Salvador Galí

Kinetics of dolomite-portlandite reaction: application to portland cement concrete

The dedolomitization reaction kinetics are studied through several long-term experiments consisting of an aqueous dispersion of fine powders of dolomite and portlandite with different alkalinity, temperature and silica content. The experimental results are reproduced through computer simulation, which allows the estimation of the apparent dissolution constant rates for dolomite. These are discussed together with other parameters influencing the kinetics, in particular the modification of the specific surface of dolomite. The parameters obtained make it possible to predict the behavior of the system beyond experimental periods. Both experimental and simulated results are discussed in connection with the expansion and cracking occurrences observed in portland mortars and concretes made with dolomitic limestone.

Quantitative Rietveld analysis of aluminous cement clinker phases

Abstract The mineralogical composition of high alumina cement (HAC) determines its ultimate properties once it has been mixed with water. The mineralogy of HAC cannot be given from the chemical composition because the thermodynamic equilibrium usually is not reached during the production process. The Rietveld method permits relatively quick quantitative phase analysis by fitting the calculated X-ray diffraction (XRD) profile with the observed one. Synthetic mixtures of pure phases were analyzed with this method in order to obtain a set of “empirical corrections” to be applied to samples of HAC clinker without previous chemical treatment. Chemical composition estimated from the weight percent of phases was in good agreement with composition obtained by X-ray fluorescence (XRF).

Thermal Decomposition of Hydrated Alumina Cement (CAH10)

It is known that the initial product of hydration of high-alumina cement at low temperatures, often labeled “CAH 10,” contains both crystalline and amorphous portions. The thermal decomposition of this product was studied in several samples. This study was carried out by two different methods: derivative thermogravimetric analysis (DTG) and x-ray diffraction (XRD) combined with temperature. The DTG signal of the compound was decomposed in four different kinetic processes. The nature of the water liberated in each process was characterized by the shape parameters of the process. The x-ray diffractograms at different temperatures showed two volume variations of the CAHx compound, related to contractions in the c parameter before the collapse of the structure to yield an amorphous phase. Two of the four observed processes are related to the dehydration of the crystalline portion and the other two to the amorphous dehydration.

Dedolomitization in different alkaline media: Application to Portland cement paste

Abstract The dedolomitization reaction kinetics is studied through several long-term experiments consisting of an aqueous dispersion of fine powders of dolomite and portlandite with alkalinity between 0.1 and 1 M KOH, at 25 and 75 °C. The experimental results are numerically simulated to calculate the apparent dissolution constant rates for dolomite, k dol . At low temperature, two dissolution stages were observed. In an early stage, part of dolomite powder dissolves quickly until an apparent steady is reached. After several days, the reaction continues at a lower rate. The calculated dissolution rate for dolomite in the first stage is one order of magnitude higher than that of the second stage. At 75 °C, the k dol is two orders of magnitude higher than at 25 °C. The addition of alkali increases the k dol at high temperature, but reduces it at room temperature.

Crystal Structure Solution of Hydrated High-Alumina Cement from X-ray Powder Diffraction Data**

This work was supported by the Direccion General de Ensenanza (DGES) of the Spanish Goverment (project PB95-0115) and by the Direccio General de la Recerca (DGR) of the Catalan Goverment (grant SGR-00460, 1995).

Cation distribution and magnetization of BaFe12−2xCoxSnxO19 (x=0.9,1.28) single crystals

The distribution of Sn4+ cations within the five crystallographic sites of the magnetoplumbite (M) ‐like compound BaFe12−2xCoxSnxO19 has been analyzed using single‐crystal x‐ray‐diffraction data. The species Fe3+ and Co2+ cannot be distinguished using x rays because of their very similar atomic numbers; however, the calculation of the apparent valencies for the different sites allows an insight into the Co2+ cation segregation. The use of previous data from neutron powder diffraction allows a precise picture of the cation distribution, which indicates a pronounced site selectivity for both Sn4+ and Co2+ cations. The Sn4+ cations prefer the 4f2 sites and to a much lower extent the 12k sites, while they do not enter the octahedral 2a sites at all. Co2+ cations are distributed among tetrahedral and octahedral sites displaying a clear preference for the tetrahedral 4f1 sites. Magnetic measurements indicate that the compound still exhibits uniaxial anisotropy with the easy direction parallel to the c axis. Nev...

Serpentinites and serpentinites within a fossil subduction channel: La Corea mélange, eastern Cuba

A variety of metaultramafic (serpentinite) rocks in La Corea melange, Sierra de Cristal, eastern Cuba, show differences in chemical, textural and mineralogical characteristics demonstrating a variety of protoliths. The melange originated during the Cretaceous as part of the subduction channel associated with the Caribbean island arc. This melange contains high pressure blocks in a serpentinite matrix and occurs at the base of the large tabular Mayari-Cristal ophiolite. Two principal groups of serpentinites have been identified in the melange: a) antigorite serpentinite, mainly composed of antigorite and b) antigorite-lizardite serpentinite, composed of mixtures of antigorite and lizardite and bearing distinctive porphyroblasts of diopsidic clinopyroxene. Antigorite serpentinites are closely related to tectonic blocks of amphibolite (representing subducted MORB) and constitute deep fragments of the serpentinitic subduction channel formed during hydration of the mantle wedge. The composition of the antigorite-lizardite serpentinites and the presence of clinopyroxene porphyroblasts in this type of rock suggest that abyssal lherzolite protoliths transformed into serpentinite before and during incorporation (as tectonic blocks) in the shallow part of the subduction channel. Although the studied rocks have different origin, mineralogical compositions and textures, they display similar PGE compositions, suggesting that these elements experienced no significant redistribution during metamorphism. Both types of serpentinites were exposed together in the La Corea melange during the Late Cretaceous, during obduction of the overriding Mayari-Baracoa ophiolitic belt that led to exhumation of the subduction channel (melange)

X‐ray diffraction studies of the epitaxy of a/b‐axes oriented YBa2Cu3O7−δ films grown by liquid phase epitaxy

A detailed x‐ray diffraction study is reported for a/b‐axes oriented YBa2Cu3O7−δ films obtained by the liquid phase epitaxy technique. The films were grown epitaxially in the tetragonal state on (110) NdGaO3 substrates so that [001]film∥ [110]subs and [100]film∥ [001]subs, 90°‐[100]/[010] boundaries were almost absent. Below the tetragonal‐to‐orthorhombic transition temperature, the film undergoes intensive {hh0}‐type twinning. The volume of the a‐axis oriented material is similar to that of the b‐axis oriented fraction and the presence of both orientations is likely to be controlled by {hh0}‐type twinning. High temperature diffraction of the substrate indicates that nucleation of YBa2Cu3O7−δ on (110) NdGaO3 takes place on a square two‐dimensional lattice and therefore the observed strong in‐plane alignment is probably controlled by impurity action and/or surface relaxation rather than simple mismatch effects. The difference between the thermal expansion coefficients of the film and the substrate (αfilm[1...

Ni ENRICHMENT AND STABILITY OF Al-FREE GARNIERITE SOLID-SOLUTIONS: A THERMODYNAMIC APPROACH

Garnierites represent significant Ni ore minerals in the many Ni-laterite deposits worldwide. The occurrence of a variety of garnierite minerals with variable Ni content poses questions about the conditions of their formation. From an aqueous-solution equilibrium thermodynamic point of view, the present study examines the conditions that favor the precipitation of a particular garnierite phase and the mechanism of Ni-enrichment, and gives an explanation to the temporal and spatial succession of different garnierite minerals in Ni-laterite deposits. The chemical and structural characterization of garnierite minerals from many nickel laterite deposits around the world show that this group of minerals is formed essentially by an intimate intermixing of three Mg-Ni phyllosilicate solid solutions: serpentine-népouite, kerolite-pimelite, and sepiolite-falcondoite, without or with very small amounts of Al in their composition. The present study deals with garnierites which are essentially Al-free. The published experimental dissolution constants for Mg end-members of the above solid solutions and the calculated constants for pure Ni end-members were used to calculate Lippmann diagrams for the three solid solutions, on the assumption that they are ideal. With the help of these diagrams, congruent dissolution of Ni-poor primary minerals, followed by equilibrium precipitation of Ni-rich secondary phyllosilicates, is proposed as an efficient mechanism for Ni supergene enrichment in the laterite profile. The stability fields of the solid solutions were constructed using [{iexxxsbap19011700254003}] (predominance) diagrams. These, combined with Lippmann diagrams, give an almost complete chemical characterization of the solution and the precipitating phase(s) in equilibrium. The temporal and spatial succession of hydrous Mg- Ni phyllosilicates encountered in Ni-laterite deposits is explained by the small mobility of silica and the increase in its activity.

Structure solution from powder data of the phosphate hydrate tinticite

The crystal structure of the mineral tinticite has been solved by direct methods from integrated intensities of X-ray powder diffraction data and subsequently refined with the Rietveld technique. The sample used for the structure solution comes from the Gava-Bruguers area (20 km SW of Barcelona), which contains a large variety of phosphates, some of which were exploited in gallery mines during the ancient neolithic. Tinticite crystallizes in the triclinic space group P 1 with unit cell parameters a = 7.965(2) A, b = 9.999(2) A, c = 7.644(2) A, α = 103.94(2)°, β = 115.91(2)°, ω = 67.86(2)° and cell content Fe 3+ 5.34 (PO 4 ) 3.62 (VO 4 ) 0.38 (OH) 4 ·6.7 H 2 O; ρ exp = 2.94 g/cm 3 ; ρ cale = 2.88 g/cm 3 . The Rietveld refinement of the data set converged to R wp = 13.1 % and χ 2 = 3.3. Due to the complexity of the disorder in this structure, the refined structure model could only account for part of it. The octahedrally coordinated Fe 3+ ions form dreier single chains of general formula ∞ 1 [Fe 3 O 14 ] at y = 0 and trimers of type cis-[Fe 3 O 14 placed at y = 1/2. While the dreier single chains are linked to each other by fully occupied PO 4 groups yielding in this way predominantly ordered layers, the trimers arc partially disordered and connected to each other and to the ordered layers both by PO 4 groups and through H-bonds. The higher stability of the ordered layers is consistent with the observed platy nature of the microcrystals of tinticite.