Utilizing Aggregates Characteristics to Minimize Cement Content in Portland Cement Concrete

Aggregate, the main constituent of concrete, constitutes 60 to 80% of the total volume of concrete. Proper selection of the type and particle size distribution of the aggregates affects the workability and the hardened properties of the concrete. There are two main reasons for increasing the amount of aggregates in concrete. The first is that cement is more expensive than aggregate, so using more aggregate reduces the cost of producing concrete. The second is that most of the durability problems, e.g. shrinkage and freezing and thawing or hardened concrete, are caused by cement. Generally, concrete shrinkage increases with increase in cement content; aggregates, on the other hand, reduce shrinkage and provide more volume stability. Furthermore, cement production is a key source of carbon dioxide (CO2) emissions, and reducing its usage should be a goal for concrete production. Various projects have explored methods of minimizing cement in concrete; among the most common of those is replacing cement with cementitious and pozzolanic materials such as fly ash.Aggregates Foundation for Technology, Research, and Education (AFTRE)Civil, Architectural, and Environmental Engineerin

Bond angles for O-H defects in SnO2 from polarization properties of their vibrational modes

Infrared absorption experiments made with polarized light yield significant insights into the possible structures of one- and two-O-H defects in SnO2 that are produced by thermal annealing treatments. These polarized absorption results reveal that a two-O-H defect must involve symmetry-equivalent O-H sites and that the axes of both one- and two-O-H defects are 63 degrees-68 degrees from the c axis of the rutile structure. These O-H bond angles found by experiment restrict the microscopic defect structures that are possible and suggest structures associated with either a metal atom substituting for Sn or an interstitial metal atom (such as Sn)

Wettability and capillary behavior of fibrous gas diffusion media for polymer electrolyte membrane fuel cells

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.jpowsour.2009.04.052 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/The relationship of capillary pressure to liquid saturation for the water-air fluid pair in two different types of gas diffusion media (GDM) used in polymer electrolyte membrane fuel cell (PEMFC) electrodes is elucidated. It is experimentally demonstrated that GDM samples with and without treatment with poly(tetrafluoroethylene) (PTFE) ubiquitously display permanent capillary pressure hysteresis. Water does not imbibe spontaneously into a dry GDM, neither is it ejected spontaneously from a water-saturated GDM. Rather, positive displacement pressure is required to force both water and air into GDMs, whereas the main effect of adding PTFE is to increase the amount of work required for forcing water into the GDM. and to decrease the work required for water removal. Irrespective of PTFE content, the GDM samples tested are generally shown to behave as materials of intermediate (neutral) wettability. The US Bureau of Mines (USBM) wettability index nevertheless shows that water is the preferentially non-wetting phase in PTFE-treated GDMs and the preferentially wetting phase in untreated GDMs. Water-air capillary pressure curves are found to depend on sample thickness, clearly demonstrating that finite size effects are important. Finally, compression of the GDM is found to increase the capillary pressures for water injection and decrease the capillary pressures required for water withdrawal. These results should aid the design of GDMs with improved water management properties and the modeling of PEMFC electrodes in general. (C) 2009 Elsevier B.V. All rights reserved.Natural Science and Engineering Research Council of Canada (NSERC

On the role of the microporous layer in PEMFC operation

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.elecom.2008.12.053 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/The condition of liquid water breakthrough at the cathode of polymer electrolyte fuel cells (PEMFC) is studied experimentally and data on corresponding water saturation and capillary pressure are provided for gas diffusion layers (GDL) with and without a microporous layer (MPL). The data demonstrate that the GDL saturation at water breakthrough is drastically reduced from ca. 25% to ca. 5% in the presence of MPL This observation is consistent with considerations of invasion percolation in finite-size lattices and suggests an explanation for the role of MPL in improving PEMFC performance at high current densities

Impact of Liquid Water on Reactant Mass Transfer in PEM Fuel Cell Electrodes

Published by Electrochemical Society. Final version available at: http://dx.doi.org/10.1149/1.3291977The breakthrough conditions (capillary pressure and liquid water saturation) in a fibrous gas diffusion medium (GDM) used in polymer electrolyte membrane (PEM) fuel cell electrodes have been studied experimentally by two independent techniques and numerically by pore network modeling. Experiments show that treatment of the GDMs with a hydrophobic polymer coating reduces the water saturation at a breakthrough by 50%. Invasion percolation modeling is employed to simulate the breakthrough process and to determine mass-transfer rates through the partially saturated network. This model shows that the water saturation at breakthrough is drastically reduced when a microporous layer (MPL) is incorporated into the GDM, agreeing with experiments. However, the simulations yield limiting currents significantly higher than those observed in practice whether or not an MPL is present. Further calculations to include the contribution of condensation to water saturation within the GDM also result in unrealistically high limiting currents and suggest that mass-transfer resistance in the catalyst layer that is not included in the model plays an important role. If condensation is the principal mode for water accumulation within the GDM, simulations show that the MPL has only a small impact on liquid water distribution and does not improve performance, contrary to expectation.Natural Science and Engineering Research Council of Canada (NSERC

Pore network modeling of fibrous gas diffusion layers for polymer electrolyte membrane fuel cells

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.jpowsour.2007.04.059 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/A pore network model of the gas diffusion layer (GDL) in a polymer electrolyte membrane fuel cell is developed and validated. The model idealizes the GDL as a regular cubic network of pore bodies and pore throats following respective size distributions. Geometric parameters of the pore network model are calibrated with respect to porosimetry and gas permeability measurements for two common GDL materials and the model is subsequently used to compute the pore-scale distribution of water and gas under drainage conditions using an invasion percolation algorithm. From this information, the relative permeability of water and gas and the effective gas diffusivity are computed as functions of water saturation using resistor-network theory. Comparison of the model predictions with those obtained from constitutive relationships commonly used in current PEMFC models indicates that the latter may significantly overestimate the gas phase transport properties. Alternative relationships are suggested that better match the pore network model results. The pore network model is also used to calculate the limiting current in a PEMFC under operating conditions for which transport through the GDL dominates mass transfer resistance. The results suggest that a dry GDL does not limit the performance of a PEMFC, but it may become a significant source of concentration polarization as the GDL becomes increasingly saturated with water

Direct measurement of the capillary pressure characteristics of water-air-gas diffusion layer systems for PEM fuel cells

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.elecom.2008.08.008 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/A method and apparatus for measuring the relationship between air-water capillary pressure and water saturation in PEMFC gas diffusion layers is described. Capillary pressure data for water injection and withdrawal from typical GDL materials are obtained, which demonstrate permanent hysteresis between water intrusion and water withdrawal. Capillary pressure, defined as the difference between the water and gas pressures at equilibrium, is positive during water injection and negative during water withdrawal. The results contribute to the understanding of liquid water behavior in GDL materials which is necessary for the development of effective PEMFC water management Strategies