Showing papers on "Ettringite published in 2017"

Zeolite formation in the presence of cement hydrates and albite

Abstract: Zeolite formation caused by interactions between cement hydrates and rock forming minerals was investigated by two sets of batch experiments and supported by thermodynamic modelling. The first set of batch experiments investigated the interaction between calcium silicate hydrates (C-S-H) (Ca0.8SiO2.8·32H2O) and ettringite (Ca6Al2(SO4)3(OH)12(H2O)26) as cement hydrate minerals and albite (NaAlSi3O8) as a rock forming mineral at 20, 50 and 80 °C. The dissolution of C-S-H, ettringite and albite led to relatively high calcium and low silicon and sodium concentrations and to the formation of zeolite P(Ca) (Ca2Al2Si2O8·4.5H2O) and natrolite (Na2Al2Si3O10·2H2O). The second set of experiments used ettringite and silica fume as cement phases and NaAlO2 to represent a rock forming mineral. High initial sodium, hydroxide and aluminium concentrations were observed leading to the precipitation of zeolite X (Na2Al2Si2.5O9·6.2H2O) and C-S-H gel at 20 and 50 °C where only 40–60% of the silica had reacted after 3 years. At 80 °C where more silica fume had reacted, the formation of SiO2-rich zeolite Y (Na2Al2Si4O12·8H2O) and chabazite (CaAl2Si4O12·6H2O) was observed. Solubility products for the zeolite P(Ca), natrolite, chabazite, zeolite X and zeolite Y were obtained from the measured concentrations. Comparison with values published in the literature shows a high variability due to the flexibility of the Si to Al ratio in zeolite structures and underlines the need for systematic experimental determination of the solubility of different zeolites.

Micro-mechanical Properties Of Calcium Sulfoaluminate Cement And The Correlation With Microstructures

Abstract: To investigate the micro-mechanical properties of calcium sulfoaluminate cement and the correlation with the microstructures, we apply a variety of advanced techniques of microstructural and micro-mechanical characterization, including scanning electron microscopy with backscattered electron and energy-dispersive X-ray spectroscopy detectors, X-ray fluorescence, X-ray diffraction and nanoindentation. For the first time, the micro-mechanical properties of material microstructures present in a calcium sulfoaluminate cement are estimated. In the calcium sulfoaluminate cement used in this research, two type of hydration product microstructures with the differentiable microstructural morphologies, compositions and micro-mechanical properties are identified and investigated. The correlation of the micro-mechanical properties with the microstructures shows that the hydration product microstructure containing more ettringite has lower indentation modulus and hardness than that containing more aluminum hydroxide.

Synthesis of low-temperature calcium sulfoaluminate-belite cements from industrial wastes and their hydration: Comparative studies between lignite fly ash and bottom ash

Abstract: The aim of this research was to study the production of calcium sulfoaluminate-belite (CŜAB) cement from industrial waste materials via hydrothermal-calcination process. Lignite fly ash and bottom ash were used as starting materials for comparison. Other waste materials viz., Al-rich sludge and flue gas desulfurization gypsum were also key players in raw mixes for the synthesis of CŜAB cement. For lignite fly ash as a starting material, mixed phases between ye'elimite and larnite were obtained, whereas for lignite bottom ash as starting material, only ye'elimite was obtained The hydration reaction was studied in terms of heat evolution, setting time, compressive strength and hydration product formation with various gypsum contents. The results showed a rapid formation of ettringite as a main hydration product mixed with calcium silicate hydrate, monosulfate and stratlingite phases as minority, with a fast final setting time of 24–26 min and high early compressive strength of 16.0 and 18.0 MPa in 1 day for CŜAB cements made of fly ash and bottom ash, respectively.

Influence of curing temperatures on the hydration of calcium aluminate cement/Portland cement/calcium sulfate blends

Abstract: In this paper, the effects of curing temperature on the hydration of calcium aluminate cement (CAC) dominated ternary binders (studied CAC: Portland cement: calcium sulfate mass ratio were 22.5: 51.7: 25.8) were estimated at 0, 10, 20 and 40 °C, respectively. Both α-hemihydrate and natural anhydrite were employed as the main source of sulfate. The impacts of temperature on the phase assemblages, morphology and pore structure of pastes hydrated up to 3 days were determined by using X-ray diffraction (XRD), backscattered electron imaging (BEI) and mercury intrusion porosimetry (MIP). Results reveal that the main hydration products are firmly related to calcium sulphoaluminate based phases. Increasing temperature would result in a faster conversion from ettringite to plate-like monosulfate for both calcium sulfate doped systems. When the temperature increases to 40 °C, an extraordinary formation of stratlingite (C 2 ASH 8 ) and aluminium hydroxide is observed in anhydrite doped pastes. Additionally, increased temperature exerts different effects on the pore structure, i.e. the critical pore diameter shifts to finer one for pastes prepared with α-hemihydrate, but changes to coarser one for those made with anhydrite. From the mechanical point of view, increased temperature accelerates the 1-day strength development prominently, while exerts marginal influence on the development of 3-day strength.

Phase assemblage in ettringite-forming cement pastes: A X-ray diffraction and thermal analysis characterization

Abstract: The study attempts to describe the evolution of the solid phase composition with ongoing hydration in three different calcium aluminate rich cement paste mixtures by means of XRD and thermal analysis. The phase assemblage was followed quantitatively at discrete ages of 1, 7, 28, 56 and 90 days. Ettringite was the main crystalline hydration product. Quantification of amorphous fractions using the external standard method was performed and relatively high amounts of amorphous fractions were reported in all the cases. Thermal analysis revealed that the X-ray amorphous hydrate fraction was mainly composed of monosulphate, AH 3 and C-S-H. The presence of stratlingite, was not clearly manifested in any of the DTG curves. A mass balance calculation based on stoichiometric reactions was performed in order to estimate the amounts of monosulphate, AH 3, and C-S-H. The quantities of the amorphous portions obtained from QXRD were observed to be higher as those estimated from mass balance calculations. Additional calculations from oxide balance suggested that besides AH 3 monosulphate and C-S-H, an X-ray amorphous AFm or/and C-S-H type like phase might form in the early age of hydration. During Rietveld refinement, the impact of the number of Chebyshev background polynomials in the determination of amorphous content was investigated.

The effect of the partial cement substitution with fly ash on Delayed Ettringite Formation in heat-cured mortars

Abstract: Delayed Ettringite Formation (DEF) is seen as a form of internal sulfate attack of cementitious materials, caused by early age heating to a temperature higher than 70°C. In this paper, the effect of fly ash on delayed ettringite formation of heat-cured cement-based mortars was investigated. To fulfil the aim of this study, a portion of cement was replaced by class F-fly ash, with three different dosages (10, 20 and 30%). The mortars were heat-cured at early-age, and the tests of expansion, mechanical strength, dynamic elastic modulus, mercury porosity and thermogravimetric analysis were carried on these mortars along a period of 650 days. Additionally, Scanning Electron Microscopy (SEM) observations were realized. The results obtained highlighted the mitigation effects of fly ash on DEF: A replacement of the cement used with 20% to 30% of fly ash was efficient to eliminate the long-term swelling due to DEF. This positive impact was explained by the combined effects of the high Al2O3 content of fly ash and the portlandite consumption induced by the pozzolanic reactions.

External sulfate attack of cementitious materials: New insights gained through numerical modeling including dissolution/precipitation kinetics and surface complexation

Abstract: In this study, a new physically and chemically coupled model taking into account multiionic diffusion, precipitation/dissolution kinetics and surface complexation is proposed to predict the ingress of sulfate ions into saturated cementitious materials. The results are compared to experimental data and numerical profiles that were previously obtained on the total sulfate concentration profile in the material. The results show good agreement between experimental and numerical results. They also show that surface complexation has to be taken into account in order to reproduce the sulfate quantity peak experimentally observed. In addition, dissolution and precipitation kinetics are indispensable to retrieve the sulfate quantity amplitude and still more the slow decay of the sulfate quantity after the peak. Finally, the model may explain the supersaturation of the sulfate concentration with respect to ettringite because of the dissolution of monosulfoaluminate and ettringite precipitation kinetics. Moreover, it shows that the supersaturation increases with the concentration of the aggressive sulfate solution without significant increase in the total quantity of sulfate in the material. These results are coherent with the crystallization pressure theory.

Resolution of the Two Aluminum Sites in Ettringite by 27Al MAS and MQMAS NMR at Very High Magnetic Field (22.3 T)

Abstract: Ettringite (Ca6[Al(OH)6]2(SO4)3·26H2O) is the first hydration product formed during Portland cement hydration. 27Al MAS NMR has been used in a wide number of studies to detect and quantify ettringite in hydrated cement blends by the observation of a single, narrow resonance at 13–14 ppm. This work reports the first observation of resonances from two distinct Al sites in octahedral coordination for ettringite, employing 27Al MAS and MQMAS NMR at an ultrahigh magnetic field (22.3 T). Thereby, the 27Al NMR spectra are in agreement with the most accepted trigonal model for the ettringite structure. 27Al quadrupole coupling parameters and isotropic chemical shifts for the two Al sites are determined from simulations and least-squares optimization of slow-speed 27Al MAS NMR spectra of the satellite transitions. These data reveal that the local environments for the two octahedral Al sites are very similar, in accord with the most recent XRD refinements of the ettringite structure. Finally, the significant improv...

Strength development and microstructural characteristics of barium hydroxide-activated ground granulated blast furnace slag

Abstract: In this study, barium hydroxide was proposed as a main activator for ground granulated blast-furnace slag (GGBFS) to produce a strong binder, and it was compared to calcium hydroxide in terms of strength development, reaction products, and microstructure. The Ba(OH)2-activated GGBFS (BHAS) achieved a significantly higher compressive strength than Ca(OH)2-activated GGBFS (CHAS), except at 3 days, mainly due to (1) the more formation of hydration products, leading to a notable reduction in pore sizes and volume, and (2) the higher solubility of Ba(OH)2, resulting in a higher dissolution of GGBFS than that of Ca(OH)2. Although calcium silicate hydrate (C-S-H) was a major reaction product in both mixtures, the Ca/Si ratios were much different. In the BHAS, the presence of barium ions prohibited the synthesis of ettringite and monocarboaluminate, which formed in the CHAS mixtures, but it induced Ba-bearing products, stratlingite, and the hydrotalcite-like phase. The removal of ettringite was the cause of the lower strength of the BHAS at 3 days compared to that of the CHAS.

Effect of Friedel’s salt on strength enhancement of stabilized chloride saline soil

Abstract: In the field of soil stabilization, only calcium silicate hydrate (CSH) and ettringite (AFt) as hydration products have been reported to directly contribute to the strength enhancement of the soil. A chloride dredger fill, an artificial chloride saline soil, and a non-saline soil were stabilized by Portland cement (PC) and PC with Ca(OH)2 (CH) with different contents. A series of unconfined compressive strength (UCS) tests of stabilized soil specimen after curing for 7 d and 28 d were carried out, and the hydration products and microstructure of the specimens were observed by X-ray diffractometry (XRD), scanning electronic microscopy (SEM), and energy-dispersive X-ray analysis (EDXA). The results showed that the strengths of PC+CH-stabilized chloride saline soils were much higher than those of PC-stabilized soils. A new hydration product of calcium aluminate chloride hydrate, also known as Friedel’s salt, appeared in the PC+CH-stabilized chloride saline soils. The solid-phase volume of Friedel’s salt expanded during the formation of the hydrate; this volume filled the pores in the stabilized soil. This pore-filling effect was the most important contribution to the significantly enhanced strength of the PC+CH-stabilized chloride saline soils. On the basis of this understanding, a new optimized stabilizer was designed according to the concept that the chloride in saline soil could be utilized as a component of the stabilizer. The strength of the chloride saline soils stabilized by the optimized stabilizer was even further increased compared with that of the PC+CH-stabilized soils.