M. Labrador

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.

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.

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.

Interactive PDF files with embedded 3D designs as support material to study the 32 crystallographic point groups

Crystallography and X-ray diffraction techniques are essential topics in geosciences and other solid-state sciences. Their fundamentals, which include point symmetry groups, are taught in the corresponding university courses. In-depth meaningful learning of symmetry concepts is difficult and requires capacity for abstraction and spatial vision. Traditionally, wooden crystallographic models are used as support material. In this paper, we describe a new interactive tool, freely available, inspired in such models. Thirty-two PDF files containing embedded 3D models have been created. Each file illustrates a point symmetry group and can be used to teach/learn essential symmetry concepts and the International Hermann-Mauguin notation of point symmetry groups. Most interactive computer-aided tools devoted to symmetry deal with molecular symmetry and disregard crystal symmetry so we have developed a tool that fills the existing gap. Display Omitted PDF is a self-contained format that supports 3D designs and multi-layer structure.We use embedded 3D designs in PDF files to illustrate point groups.We use crystallographic notation to set a structure of interactive buttons in PDF files.

Non-stable mixtures of molecular alloys The p-bromoiodobenzene/p-diiodobenzene phase diagram

Abstract The p-bromoiodobenzene/p-diiodobenzene phase diagram has been determined from samples prepared by quenching from the melt, from room temperature to the melting. The solid state miscibility between the monoclinic p-bromoiodobenzene and the orthorhombic p-diiodobenzene is extensive and it is only interrupted by a narrow region of demixing derived from a peritectic invariant. In the course of this investigation non-stable mixtures of two molecular alloys have been identified from X-ray diffraction experiments on samples of the central compositions prepared by precipitation from solution. The phenomenon can be easily confused with a region of demixing when observed by X-ray diffraction, particularly in the case of two pure compounds with very close crystalline unit cells. This finding could be the key for the interpretation of the contradictions found in the 10 binary systems’ group of the monoclinic p-dihalobenzenes. In five of these systems, demixing phenomena not predicted by thermodynamic analysis were deduced from X-ray diffraction experiments. The thermodynamic analysis of the experimental p-bromoiodobenzene/p-diiodobenzene phase diagram confirms the absence of any demixing phenomenon for the central compositions where the non-stable mixtures were found. The coefficient of crystalline isomorphism e M is more indicative of the observed solid state miscibility than Kitaigorodskys coefficient e K of molecular shape similarity. A detailed formalism is presented for extracting the characteristic temperatures of the DSC signals by means of two shape-factors.

The application of DSC to the study of phase segregation during the thermal cycling of para-dihalosubstituted derivatives of benzene and their alloys

Abstract Calorimetric techniques are most commonly used in our research into organic compounds and their alloys, both for the initial characterization of the products and for the determination of phase diagrams that may be established. They are equally useful for the cycling of materials used for energy storage by latent heat, and for the study of their behaviour. This paper also demonstrates the use of the DSC technique to study the homogeneity of these materials when they are subjected to continuous thermic cycling.

Molecular alloys in the series of para-disubstituted benzene derivatives. Part 8. The para-dichlorobenzene + para- bromoiodobenzene system

Experimental data are given for the solid-liquid equilibrium in the system (1 − X)p-dichlorobenzene + X p-bromoiodobenzene. The significance of the data is supported by a thermodynamic phase-diagram analysis.