Water-entrained cement-based materials : II. Experimental observations

This paper concerns a new concept for the prevention of self-desiccation in hardening cement-based materials. The concept is based on using fine, superabsorbent polymer (hydrogel, SAP) particles as a concrete admixture. This permits a controlled formation of water-filled macropore inclusions—water entrainment—in the fresh concrete. Consequently, the pore structure is actively designed to control self-desiccation. In the paper, experimental observations in relation to this technique are described and discussed. The observations show that self-desiccation can be controlled by water entrainment. The paper forms the second part of a series. In the first part, the theoretical background was presented [Cem. Concr. Res. 31(4) (2001) 647].     

Polyaniline fibres as electrodes.: Electrochemical characterisation in acid solutions

Polyaniline fibre microelectrodes prepared from a doped solution of polyaniline protonated with 2-acrylamido-2-methyl-1-propanesulphonic acid in dichloracetic acid were characterized electrochemically for the first time. Low scan rate cyclic voltammetry was used for characterisation in different acid electrolyte solutions, hydrochloric, nitric, perchloric, sulphuric and phosphoric, at low pH values with varying positive potential limits. Electrochemical impedance spectroscopy was also utilised. The electrochemical behaviour of polyaniline (PANI) fibres was found to be similar to that of PANI films obtained by electropolymerisation on metallic electrode substrates. The conduction potential window was found to be from +0.20 to +0.60 V versus SCE, with small variations in the different acid solutions as well as with pH. The standard electrochemical redox couple hexacyanoferrate(III), was found to behave quasi-reversibly in the conduction potential region and rate constants were evaluated.     

ZnFe anomalous electrodeposition:: stationaries and local pH measurements

The kinetics of ZnFe codeposition was investigated in acid solutions. The effects of solution composition and pH were analyzed. Inhibition of H⁺ reduction and Fe deposition occurs with increasing Zn⁺⁺ concentration in sulfate solution. An activation of Zn deposition is also observed. Increasing pH causes Zn deposition activation during ZnFe codeposition. The anomalous codeposition is also favored in chloride medium. When alloy deposition becomes the main process, the interfacial pH is governed by the individual metal deposition that controls the kinetic behavior. The interfacial pH increases during separate Fe deposition, meaning that it occurs with simultaneous consumption of H⁺. Individual Zn deposition brings about an H⁺ inhibition. A correlation between the codeposition behavior of ZnFe and ZnNi in sulfate and chloride solutions suggests that the prevailing cathodic reaction governs the interfacial pH. Anomalous codeposition process does not seem to be associated with a saturation of any thermodynamic species at the electrode surface. It can only be described by kinetic arguments.     

Gel polymer electrolytes based on PMMA : III. PMMA gels containing cadmium

Gel polymer electrolyte containing cadmium perchlorate was prepared by polymerisation of methylmethacrylate with a solution containing Cd perchlorate in propylene carbonate (PC). Electric conductivity, fundamental electrochemical properties and Cd electrodeposition were investigated. The deposition of Cd was fairly homogenous.     

New quality criteria in wastewater reuse : The case of Gran Canaria

Wastewater reuse plays a key role in this vital cycle of water because it is able to reduce the wastewater spilled. Simultaneously, the supply of water for specific uses is increased. A new regulation (Royal Decree 1620/2007) came into force in Spain in December 2007 and regulates the basic conditions for the wastewater reuse and establishes the required criteria of quality to waters according to use.In the present paper, different kinds of tertiary treatments in reuse of wastewater are described. We focused on the applied tertiary treatments in most wastewater treatment plants of Gran Canaria Island. Particularly, we analysed Hoya del Pozo wastewater treatment plant due to its wide number of technologies and processes.Finally, we reviewed the quality criteria used in the regenerated waters, in agreement with the new regulations.     

Pyrrole nanoscaled electropolymerization: Effect of the proton

Polypyrrole (Ppy) nanowires are electrochemically synthesized using the nanochannels of a proton-modified natural zeolite clinoptilolite (HNZ). The synthetic inorganic structures Y zeolite and clay montmorillonite (M) in acid form (HY and HM) are also employed for pyrrole polymerization. The generation of Ppy with electrochemical activity is favored in a strongly acidic nanoscaled environment, in comparison with the polymer synthesized in the hosts without previous proton modification. It is proposed that the reduction/oxidation responses are a consequence of the polymer protonation/deprotonation where the H⁺ ions possibly act as charge transfer promoters. The well defined redox signals of Ppy included in HNZ, compared to the HY and HM, suggest that the polymeric units are held more tightly to the channels walls due to its complex framework. The TEM image of HNZ-Ppy reveals the presence of the polymer nanowires inside the natural zeolite framework. It is also found that the one electron oxidation process do not depend on first order monomer concentration but lower (0.38) for Ppy growth in HNZ, which is related to the spatial restriction of the host.     

Characterization of membrane-electrode assembly with hydrogen-hydrogen cell and ac-impedance spectroscopy: Part II. Modeling

A two-dimensional, one-phase model for hydrogen and water vapor transport inside a hydrogen-hydrogen cell is presented. The model was used to analyze the results of water transport measurements. As a result, the concentration dependence of the diffusion coefficient of water in membrane of Gore^™ Primea^® Series 58 MEA was achieved. Temperature differences inside a hydrogen-hydrogen cell were studied with modeling approach and it was found that the cell can be treated as an isothermal system.     

Characterization of membrane electrode assembly with hydrogen-hydrogen cell and ac-impedance spectroscopy: Part I. Experimental

A measurement system based on a symmetrical hydrogen-hydrogen cell was developed and presented. The net water flux through and impedance of Gore™ Primea^® Series 58 MEA were measured under different humidity conditions. An equivalent circuit approach was used to analyze impedance measurement data and the dependences of membrane conductivity and reaction kinetics on the humidity were obtained. Modeling work was found necessary to estimate the diffusion coefficient of water in the membrane, because humidity profile inside the cell was uneven and unknown during the net water flux measurements. Gas permeabilities of different cell components and water uptake of the MEA were measured to get parameter values for the modeling work. The actual modeling work was not conducted in this contribution, but is explained in Part II of this paper.     

Water electrolysis under microgravity: Part 1. Experimental technique

Water electrolysis was conducted in both alkaline (25 wt.% KOH, 2 wt.% KOH) and acid (0.1N H₂SO₄) solutions for 8 s under microgravity environment realized in a drop shaft. The gas bubble formation of hydrogen and oxygen on platinum electrodes was observed by CCD camera. In alkaline solutions, a bubble froth layer grew on the electrode surface. Hydrogen bubble size was smaller than that of oxygen. The current density at constant potential decreased continually with time. In spite of the growth of a bubble froth layer on the electrode, the electrolysis never stopped, apparently because fresh electrolyte is supplied to the electrode surface by microconvection induced by the gas bubble evolution. In acid solution, hydrogen gas bubbles frequently coalesced on the cathode surface, yielding a larger average bubble than that of oxygen. The current density did not vary at constant potentials from -0.4 to −0.8 V versus reversible hydrogen electrode (RHE), because the effective electrode surface area was significantly reduced by the larger bubble size compared to alkaline electrolyte. The present experiments indicate that, especially in a microgravity environment, the bubble evolution behavior and the resultant current-potential curves are significantly influenced by the wettability of the electrode in contact with the electrolyte.     

Trace elements in clinker: I. A graphical representation

The trace element content of clinkers (and possibly of cements) can be used for the qualitative identification (i.e. manufacturing works). Several hundred clinker sorts have been analysed (by replicated quarterly samples, collected from all Hungarian cement factories, as well as from factories in eight foreign countries) to determine their Mg, Sr, Ba, Mn, Ti, Zr, Zn and V content. The first six elements come from the main raw materials and are of dactylogrammatic value, while the last two elements mainly come from fuel (used tires and heavy fuel oil, respectively) and cannot be used for identification. In this paper, a graphical method is presented to facilitate the visualisation of the trace element content.     

Thin-film particles of graphite oxide 1:: High-yield synthesis and flexibility of the particles

Thin-film particles of graphite oxide were synthesized in high yield by the modified Hummers’ method, in which a very long oxidation period was combined with a high purity purification process. The particles obtained had an average thickness of several nanometers and an average width of about 20 μm. The yield was 122 wt% based on the raw graphite and the recovery of carbon was 68%. Moreover, excellent flexibility of the particles was observed for the original particle(s) itself, for the particle in matrix polymer, and for two kinds of secondary conformations (lamination-layer-aggregate and random-shape-aggregate). As generally expected and already observed partially, if the affinity between the particle and the dispersion medium was very high, the thin-film particle extended well, though a local crease or a large bend could be generated. At a medium degree of affinity, the particle slightly bent, and at very low affinity, the particle(s) randomly aggregated.     

Polyaniline as cathodic material for electrochemical energy sources: The role of morphology

Polyaniline layers of different morphologies ranging from open and “sponge-like” structures to compact and “pebble-like” surfaces were synthesized from perchlorate solutions and employed as cathode in the galvanic cell with Zn anode and NH₄Cl/ZnCl₂ electrolyte. Cathodic properties of synthesized layers were investigated by the constant current charging/discharging method in 500 cycles. Specific charge capacities and specific energies obtained form the current-time curves strongly depend on the morphology of investigated layers and discharge conditions. The results unambiguously show that charging/discharging reaction of polyaniline layers is limited to relatively thin layer at polymer/solution boundary. Specific charge capacities are inversely related to both the polymer thickness and the discharge current density. In the limit of zero current densities the specific charge capacity as high as 245 A h kg⁻¹ could be achieved for porous structures of polyaniline layers. Specific capacitance higher than 400 F g⁻¹ obtained at 2 mA cm⁻² current density makes polyaniline a promising material for the application in electrochemical supercapacitors. The electrochemical behaviour of the layers was investigated by cyclic voltammetry and electrochemical impedance spectroscopy before and after 500 cycles of charging/discharging experiments. Both, cyclic voltammetry and electrochemical impedance spectroscopy showed that some polyaniline layers develop an increased charged transfer resistance at the carbon support/polymer interface during charging/discharging process. The increased charge transfer resistance does not affect the overall specific charge of the layers. The low-frequency capacities in impedance spectra are attributed to charging/discharging of polymer/electrolyte interface and seem to be related to the specific charge capacities obtained by extrapolation to zero current density discharge reaction.     

Electrochemistry of non-aged 90-10 copper-nickel alloy (UNS C70610) as a function of fluid flow: Part 1: Cathodic and anodic characteristics

The polarisation behaviour of freshly polished UNS C70610 (CN 102) 90-10 copper-nickel has been examined in fully characterised seawaters using the rotating disc electrode (RDE) and the rotating cylinder electrode (RCE) geometries. The charge and mass transport controlled responses of both the cathodic and anodic reactions are presented as a function of both laminar and turbulent undisturbed fluid flow. At low values of polarisation applied over short exposure periods (<1 h), the anodic behaviour of non-aged material is controlled by the selective dissolution of the copper component of the alloy. Under conditions of complete mass transport control, oxygen reduction proceeds via the irreversible direct four-electron reduction to the hydroxide ion. Above a critical range of Reynolds numbers the rate of both the reduction and oxidation reactions tended towards a more reversible character. This change in mechanism, however, was not observed for unalloyed copper and was attributed to a convective-diffusion-based modification of the corrosion product film.     

Investigation of zinc chromatation: Part II. Electrochemical impedance techniques

A mechanism to explain the formation of a chromate layer on zinc is proposed. It assumes that a ZnO inner film blocks the zinc surface on which the chromate layer grows. This layer has gel-like properties. The diffusion of the protons across the chromate layer and across the solution is supposed to be the kinetically limiting steps. This model was derived and experimentally tested in terms of impedance. The influences of the immersion time, mass transport, and pH of the chromatation solution were examined. A rather good agreement was found between the predictions of the model and the experimental results.     

Synthetic carbons activated with phosphoric acid: II. Porous structure

Synthetic activated carbons were prepared by H₃PO₄ activation of a chloromethylated and sulfonated copolymer of styrene and divinylbenzene, using an impregnation weight ratio of 0.75 and carbonization temperatures in the 400-1000 °C range. Other impregnation ratios (0.93 and 1.11) were also used at a carbonization temperature of 800 °C. The porous texture of the resulting carbons was characterized by N₂ adsorption at −196 °C and CO₂ adsorption at 0 °C. All carbons exhibited a multimodal pore size distribution with maxima in the micropore and meso/macropore regions. Maxima in pore volume were attained at 900 °C for micropores and at 500 and 900 °C for mesopores. The mesopore volume was less sensitive than the micropore volume to changes in the impregnation ratio. It is concluded that the porous texture is not a prime factor in determining the outstanding cation exchange capacities of these carbons.     

Alkali-silica reaction: A method to quantify the reaction degree

We propose a new chemical method for quantitative measurement of the reaction degrees of the alkali-silica reaction (ASR). We apply this method to a crushed natural reactive aggregate kept in contact with an alkaline solution, lime saturated by an appropriate amount of portlandite. This chemical system is designed to model the concrete capillary pores alkaline solution, in contact with reactive aggregates. Two reaction steps are taken into account in the mechanism: formation of Q₃ sites made by breaking up siloxane bonds of the reactive silica and dissolution of these Q₃ sites. The dissolution degree is measured by a selective acid treatment, and the nature of silica into solution is characterised by liquid NMR spectroscopy. The remaining silica is composed of Q₄ tetrahedrons and Q₃ protonated sites identified by solid NMR spectroscopy. These Q₃ protonated sites are measured by thermogravimetry analysis. We show that the formation Q₃ sites prevails on dissolution as the reaction progresses and contributes to an internal silica gel generation. The limiting step is the siloxane breaking up. Petrographic observations show that the reaction front penetrates in the aggregate through its porosity.     

Characterization of the ASR rim: Application to the Potsdam sandstone

Chemical properties of the reaction rim associated with alkali-silica reaction (ASR) were investigated using microprobe and scanning electron microscope (SEM). The studied aggregate is the Potsdam sandstone, a Cambrian siliceous sandstone well known for its reactivity. This particular rock is composed of well-crystallized quartz grains surrounded by a poorly crystallized siliceous cement that is considered to be the reactive constituent. Research was conducted on laboratory concrete specimens having reached various expansion levels and on some samples taken from an ASR-affected dam. Results indicate that the dark rim surrounding reactive particles is mainly composed of silica. This suggests that the reaction rim is formed by the precipitation of dissolved silica. Some alkalis and calcium were detected inside the thin intergranular joints in concentration ranging from 1% to 10%. These ions come from the cement paste and play a major role in dissolving original reactive silica.