Miguel A. Climent

The behaviour of platinum single-crystal electrodes in neutral phosphate buffered solutions

Abstract The voltammetric behaviour of the three basal Pt orientations (100), (110) and (111) in neutral phosphate buffered solutions are presented and discussed. The voltammograms corresponding to the hydrogen adsorption-desorption reaction and the influence of oxygen monolayer adsorption on these faces show some similarities with the corresponding results in acid media; in general they are intermediate between those observed in sulphuric and perchloric acids. The electric charges measured in the hydrogen adsorption- desorption potential range are higher than the corresponding theoretical values of the unreconstructed surfaces, particularly in the cases of Pt(100) and Pt(111). This is interpreted as being due to a reconstruction of the Pt(100) surface towards the (5 × 20) structure. For Pt(111) hydrogen is adsorbed in two separate potential ranges; however, the excess charge found for the state at higher potential suggests that oxygenated species are present on the surface in this range. The adsorption of bromide ions suppresses the adsorption of oxygenated species. In contrast, the Pt(110) orientation seems to experience a lower degree of initial reconstruction to a (1 × 2) structure in neutral phosphate buffer than in sulphuric acid, but it shows a progressive reconstruction of the type mentioned above upon cycling in a potential range including oxygen adsorption-desorption.

Behaviour of the Cr(III)/Cr(II) reaction on goldgraphite electrodes. Application to redox flow storage cell

Abstract With the aim of increasing the efficiency of an Fe/Cr redox flow battery, the behaviour of the Cr(III)/Cr(II) redox reactions and the hydrogen evolution on goldgraphite electrodes has been studied. Different metals with high hydrogen overvoltage have been used. An enhancement in the reversibility of the redox couple is produced with these types of goldgraphite electrodes, though the energetic efficiency of a monopolar Fe/Cr redox cell constructed with these electrodes is not improved due to the greater hydrogen evolution presented by the goldgraphite electrodes in relation with graphite electrodes.

Long-Term Behaviour of Fly Ash and Slag Cement Grouts for Micropiles Exposed to a Sulphate Aggressive Medium

Nowadays, one of the most popular ways to get a more sustainable cement industry is using additions as cement replacement. However, there are many civil engineering applications in which the use of sustainable cements is not extended yet, such as special foundations, and particularly micropiles, even though the standards do not restrict the cement type to use. These elements are frequently exposed to the sulphates present in soils. The purpose of this research is to study the effects in the very long-term (until 600 days) of sulphate attack in the microstructure of micropiles grouts, prepared with ordinary Portland cement, fly ash and slag commercial cements, continuing a previous work, in which these effects were studied in the short-term. The microstructure changes have been analysed with the non-destructive impedance spectroscopy technique, mercury intrusion porosimetry and the “Wenner” resistivity test. The mass variation and the compressive strength have also been studied. The impedance spectroscopy has been the most sensitive technique for following the sulphate attack process. Considering the results obtained, micropiles grouts with slag and fly ash, exposed to an aggressive medium with high content of sulphates, have shown good behaviour in the very long-term (600 days) compared to grouts made with OPC.

Effect of Type of Anodic Arrangements on Efficiency of Electrochemical Chloride Removal from Concrete

Electrochemical techniques can be used to arrest or prevent reinforced concrete corrosion. This article reports on a study that evaluated the effect of the type of anodic arrangement on the efficiency of the technique of electrochemical chloride removal (ECR) applied to reinforced concrete structures. Two different arrangements were tested for maintaining the electrolytic medium in contact with the concrete surface: complete immersion of the system (structure and anode) in water during the electrochemical treatment; and the wrapping of the concrete structure with a sandwich-like anodic system, which consisted of two layers of polypropylene felt embedding the anodic material. Results demonstrate that the efficiency of an ECR treatment applied with a sandwich-like anodic system may be practically equal to that obtained with a liquid electrolyte (immersion) if a good contact between the concrete surface and the polymeric system is ensured. The use of a continuous graphite felt for the sandwich-like anodic system, instead of a discontinuous and more rigid metallic wire mesh anode, makes it easier to achieve the necessary electrical contact. The authors caution that the net influence of carbonation on the efficiency of ECR may vary for different experimental conditions.

Influence of Silica Fume Addition in the Long-Term Performance of Sustainable Cement Grouts for Micropiles Exposed to a Sulphate Aggressive Medium

At present, sustainability is of major importance in the cement industry, and the use of additions such as silica fume as clinker replacement contributes towards that goal. Special foundations, and particularly micropiles, are one of the most suitable areas for the use of sustainable cements. The aim of this research is to analyse the effects in the very long-term (for 600 days) produced by sulphate attack in the microstructure of grouts for micropiles in which OPC (ordinary Portland cement) has been replaced by 5% and 10% silica fume. This line of study is building on a previous work, where these effects were studied in slag and fly ash grouts. Grouts made using a commercial sulphate-resisting Portland cement were also studied. The non-destructive impedance spectroscopy technique, mercury intrusion porosimetry, and Wenner resistivity testing were used. Mass variation and the compressive strength have also been analysed. Apparently, impedance spectroscopy is the most suitable technique for studying sulphate attack development. According to the results obtained, grouts for micropiles with a content of silica fume up to 10% and exposed to an aggressive sulphate medium, have a similar or even better behaviour in the very long-term, compared to grouts prepared using sulphate-resisting Portland cement.

Performance of Sustainable Fly Ash and Slag Cement Mortars Exposed to Simulated and Real In Situ Mediterranean Conditions along 90 Warm Season Days

Nowadays, cement manufacture is one of the most polluting worldwide industrial sectors. In order to reduce its CO2 emissions, the clinker replacement by ground granulated blast–furnace slag and fly ash is becoming increasingly common. Both additions are well-studied when the hardening conditions of cementitious materials are optimum. Therefore, the main objective of this research was to study the short-term effects of exposure, to both laboratory simulated and real in situ Mediterranean climate environments, on the microstructure and durability-related properties of mortars made using commercial slag and fly ash cements, as well as ordinary Portland cement. The real in situ condition consisted of placing the samples at approximately 100 m away from the Mediterranean Sea. The microstructure was analysed using mercury intrusion porosimetry. The effective porosity, the capillary suction coefficient and the non-steady state chloride migration coefficient were also studied. In view of the results obtained, the non-optimum laboratory simulated Mediterranean environment was a good approach to the real in situ one. Finally, mortars prepared using sustainable cements with slag and fly ash exposed to both Mediterranean climate environments, showed adequate service properties in the short-term (90 days), similar to or even better than those in mortars made with ordinary Portland cement.

Voltammetric and subtractively normalized interfacial FTIR study of the adsorption and oxidation of L(+)-ascorbic acid on Pt electrodes in acid medium: effect of Bi adatoms

The adsorption and oxidation of L(+)-ascorbic acid on bare and Bi-covered Pt electrodes in a sulfuric acid medium have been studied by cyclic voltammetry and in situ FTIR spectroscopy. The latter technique has allowed the direct detection of COads from ascorbic acid adsorption, both at open-circuit and controlled potential, for the bare Pt surface. Under these latter conditions both linearly bonded and multi-bonded CO were detected. The existence of other strongly adsorbed species which are oxidized in the oxygen adsorption region has been demonstrated by cyclic voltammetry. The formation of all these adsorbed species is completely suppressed on the Bi-covered Pt surface under UPD conditions. Ascorbic acid oxidation was monitored by following the variation of the IR spectra in the presence and in the absence of Bi3+ in solution. The electrocatalytic effect of the Bi adlayer can be explained mainly through the suppression of any dissociative adsorption step leading to the formation of strongly adsorbed species.

Influence of Environment on Durability of Fly Ash Cement Mortars

In this study, the durability-related properties of mortars prepared using two types of cement with different contents of fly ash were tested. One of the most important ways of ingress of aggressive substances in cementitious materials is through water. Therefore, to characterize their durability, the capillary suction and the resistance against chloride ingress of the fly ash mortars were studied. The mortars were exposed to four different constant temperatures and relative humidities (RHs) for 365 days. The measured properties of both cement types were affected by the hardening environment. For each environment studied, the durability-related parameters of the fly ash cement mortars studied were quite similar at high hardening ages, independent of their fly ash content. In spite of the different resistance classes of cements used, the durability related properties of both cement types were similar at high hardening ages, regardless of the environment.