Hongyan Ma

Interactions between Organic and Inorganic Phases in PA- and PU/PA-Modified-Cement-Based Materials

In this paper, two types of acrylics, polyacrylate (PA), a copolymer of methyl methacrylate, acrylic acid, and others, and polyurethane-modified PA (PU/PA), are investigated in their interactions with inorganic phases in polymer-modified cement-based mixtures and in their mechanisms in improving both mechanical properties and durability of cement-based materials. In these investigations, the total organic carbon content (TOC) method is used to study the adsorption of polymer particles in mixtures. An aqueous tube test, x-ray diffraction (XRD), thermal gravity analysis (TGA), and Fourier transform infrared spectroscopy (FTIR) are employed to study the possible chemical reactions between the latexes and cement hydrates. Scanning electron microscopy (SEM) and energy-dispersive x-ray analysis (EDX) are used to observe the film formation and the final morphology of PA- and PU/PA-modified mortars. PU/PA latex is found to be relatively stable, and the behaviors of PU/PA-modified mixtures can be explained by Oham...

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.

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.

New Pore Structure Assessment Methods for Cement Paste

AbstractIn 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 d...

Dynamic Mechanical Properties and Microstructure of Graphene Oxide Nanosheets Reinforced Cement Composites

This paper presents an experimental investigation on the effect of uniformly dispersed graphene oxide (GO) nanosheets on dynamic mechanical properties of cement based composites prepared with recycled fine aggregate (RFA). Three different amounts of GO, 0.05%, 0.10%, and 0.20% in mass of cement, were used in the experiments. The visual inspections of GO nanosheets were also carried out after ultrasonication by transmission electron microscope (TEM) atomic force microscope (AFM), and Raman to characterize the dispersion effect of graphite oxide. Dynamic mechanical analyzer test showed that the maximum increased amount of loss factor and storage modulus, energy absorption was 125%, 53%, and 200% when compared to the control sample, respectively. The flexural and compressive strengths of GO-mortar increased up to 22% to 41.3% and 16.2% to 16.4% with 0.20 wt % GO at 14 and 28 days, respectively. However the workability decreased by 7.5% to 18.8% with 0.05% and 0.2% GO addition. Microstructural analysis with environmental scanning electron microscopy (ESEM)/backscattered mode (BSEM) showed that the GO-cement composites had a much denser structure and better crystallized hydration products, meanwhile mercury intrusion porosimetry (MIP) testing and image analysis demonstrated that the incorporation of GO in the composites can help in refining capillary pore structure and reducing the air voids content.

New equation for description of chloride ions diffusion in concrete under shallow immersion condition

Abstract A new equation describing chloride penetration into concrete under shallow immersion condition, based on modifications to the constant diffusivity solution of the Fick’s second law, is developed in this study. When applied to real concrete material in a specific environment, the Fick’s second law with constant diffusion coefficient assumption cannot provide a precise prediction of the chloride profile in concrete. The new equation, which considers the binding of chloride ions, the time-dependency of chloride diffusion coefficient due to hydration and the influence of calcium leaching on the diffusion process in its derivation, can predict the evolution of chloride profile with high accuracy. As shown in this study, both hydration of cement in concrete (expressed by the time-dependency factor) and calcium leaching (expressed by the leaching effect factor) can retard the ingress of chloride ions into concrete.

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 (>100 S cm−1), the flexural strength (>25 MPa), the corrosion resistance (<1 μA cm−2), and gas permeability (<10−5 cm3 (s cm2)−1) were achieved as well as low cost (<5

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

per kW). The optimized formula and the detailed procedures to fabricate the MPC based composite were concluded.