Showing papers on "Portlandite published in 1997"

Behaviour of crystallised phases of Portland cement upon water attack

Abstract: The changes occurring in the crystallised phases during degradation of an ordinary Portland cement have been studied. Leaching of cement paste with demineralised water generates a series of dissolution boundaries. Portlandite, then AFm, ettringite and calcite dissolve successively. In the zones with no change in mineralogy, local chemical equilibrium and decreasing gradients in calcium and hydroxyls between the core and the surface explain the precipitation of the secondary phases, like AFm, ettingite and calcite. Hydrogarnets in the surface layer in contact with the aggressive solution only dissolved slightly, or not at all. The very low solubility of hydrotalcites at pH values near neutrality explains the preciptation of these magnesium-containing phases in the surface layer.

Ca X-ray absorption spectroscopy of C—S—H and some model compounds

Abstract: Calcium K-edge (4038 eV) X-ray absorption spectroscopy (XAS) of wet and dry Ca-silicate hydrate (C—S—H) samples and model compounds (calcite, xonotlite, 11 A tobermorite, and 14 A tobermorite) show that the local structure of single-phase C—S—H as viewed from the Ca-positions is similar to that of tobermorite. The only major effect of wetting and drying observed by Ca-XAS is an increase in the average Ca-Ca distance, probably due to changes in interlayer spacing. The Ca-XAS spectrum of a fully hydrated β-C2S sample, containing C—S—H and portlandite, is significantly different due to the presence of the portlandite. Ca-XAS provides unique information about the structure of C—S—H and is likely to be a useful tool for investigating the structures of a wide range of phases of importance to cement science.

Study of Hydration During Curing of Residues From Coal Combustion With Limestone Addition

Abstract: The hydration reactions that occur with CFBC materials treated via the CERCHAR hydration process and combined with PFA have been examined and compared with materials produced via the LIFAC (limestone injection into the furnace and activation of unreacted calcium) process. A wide range of chemical and physical techniques have been used to differentiate the actual speciation of cubes treated for up to 150 days. The two materials behave very differently, with FBC-derived ashes being dominated by sulphate chemistry, i.e., the formation of gypsum and ettringite. The CERCHAR-treated ashes appear to make portlandite available for sulpho-pozzolanic reactions, and this seems to be the key to understanding why these materials display superior performance in applications with PFA or cement substitution. The LIFAC materials, with much less sulphate and Fe 2 O 3 and more Al 2 O 3 , still produces ettringite, but no gypsum, and hydrated tetracalcium aluminate appears as the major hydration product.

Chemical Approach to Formation of Calcite Precipitate from Recycled Concrete Aggregate Base Layers

Abstract: The use of recycled portland cement concrete aggregate in pavement base layers has been associated with the reduction of filter fabric permittivity and the accumulation of calcite precipitate and other materials in pavement subdrainage systems A mechanism of portlandite dissolution within base-layer pore waters, followed by exposure to atmospheric carbon dioxide via subdrainage systems, and subsequent calcite precipitation is explored using chemical thermodynamic techniques Ion activities and pH levels are estimated for the equilibrium states of rainwater entering the pavement, base-layer pore water isolated from the atmosphere, and subdrainage water exposed to the atmosphere Results derived agree with field and laboratory observations of pH levels

Characteristics of morphology of the interface between cement paste and fly ash

Abstract: Using SEM and EDS, the characteristics of morphology within the interfacial zone between fly ash and cement paste added with fly ash was studied. The results show that there are three states for the surface of fly ash particles in the early period of hydration: (1) enveloped with CSH mono film, (2)enveloped with CH CSH duplex film and (3) inserted in the block of portlandite. However, in the later age of hydration, the surface of fly ash particles has been reacted fully with hydration products of cement, and formed some reaction products, e. g. densed granular or ring shaped gel.

Contribution of 1H combined rotation and multipulse spectroscopy nuclear magnetic resonance to the study of tricalcium silicate hydration

Abstract: Hydration products of tricalcium silicate (C3S) are the calcium silicate hydrates (C–S–H) and portlandite. Silica fume, added to anhydrous cement in industrial formulations, reacts with portlandite and leads to C–S–H slightly different from the previous one (this reaction is called the pozzolanic reaction). C3S hydration at 120 °C with and without silica fume has been studied by two 1H nuclear magnetic resonance techniques: combined rotation and multi-pulse spectroscopy (CRAMPS) and magic angle spinning (MAS). The static spectra are broadened by the proton–proton dipolar interaction. The 1H MAS technique does not allow us to remove the effects of this interaction completely and cannot be in this case a quantitative method. Therefore the CRAMPS technique, which can remove the broadening of the interaction because of the use of a multipulse sequence associated with the sample rotation, was used. It is shown that the CRAMPS NMR spectra allow us to describe the action of silica fume on the hydrates and to reveal the competition between portlandite formation around the C3S grains and portlandite dissolution near the silica grains until the complete dissolution of portlandite.

Analysis of different cement based pastes in s02 and n02 atmospheres

Abstract: The paper explains the behaviour of cements of different composition in atmospheres rich in sulphur-dioxide and nitrogen-dioxide. As a result of 180 day long exposition to sulphur-dioxide rich atmosphere the minimal increase of the sulphate ion compound (4%) occurred in cements containing 20% fly ash, while the maximum increase (7%) was found in cements with 40% slag content. The results for portland cements were between these two extreme values. While on the surface of portland cement samples a well-adhesive salt layer appeared, the outer 0.8-1.0 mm of samples with 40% slag peeled off by the end of the ISO day long treatment period. The main constituent of both the salt and the peeled off layer was gypsum, formed from the portlandite and the carbonate phase of the samples. The nitrate content in ISO day old samples stored in nitrogen-dioxide rich atmosphere turned out to be between 1.1-1.4% unaffected by the composition of the cement.