of cement hydration and no specialised equipment other than the ESEM is required. The hydration process can be observed at any time without the need to halt the hydration process prior to specimen preparation. Subsequently, artefacts associated with specimen preparation, such as water loss and desiccation, are now avoided. In the absence of sulphate, amorphous gel, poorly crystalline hexagonal calcium aluminate hydrate (? C4AH19) and cubic calcium aluminate hydrate (C3AH6) are observed on the surface of C3A grains. When small amounts of sulphate (2% gypsum) are present the same phases are observed. If larger amounts of sulphate (25% gypsum) are added to the system amorphous gel products, crystalline ettringite (C6AS¯3H32) and monosulphate (C4AS¯H12) are observed. The crystalline products grow both within the amorphous gel and, where space allows, in interstices suggesting a through solution mechanism of transport.

Durability of ternary binders based on Portland cement, calcium aluminate cement and calcium sulfate

Abstract: The applications of systems based on Portland Cement (PC), Calcium Aluminate Cement (CAC) and Calcium Sulphate (C$) are usually limited to indoor uses in the field of Building Chemistry because no durability data exist on the outdoor uses. These ternary binders have different phase compositions and microstructure than those of the products based purely on Portland cement. Whereas Portland cement is widely studied, there exists very little published data on the performance of these materials over time. This research work aimed at understanding the development of the microstructure of these ternary systems and identifying the degradation mechanisms under controlled atmosphere and under natural weathering. The influence of the formulation was thoroughly studied with respect to hydration mechanism, porosity and transport properties. SEM (BSE observations and EDS microanalysis) was the main analytical technique to understand the hydration mechanisms and was also coupled with XRD, TGA and 27Al NMR. Porosity was examined by solvent exchange and desorption isotherm. Transport properties were studied through oxygen diffusion, oxygen permeability and water sorption. In parallel, performance of different systems were tested with respect to the storage, carbonation, acid corrosion and external sulphate attack. Alongside with that study, the degradation mechanisms after 3 years of natural weathering were examined. The microstructural investigations of the cementitious matrix show that the hydration mechanisms of Portland-rich systems depend on the PC/CAC and CAC/C$ ratios but all present a delay of the silicates' hydration. The CAC-C$ rich binders lead to ettringite and AH3 formation. Despite different hydration mechanisms, Portland and CAC-C$ rich binder at similar porosity have similar transport properties. A first durability "map" was drawn with the data from accelerated ageing. It was found that binders containing portlandite provide superior resistance to carbonation but were more sensitive to sulphate attack, those with AH3 perform better in acidic media. The 3 years natural weathering reveals that samples underwent leaching and carbonation and the mechanical strengths remain the same from 1 to 3 years.

Effect of Remineralization on Heavy-Metal Leaching from Cement-Stabilized/Solidified Waste

Abstract: Crushed samples of stabilized/solidified (s/s) waste were leached at constant leachate pH in the pH range 4-7 with nitric acid solutions to evaluate the influence of remineralization on metal release. The s/s waste consisted of synthetic heavy-metal sludge containing 0.1 mol L(-1) copper nitrate, 0.1 mol L(-1) zinc nitrate, and 0.1 mol L(-1) lead nitrate mixed with ordinary Portland cement. Unleached and leached particles were characterized by scanning electron microscopy and energy-dispersive X-ray spectrometry. Two consecutive leaching fronts advancing from the surface of the particles toward the center were identified: the first front was associated with the dissolution of portlandite and partial reaction of the calcium silicate hydrate gel, while the second front was associated with the dissolution of calcium-aluminum hydroxy sulfates such as ettringite and monosulfate. At pH 4 and 5, a remineralization zone rich in heavy metals formed immediately behind the second leaching front. The shell extending from the remineralization zone to the surface of the particles was depleted in calcium, sulfate, and heavy metals. As a result of remineralization, heavy-metal releases to the leachate were reduced by factors ranging between 3.2 and 6.2 at pH 4 and between 74 and 193 at pH 5. At pH 6 and 7, remineralization of Pb and Zn occurred further behind the second leaching front and closer to the surface of the particles. The amount of heavy-metal release depended on both the leachate pH and the remineralization factor.

Role of ettringite in the reuse of hydrated fly ash from fluidized-bed combustion as a sulfur sorbent: A hydration study

Abstract: Waste from fluidized-bed combustion (FBC) has a low potential for reuse. One possibility for its recycling lies in a hydration process aimed at reactivating the SO2 sorption ability of the unconverted lime. The formation of ettringite, as well as calcium hydroxide, in the hydrated FBC fly ash to be reinjected into the reactor could be of importance because ettringite is able to play a chemical and physical role in SO2 capture. The aim of this paper is to investigate the conditions under which ettringite is formed by the liquid-phase hydration of FBC waste. To this end, two industrial FBC fly ashes were hydrated at temperatures of 20 and 70 °C, for curing times ranging from 30 min to 96 h. Ettringite concentrations and hydration levels of up to 50 and 74%, respectively, were measured. The experimental data were also employed to set up a simple kinetic model for ettringite synthesis. With respect to the CaSO4 concentration, this reaction was of first order for hydration times up to 8 h and of second order t...

Relationship between delayed ettringite formation and delayed expansion in massive shrinkage-compensating concrete

Abstract: The possibility of delayed expansion induced by delayed ettringite formation in massive shrinkage-compensating concrete was investigated. Mortars were prepared using ordinary Portland cement and a sulfoaluminate-based expansive agent. They were cured in three different moist conditions at elevated temperature in a temperature match conditioning process to simulate the temperature development in the interior of massive concrete, and then at ambient temperature. Changes in quantity and morphology of ettringite were examined using semi-quantitative X-ray diffraction and scanning electron microscopy. The time-dependent length change of restrained mortars was also measured. The results show that, in addition to the quantity of ettringite, the structure of mortar, morphology of ettringite and curing conditions also influenced expansion of mortars induced by delayed ettringite formation. The expansion mechanism of delayed ettringite formation and the relationship between curing conditions and length change, delayed expansion and damage to concrete structure are discussed.

Investigation of the CaO-Al2O3-SiO2-CaSO4-CaCO3-H2O system at 25 degrees C by thermodynamic calculation

Abstract: The phase diagram of the CaO–Al2O3–SiO2–CaSO4–CaCO3–H2O system relevant to cement has been calculated at 25°C and 1 bar pressure. It is composed of 331 stable phase assemblages in equilibrium with the aqueous phase and contains 30 invariant points with 15 solids. Thaumasite appears to be a stable phase with a wide stability domain. Compared to ettringite, thaumasite is stable to lower pH and generally demands higher sulphate concentrations to be stable.

Effect of MgO and gypsum content on long-term expansion of low heat Portland slag cement with slight expansion

Abstract: The expansion of low heat Portland slag cement with slight expansion (LSE cement) was studied by XRD, SEM and test methods for strength and expansion. Results indicated that under the condition of 4.5–5.0% MgO in clinker and 2.8–3.4% SO 3 in cement, ettringite expansion and brucite expansion produced by periclase hydration in the paste had continuity, entirety and stability. Periclase hydration in the paste started at about 60 days and was completed up to 2000 days and ettringite was stable from 3 to 2000 days. At the ages of 28, 90, 365, 730 and 2000 days the expansion of the paste reached 0.08–0.13%, 0.09–0.14%, 0.12–0.17%, 0.13–0.18% and 0.15–0.21%, respectively. At the ages of 2, 28 and 180 days the autogenous volume deformation of mass concrete made out of LSE cement was positive and was 0.0042%, 0.0050% and 0.0066%, respectively and the prestress of the concrete with 2.0% steel bar content was 0.069, 0.060 and 0.082 MPa, respectively. The results suggest that using this cement in mass concrete may compensate for a part of its thermal shrinkage and autogenous shrinkage.

Formation of Ettringite from Monosubstituted Calcium Sulfoaluminate Hydrate and Gypsum

Abstract: The formation of ettringite (3CaO·Al2O3·3CaSO4·32H2O) from monosulfate (3CaO·Al2O3·CaSO4·12H2O) and gypsum (CaSO4·2H2O) was investigated by isothermal calorimetry and X-ray diffraction (XRD) analyses Hydration was carried out at constant temperatures from 30° to 80°C using deionized water and 02M, 05M, and 10M sodium hydroxide (NaOH) solutions Ettringite was found to be the dominant crystalline phase over the entire temperature range and at all sodium hydroxide concentrations A sodium-substituted monosulfate phase was formed as a hydration product in the 10M sodium hydroxide solution regardless of temperature XRD and calorimetry demonstrate that hydration in increasing sodium hydroxide concentrations decreases the amount of ettringite formed and retards the rate of reaction The apparent activation energy for the conversion of the monosulfate/gypsum mixture to ettringite was observed to vary depending on the sodium hydroxide concentration Ettringite formation was observed to depend upon the concentration of calcium in solution; thus the formation of calcium hydroxide and sodium-substituted monosulfate phase competes with ettringite formation

Cement stabilization of heavy-metal-containing wastes

Abstract: Cement has found wide usage in the stabilization of heavy-metal-containing wastes as cement minerals can substantially reduce heavy metal solubility as a result of precipitation, adsorption to the surfaces and incorporation. The solubility of some heavy metal cations is limited by the precipitation of hydroxides, while that of some oxyanions is limited by the formation of Ca salts. Only ions that are sufficiently soluble in basic media will be incorporated in or sorbed to hydrated cement minerals to a significant degree. Heavy metal cations may sorb quite strongly to calcium silicate hydrate (C-S-H). The cations diffuse into the C-S-H particles where they are probably sorbed to the silicate chains. Pure phases of oxyanion-substituted ettringite (3CaO·Al 2 O 3 ·3(CaSO 4 )·32H 2 O, an AFt phase) and monosulphate (3CaO·Al 2 O 3 ·CaSO 4 ·12H 2 O, an AFm phase) phases and solid solutions with SO 4 2 − and OH − have been synthesized. Some thermodynamic data are available for the pure phases. For most elements an approximate range of solubility has recently become known. However, it is not possible to predict solubility from the available data.