Design of Magnesium Phosphate Cement Based Composite for High Performance Bipolar Plate of Fuel Cells

In this work, we report a comprehensive study on a magnesium phosphate cement (MPC) based composite as the construction material for high performance bipolar plates of fuel cells. MPC with partial replacement of fly ash was employed as the binding matrix. Some carbon-based materials, such as graphite, carbon black, carbon fiber, and multi-walled carbon nanotubes were used to construct the conductive phase. A simple hot-press process was applied to produce the composite. The formula and the structure of the composite was modified and adjusted to optimize the properties of the composite to meet the US DOE 2015 technical targets, including the introducing of a reinforcement support. Finally, all the technical targets such as electrical conductivity (\u3e100 S cm-1), the flexural strength (\u3e25 MPa), the corrosion resistance ( \u3c 1 μA cm-2), and gas permeability ( \u3c 10-5 cm3 (s cm2)-1) were achieved as well as low cost ( \u3c 5 $ per kW). The optimized formula and the detailed procedures to fabricate the MPC based composite were concluded

Realistic pore structure of Portland cement paste: experimental study and numerical simulation

In this study, the pore structure of Portland cement paste is experimentally characterized by MIP (mercury intrusion porosimetry) and nitrogen adsorption, and simulated by a newly developed status-oriented computer model. Cement pastes with w/c= 0.3, 0.4 and 0.5 at ages from 1 day to 120 days are comprehensively investigated. It is found that MIP cannot generate valid pore size distribution curves for cement paste. Nevertheless, nitrogen adsorption can give much more realistic pore size distribution curves of small capillary pores, and these curves follow the same distribution mode. While, large capillary pores can be effectively characterized by the newly developed computer model, and the validity of this model has been proved by BSE imaging plus image analysis. Based on the experimental findings and numerical simulation, a hypothesis is proposed to explain the formation mechanism of the capillary pore system, and the realistic representation of the pore structure of hydrated cement paste is established

Microstructures and Mechanical Properties of Polymer Modified Mortars under Distinct Mechanisms

In this study, two types of acrylic latexes, PA (polyacrylate) and PU/PA (polyurethane modified PA), are investigated in their influences on mechanical properties of mortar under different interaction mechanisms. In light of a previous study, the polymer-cement hydrates interaction mechanisms in PA and PU/PA modified mortars are illustrated respectively, and the microstructures are simulated using a computer model. Through mechanical experiments, it is revealed that the incorporation of polymer tends to reduce the compressive strength and elastic modulus except PA at low P/C ratio, while improve the flexural strength and toughness. As compared with PA, PU/PA is more effective in these influences. All of the influences of PA and PU/PA on mechanical properties can be explained successfully based on the interaction mechanisms and microstructures. In addition, it\u27s also found that the compressive strength of polymer modified mortar can be roughly estimated based on a modified gel/space ratio, and the incorporation of polymers does not change the relationship between elastic modulus and compressive strength. A high-temperature curing procedure is concluded to be suitable for preparation of high-performance cement composites in short period

Multi-Aggregate Approach for Modeling Interfacial Transition Zone in Concrete

Interfacial transition zone (ITZ) has long been of particular interest in concrete technology. The limited sensitivity of experimental techniques makes it attractive to study ITZ using computer simulations. In this paper a multi-aggregate approach is proposed to simulate the formation of ITZ in concrete. In light of a modified status-oriented computer model for simulating cement hydration, the evolution of ITZ is also simulated in this approach. Through simulations, the influences of several factors related to concrete mixture proportion on ITZ are investigated. It is found that ITZ thickness, as defined by the overall average porosity, can be reduced by using finer aggregate, increasing aggregate volume fraction, reducing water-cement ratio (w/c), or making the binder system finer Following hydration, the ITZ thickness decreases continuously, but the difference of porosity between ITZ and bulk paste keeps almost constant at mature ages

Two-Scale Modeling of Transport Properties of Cement Paste: Formation Factor, Electrical Conductivity and Chloride Diffusivity

Predicting transport properties of cement-based materials directly from the microstructure is very challenging, due to the problems of bridging length scales and the difficulties of realistically representing the microstructure. Based on a two-scale representation of the microstructure, a scheme is proposed in this paper to model the transport properties of cement paste through two-scale random walk simulation. A random walk algorithm is firstly applied at the sub-micro-scale to determine the diffusion tortuosity of the outer C-S-H layer. This is then up-scaled to the micro-scale to compute the diffusion tortuosity of cement paste. Based on physical laws, the diffusion tortuosity is transformed into the formation factor, and further into the electrical conductivity and the chloride diffusion coefficient of cement paste, and subsequently validated. It is proven that a more realistic representation of the microstructure makes it possible to derive transport properties of cement paste, directly and accurately, from the microstructure

Influence of Magnesia-to-Phosphate Molar Ratio on Microstructures, Mechanical Properties and Thermal Conductivity of Magnesium Potassium Phosphate Cement Paste with Large Water-to-Solid Ratio

This paper describes the influence of the magnesia-to-phosphate (M/P) molar ratios ranging from 4 to 12, on the properties and microstructures of magnesium potassium phosphate cement (MKPC) pastes with a large water-to-solid ratio (w/s) of 0.50. The setting behavior, compressive strength, tensile bonding strength and thermal conductivity of the MKPC pastes, were investigated. The results show that an increase in the M/P ratio can slow down the setting reaction, and clearly degrade the mechanical strengths, but clearly improve the thermal conductivity of MKPC pastes. Furthermore, micro-characterizations including X-ray diffraction, scanning electron microscopy and thermogravimetric analysis, on the MKPC pastes reveal that a lower M/P ratio can facilitate better crystallization of the resultant magnesium potassium phosphate hexahydrate (MKP) and a denser microstructure. Moreover, strong linear correlations are found between the mechanical strengths and the MKP-to-space ratio, and between thermal conductivity and the volume ratio of the unreacted magnesia to the MKP

New Pore Structure Assessment Methods for Cement Paste

In this study, two new approaches for pore structure assessment of cement paste are investigated and compared with mercury intrusion porosimetry (MIP)-based methods. One is based on a status-oriented computer model; the other is based on fractal analysis on the impedance measured by a noncontact impedance measurement system. In the computer model, cement paste microstructure is simulated as a function of cement properties, water-to-cement ratio (w/c), and degree of hydration. With the simulated microstructure, large capillary pores are characterized by image processing. A model developed based on nitrogen adsorption results and physical rules is applied to predict the small capillary pore structure, as a complementation to cover the whole range of capillary pores. In the impedance measurement based approach, fractal dimensions corresponding to specific pore size ranges are determined from the fractal networks and the impedance responses of the material. Then the pore size distribution curves are deduced. The former approach is capable of providing much more realistic representation of the pore structure as compared with the traditional MIP method, while the latter is also considered as a promising method for pore structure assessment, especially in large pore size range

Ultrasonic Monitoring of the Early-Age Hydration of Mineral Admixtures Incorporated Concrete using Cement-Based Piezoelectric Composite Sensors

The early-age hydration processes of concretes with mineral admixtures have been monitored and evaluated by a newly developed ultrasonic method based on embedded cement-based piezoelectric composite sensors. With the embedded ultrasonic (P-wave) measurement system, the waveform, wave velocity, attenuation coefficient index, and frequency-domain spectrum of detected ultrasonic waves during hydration can be recorded. The mineral admixtures examined include fly ash, slag, and silica fume, which replace part of the cement in concrete mixtures. It is found that the ultrasonic transmission parameters can be related to the microstructure changes of the concrete. Both the acceleration effects of silica fume and the retardation effects of fly ash and slag on the early hydration of concrete can be determined and explained through the analysis and comparison of the characteristics of the velocity curves. The attenuation coefficient index curve provides additional observation for the study of hydration kinetics. Moreover, the function of fresh concrete in filtering the high-frequency component of the wave varies with time, and concrete can be considered as low-pass frequency spectral filter. Frequency spectra analysis at different ages of fresh concrete provides useful information to reveal the early-age hydration process