E. Tauler

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.

Ni-sepiolite-falcondoite in garnierite mineralization from the Falcondo Ni-laterite deposit, Dominican Republic

Abstract New chemical and structural data on sepiolite-falcondoite in garnierite veins from the Falcondo Ni-laterite deposits, central Dominican Republic, are reported. Samples of Ni-sepiolitefalcondoite vary in colour from whitish green to green depending on the NiO content (wt.%) and the amount of silica present. The texture is normally schistose and friable but samples with considerable quartz and/or amorphous silica are compact and hard. Back-scattered electron images indicate that the samples are composed of at least three generations of Ni-sepiolite-falcondoite. The extreme refined cell parameters for Ni-sepiolite-falcondoite vary from 13.400(2), 27.006(4), 5.273(1) Å to 13.340(3), 27.001(6), 5.267(1) Å (space group Pncn). As the Ni content increases there is a small reduction in the a parameter. Chemical compositions determined by electron probe microanalysis cover a large interval of the Ni-sepiolite-falcondoite solid solution (Fal3 and Fal77). Individual samples show a considerable range in composition with the widest range determined in one sample from 4.63 to 22.40 wt.% NiO.

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.

Surface alteration of dolomite in dedolomitization reaction in alkaline media

Abstract The phases precipitated on the dolomite surface and the influence of alkalis on the properties of this surface were characterized. Experiments consisted of the immersion of single crystals of dolomite in saturated portlandite solutions with different alkalinity, temperature, and silica content. Most calcite forms in the solution as {104} rhombohedra. Brucite crystallizes as platelets, piles of platelets, sponge-like and {001} truncated ditrigonal pyramids attached to the dolomite surface. The morphology of brucite crystals depends on the pH of the solution. Apparent dissolution rate constants for dolomite at 75 °C are one order of magnitude higher than at room temperature. Initially, calcite grows on the dolomite surface with the same structural orientation {104}. In a second stage, when calcite has reached equilibrium with the solution, the new-formed {104} calcite surface starts playing an important role on the precipitation of brucite, while calcite continues crystallizing in the bulk solution. The calcite surface, negatively charged, can adsorb specifically metal cations. Mg2+, liberated by the dissolution of dolomite, would move attached to the surface, until they are trapped on these negatively charged surfaces, while OH− supplied by the alkaline solution favors the nucleation and growth of brucite. The attachment of Mg2+ to the surface controls the place where brucite grows.

Application of the “shape-factors method” to purity analysis of compounds by thermal methods

Abstract The shape-factors method may be applied to the determination of phase diagrams by thermal methods. In this paper, its applicability to purity evaluation of compounds (organic and inorganic) is discussed. It is a rapid and easy method which can be applied either to impurities which form a eutectic phase diagram with the main component or to impurities miscible in the solid state with the compound to be purified.

The thermal properties of three (Cl-, Br-, I-) para-halotoluenes. Formation of glassy crystals

Abstract The heat capacities of three para-halotoluenes, p -chlorotoluene, p -bromotoluene and p -iodotoluene, were measured with an adiabatic calorimeter. All three compounds showed the formation of glassy crystals. The glass transition temperatures are 220, 242 and 250 K respectively. The temperature and enthalpies of fusion are: for p -chlorotoluene, 280.69 ± 0.01 K and 13554 J mol −1 ; for p -bromotoluene, 299.94 ± 0.01 K and 15127 J mol −1 ; and for p -iodotoluene, 306.70 ± 0.05 K and 14963 J mol −1 . The heat capacity of p -iodotoluene was measured from 5 K; the derived thermodynamic properties are given. Those of the other compounds were measured from 80 K.

Molecular alloys in the series of para disubstituted benzene derivatives: Part III. The para-bromochlorobenzene-para-dibromobenzene system

Abstract This paper discusses the binary system between para-bromochlorobenzene and para-dibromobenzene. The phase diagram and crystalline parameters of the molecular alloys of this system have been determined by DSC and X-ray diffraction. The miscibility versus temperature study has shown a miscibility in all proportions in the solid state between room temperature and the melting points. The evolution of the alloy crystalline parameters with the composition shows that the deformation of the para-bromochlorobenzene (pBCB) by introducing para-dibromobenzene (pDBB) takes place mainly in the a direction.

Molecular alloys in the series of paradisubstituted benzene derivatives. IV. Parabromochlorobenzene-parachloroiodobenzene system

Abstract The stability of mixed crystals between parabromochlorobenzene (pBCB) and parachloroiodobenzene (pCIB), in all proportions, from 293 K to melting point, is established. The alloys between both components differ slightly from linear behaviour, from a thermal and crystallographic point of view, according to variations in their composition. The greater isotropic nature of the crystalline expansion caused by the introduction of the bulkier component (pCIB) into that of less reticular volume (pBCB) is revealed, with respect to the alloys that only contain Br and Cl as benzene substituents.

Behaviour during the thermal cycling of molecular alloys for the storage of thermal energy

Abstract Reliability tests carried out on certain molecular alloys and their constituents to evaluate their behaviour during thermal cycling have shown that this behaviour is reproducible during melting and crystallization (ΔH f , T f , ΔH c , T c ) once these materials have undergone a series of consecutive heatings and coolings over a temperature range broadly including melting. The results show that the behaviour of the alloys is little or no different from that of their constituents at this working scale.

A system with a less than 2 degree melting window in the range within −31 °C and −45 °C: chlorobenzene-bromobenzene

Abstract Chlorobenzene and bromobenzene, liquid substances at room temperature, crystallize with very similar structures, without any known polymorphism at ordinary pressures. They are isomorphous, showing complete solid state solubility, with little deviation from ideality. Thus, as it is confirmed by thermal cycling experiments, their alloys would be suitable as phase change materials for cold storage applications if one needs precise temperature control at a temperature between −31 and −45 °C.