Showing papers on "Ettringite published in 2006"

Cement Hydration Inhibition with Sucrose, Tartaric Acid, and Lignosulfonate: Analytical and Spectroscopic Study

Abstract: The reaction of the cement retarders tartaric acid, sucrose, and lignosulfonate with tricalcium silicate (C3S), tricalcium aluminate (C3A), and C3A/gypsum have been studied by 27Al and 29Si MAS NMR spectroscopy, SEM, XRD, and XPS to gain an understanding of the effect on the individual minerals prior to studying a typical sample of portland cement. Tartaric acid is the most effective at retarding C3A hydration and ettringite formation, while sucrose and the lignosulfonate accelerate ettringite formation but are more effective at retarding C3S hydration. We have confirmed that sucrose acts via nucleation poisoning/surface adsorption while lignosulfonates involve the formation of a semipermeable layer on the cement grains. The formation of calcium tartrate is clearly the most important step in tartaric acid inhibition; however, tartaric acid only exhibits a dissolution−precipitation mechanism for C3A. Under conditions of excess calcium, the formation of a calcium tartrate overlayer does not require the pred...

Hydration of tricalcium aluminate (C3A) in the presence and absence of gypsum—studied by Raman spectroscopy and X-ray diffraction

Abstract: Raman spectroscopy has been used to follow the hydration of one of the principal components of Ordinary Portland Cement (OPC) clinkers, tricalcium aluminate, both in the absence and in the presence of calcium sulfate. Direct in-situ analysis of the hydrating paste surface was possible. Spectra were recorded regularly during the first 24 hours of hydration, and then systematically after hydration periods of up to 28 days. X-Ray diffraction was performed to confirm sample identity, firstly after 24 hours and then regularly in coordination with Raman analysis. Hydration in the absence of sulfate was rapid and led to the initial formation of C4AH19 and subsequently C3AH6. Characteristic changes were seen in the Raman spectra and the products' identities confirmed by XRD. Hydration in the presence of sulfate led to very rapid ettringite (AFt) formation, with the first evidence of ettringite formation within 3 minutes. Ettringite gradually converted to monosulfate (AFm), with the re-formation of traces of gypsum at high sulfate contents. Ettringite was formed at the expense of C4AH19. The level of C4AH19 formed diminished greatly with increasing sulfate content. Care was taken throughout the study to exclude carbon dioxide, thus minimising formation of various carbonate species, which could also be readily distinguished by Raman spectroscopy. By combining the results obtained using the well-established technique of X-ray diffraction with Raman spectroscopy, in-situ characterisation of hydrating pastes has been achieved in real-time. In addition to demonstrating the suitability of Raman spectroscopy for analysis of these systems this study should aid the understanding of tricalcium aluminate hydration and the formation of ettringite and monosulfate.

Refined ettringite (Ca6Al2(SO4)3(OH)12*26H2O) structure for quantitative X-ray diffraction analysis

Abstract: A revised structure model of ettringite is presented, in order to provide quantitative X-ray diffraction (QXRD) of this mineral in cement pastes. The model is derived from two different existing structure models, both of which are suitable for restricted use but are inferior to the refined ettringite structure presented. In the first published ettringite structure proposed by Moore and Taylor [Acta Crystallogr. B 26, 386–393 (1970)], none of the 128 positions for H are given in the unit cell, which results in reduced scattering power for use of this model for quantification purposes. For the precise quantification of ettringite in samples together with anhydrous phases, the scattering factors of all atoms including the H positions are indispensable. The revised structure model is based on the data of Moore and Taylor, supplemented by the H positions determined by Berliner (Material Science of Concrete Special Volume, The Sydney Diamond Symposium, American Ceramic, Society, 1998, pp. 127–141) on the basis of a neutron diffraction structural investigation of deuterated ettringite at 20 K. Berliner’s (Material Science of Concrete Special Volume, The Sydney Diamond Symposium, American Ceramic Society, 1998, pp. 127–141) thermal parameter should not, however, be used, since a normal application is at room temperature. In order further to improve the structure model of ettringite, Rietveld refinement with the rigid body approach for OH and H2O molecules and SO4 tetrahedra was employed. The refined and improved ettringite structure model was tested for quantitative phase analysis by the determination of actual ettringite contents in mixtures with an internal standard. Synthesized and orientation-free prepared ettringite powders were investigated by X-ray powder diffraction analysis and quantified in four different blends with zircon. The quantification results with the new structure model demonstrate the superior quality of the revised ettringite structure.

Modification of cement hydration at early ages by natural and heated wood

Abstract: Heat treatments of wood are widely used for the reduction of wood swelling and dimensional instability of wood-cement composites. The effect of natural and heated wood on the hydration of cement at early ages was investigated by isothermal calorimetry, thermogravimetry (TGA) and Fourier transform infrared (FTIR) spectroscopy. The addition of wood strongly delays and inhibits the hydration of the silicate phases. Consequently, the amount of portlandite is lower in composites than in neat cement. Approximately 30% of the inhibition of portlandite precipitation is due to an increase of calcium carbonate content in cement paste. The absorption of a part of water by wood produces a decrease in gypsum consumption. Nevertheless, the ettringite formation is not significantly affected since a diffusion of sulfate ions from wood occurs.

Influence of cellulose ethers on the kinetics of early Portland cement hydration

Abstract: The phase-specific influence of cellulose ether (CEs) on Portland cement hydration was investigated in-situ, using synchrotron X-ray diffractometry. CE-caused retardation can be traced to the polymers adsorption behaviour. The adsorption decreases in following order: silicates and their hydrates (high), sulfates (low), ettringite (zero). The retarding effect is strong on silicates, moderate on sulfates and unspecific on alluminates. Umfang: III, 110 S. Preis: €37.50 | £35.00 | $66.00

XRD and TGA Investigation of Hardened Cement Paste Degradation

Abstract: It is generally recognized that the environmental degradation of the concrete infrastructure is a serious, large scale and costly problem in many parts of the world. In this paper a deterioration of concrete by leaching of hardened cement paste due to aggressive water action is considered. Concrete from a hydro-electric power plant’ s pipeline 30 years old was characterized with X-ray diffraction (XRD), and thermogravimetric analysis (TGA/DTA). The study highlights the capabilities of the methods for the analysis of concrete towards the determination of hardened cement paste degradation. On the basis of a visual survey of drilled cores, a number of representative spots were selected for detailed testing and verification of visual estimates. The methods ascertain a leach of hardened cement paste on samples taken from a surface concrete cover layer of few millimetres, and from flaws like cracks, segregations or cold joints. On these samples XRD results showed small quantity of ettringite, calcium carboaluminate hydrate and Friedel’ s salt, and a complete leach of portlandite, while TGA results indicated small quantities of hydrates. Samples taken from flawless inside of concrete layer showed expected quantities of hydrates for the concrete.

Behavior of B, Cr, Se, As, Pb, Cd, and Mo present in waste leachates generated from combustion residues during the formation of ettringite

Abstract: The behavior of B, Cr, Se, As, Pb, Cd, and Mo in the leachates generated from two combustion residues, coal-fired power plant fly ash and municipal solid waste incineration ash, during precipitation of ettringite is presented. Experiments also were performed using modeled waste leachates as well as controlled solutions containing all the investigated elements. Moreover, to determine the possible effect of pH, lime treatment was conducted using the waste and modeled leachates. Results indicated the removal of B, Se, and Cr from the leachates because of incorporation of their oxyanions in the ettringite structure. The removal of B could further be explained by considering the structure of ettringite and monosulfoaluminate. The removal of Pb also occurred, probably because of lime-induced precipitation of Pb(OH)2. Similarly, the removal of As was observed as a result of lime-induced precipitation of Ca3(AsO4)2 and incorporation into the ettringite structure. On the other hand, the precipitation of ettringite did not reduce the concentrations of Mo and Cd because of the complex nature of these elements in the leachates. The results also suggest that the presence of high amounts of constituents such as Cl−, an alkali metal in the solution, also affected ettringite precipitation behavior. Moreover, elemental speciations as well as the presence of other constituents in the solution affected the incorporation by ettringite.