Esperanza Menéndez

Sulfate Attack of Concrete

Cements have long been known to undergo deterioration in sulfate-rich service environments. Yet the mechanism of attack has not been universally agreed and, arguably of even greater importance, test procedures to measure sulfate attack and specifications for sulfate-resistant formulations are not agreed, or are disputed. We also lack reliable mitigation procedures.

A study of ionic defects in modified lead titanate ceramics

Pb1−xCax[(Co0.5W0.5)0·05Ti0.95]O3 ceramics with x = 0.24, 0.30 and 0.35, are prepared by a solid state reaction of oxides. Deviations from nominal chemical compositions and formation of segregated phases different from modified lead titanate perovskites, are studied. Pyro- and non-pyroelectric currents excited in the material by a thermal wave are investigated and related to mobility of electrical charges in the materials.

Caracterización de cenizas de fondo procedentes de centrales térmicas de carbón para determinar su viabilidad de uso

Coal ash from pulverized coal – fired power plants represents a big amount of total residues in many countries, especially when low quality carbon is used, producing not only lack of space but also important environmental and health problems. The ashes collected from pulverized coal – fired furnaces are fly ash (FA) and bottom ash (BA) and they are produced due to an incomplete combustion of coal. FA constitutes around the 80 % of total ashes produced and it is composed of fine particles that remain in suspension in the air stream. BA constitutes around the 20 % of total ashes and it is composed of bigger particles, which drop, melt, conglomerate and vitrify in the bottom of the furnace [1-2]. Both kind of ashes contain trace metals which could be leached to water and damage the aquatic ecosystem [3]. In Europe, it was generated 41 million tonnes of coal ashes in 2009 and around half of them were landed into landfills [4]. Although around half of the total amount of FA generated in Europe is disposed in landfills or it is not used in high – value industrial products, there are some high – value applications to reuse this waste [1-2; 4]. The reutilization of FA as Supplementary Cementitious Material (SCM) in the construction industry is an example of high – value application utterly accepted [5] which enhances cement properties [6-7] and reduces both the amount of raw materials and the CO2 emitted when cement is manufactured. Contrary to FA, coal BA has not been seen as a high – value SCM in construction industry; BA has been disposed or used as low cost replacement for more expensive sand in the The disposal of coal by products represents environmental and economical problems around the world. Therefore, the reuse and valorisation of this waste has become an important issue in the last decades. While high-value construction products containing fly ash were developed and its use is actually totally accepted as an addition to cement, the use of the bottom ash as supplementary cementitious material has not been allow. This paper examines the chemical and physical properties of fly ashes and bottom ashes from two different coal power plants in order to compare them and analyse the potential feasibility of bottom ash as cement replacement. The mechanical properties of cement mortars made with different percentages of both ashes were also study. The results obtained showed similar chemical composition of both kinds of ashes. The compressive strength values of mortars with 10 % and 25 % of cement replacement (at 28 days) were above the limits established in European standards and there were not significant differences between fly ash and bottom ash from both origins.

Internal Deterioration of Mortars in Freeze-Thawing: Non-Destructive Evaluation by Means of Electrical Impedance

In this work, different mortars with different degrees of damage brought about by the action of the freeze-thawing cycles are evaluated. Analysis of its state is carried out by different usual evaluation methods. Results obtained through these methods are compared with the electrical impedance spectroscopy. Moreover, this non-destructive technique is employed to evaluate the state of mortars and the amount of internal damage.

Coal Bottom Ash for Portland Cement Production

Because of industrialization growth, the amount of coal power plant wastes has increased very rapidly. Particularly, the disposal of coal bottom ash (CBA) is becoming an increasing concern for many countries because of the increasing volume generated, the costs of operating landfill sites, and its potential hazardous effects. Therefore, new applications of coal bottom ash (CBA) have become an interesting alternative to disposal. For instance, it could be used as a Portland cement constituent leading to more sustainable cement production by lowering energy consumption and raw material extracted from quarries. Coal fly and bottom ashes are formed together in the same boiler; however, the size and shape of these ashes are very different, and hence their effect on the chemical composition as well as on the mineralogical phases must be studied. Coal bottom ash was ground. Later, both ashes were compared from a physical, mechanical, and chemical point of view to evaluate the potential use of coal bottom ash as a new Portland cement constituent. Both ashes, produced by the same electrical power plant, generally present similar chemical composition and compressive strength and contribute to the refill of mortar capillary pores with the reaction products leading to a redistribution of the pore size.

SYNTHESIS AND CHARACTERIZATION OF NICKEL FERRITE-BARIUM TITANATE CERAMIC COMPOSITES

ABSTRACT Magnetoelectric composites were synthesized from piezoelectric BaTiO3 and ferrimagnetic NiFe2O4. These two phases are mixed, mechanically milled and heat treated. A characterization is made by means of electronic microscopy and x-ray diffraction. The results of the magnetic, electric, ferroelectric and piezoelectrical response of two different compositions are obtained. The composites are superparamagnetic in all the measured temperature range. It was observe a magnetic change induced by a structural transition of the ferroelectric phase. The composites behaves as an acceptable mechanical resonator when the content of the ferroelectric phase is equal or greater than 60 wt.%.

Non-uniform distribution of polarization in Pb0.76Ca0.24(Co0.5W0.5)0.05Ti0.95O3 ceramics produced by defects

Polarization distribution of calcium-modified lead titanate ceramics with a nominal composition of Pb0.76Ca0.24(Co0.5W0.5)0.05Ti0.95O3 was measured from the thermocurrents stimulated in these materials by a sinusoidal thermal wave. The polarization distribution was obtained and related to the development of defects in the material during processing. The deviation in the perovskite composition and the formation of segregated phases at the grain boundaries have been studied using the techniques of energy dispersion X-ray analysis, scanning electron microscopy and backscattering electron analysis.

Microstructural and poling studies in ferroelectric ceramics of pure lead titanate

Abstract PbTiO3 powder was obtained from a PbO-TiO2 gel treated at 900°C/30 min. Molding at 2000 Kgcm−2 and sintering at 1100 °C of this ceramic precursor, gives place to lead titanate ceramics with a tetragonal distortion of c/a = 1.063 and a Curie-Weiss temperature of Tc = 451 °C. The results of EDXA show that the chemical composition changes very little with the sintering time and SEM pictures reveal microstructures with two well defined grain sizes. Small decompensation in chemical compositions and the particular microstructures can be the origin of some variations in crystallographic (c/a), ferroelectric (Tc) and piezoelectric (d33) parameters.

EVALUACIÓN DE UN NUEVO CONSTITUYENTE DEL CEMENTO PORTLAND: CENIZA DE FONDO MOLIDA

Coal bottom ash is produced in electrical power stations as result of the coal combustion. Because coal fly ash and coal bottom ash are formed together in the same boiler, similar chemical and mineralogical composition is expected. The size and shape of these ashes is very different, and then, its effect on the performance must be studied. In order to get a similar grain size to that of the coal fly ash, the coal bottom ash was ground. Cement-based products are the main construction materials which manufacture requires the use of significant natural raw materials and energy. These manufacturing processes result in several types of emissions. In particular, the cement industry is under pressure to reduce CO2 emissions and some studies have shown different measures to reach CO2 reduction. For instance, reducing the clinker/cement factor will lead to a clear CO2 emission reduction. In this work, ground coal bottom ash is investigated to know its viability of being used as a new Portland cement constituent. Then, it is studied from a mechanical and durability point of view to evaluate the potential use of the coal bottom ash as an innovative binder. Ground coal bottom ash and fly ash mortars were more carbonated and exhibited a lower compressive strength than the reference mortars, but similar to each other. Keywords: coal bottom ash, Portland cement, Compressive strength, Durability

STUDY OF MISCIBILITY IN MIXED SYSTEMS OF PRESINTERED PZT

ABSTRACT In this work, we present a compositional and electrical study of mixed presinterized systems following a solid-state and sol-gel rule in order to achieve mixed morphotropic PZT compositions with other compositions in different proportions. A complete characterization of the electrical response of the different PZT compositions is presented, as is the degree of reactivity of the powder according to the route followed and the capacity to mix phases indicating the greater or lesser miscibility between compositions.