J.J. Beaudoin

Cement and Concrete Nanoscience and Nanotechnology

Concrete science is a multidisciplinary area of research where nanotechnology potentially offers the opportunity to enhance the understanding of concrete behavior, to engineer its properties and to lower production and ecological cost of construction materials. Recent work at the National Research Council Canada in the area of concrete materials research has shown the potential of improving concrete properties by modifying the structure of cement hydrates, addition of nanoparticles and nanotubes and controlling the delivery of admixtures. This article will focus on a review of these innovative achievements.

Electrical percolation phenomena in cement composites containing conductive fibres

Electrical conductivity measurements on cement composites containing carbon fibres or steel fibres were conducted. Percolation phenomena associated with electrical conductivity were observed. The conductivity of the systems studied increased by several orders of magnitude, at a specific concentration of conductive fibre, i.e. the percolation concentration. The percolation concentration is shown to be dependent on conductive fibre geometry instead of system composition. The results provide an important guide for the manufacture of conductive cement composites containing conductive fibres.

Interaction of chloride and CSH

Abstract The chloride binding properties of synthetic CSH preparations having a wide range of C/S and H/S ratios are examined. Bound chloride is separated into two types, alcohol-insoluble and tightly held. Dependence of so-called chemisorbed chloride on C S ratio, C S ratio and surface area is observed. A mechanism for chloride interaction compatible with the Feldman and Taylor models for CSH is suggested. Implications with respect to corrosion of steel in concrete are also discussed.

A.C. Impedance spectroscopy. I: A new equivalent circuit model for hydrated Portland cement paste

Abstract A.C. impedance spectroscopy has been used to investigate the mechanism of hydration of portland cement paste, from 48 to 380 hours. Interpretation of a.c. measurements obtained over a wide frequency range was complimented by equivalent circuit modelling. The proposed equivalent circuit was chosen so that its RC parameters would physically represent microstructural elements of the cement paste.

A rationalized a.c. impedence model for microstructural characterization of hydrating cement systems

Abstract An electrical conductivity model for hydrating portland cement systems has been developed. The high frequency a.c. impedance technique for characterizing the microstructure of cement paste and the hydration process of cement has been rationalized quantitatively for the first time. It is suggested that the major contribution to the a.c. impedance behavior of hydrating cement systems results from solid-liquid interfaces. All observed experimental phenomena can be explained by the proposed model. It is suggested that a.c. impedance techniques can be used for investigation of solid-liquid interfacial interaction in a porous material.

A.C. impedance spectroscopy (II): Microstructural characterization of hydrating cement-silica fume systems

Abstract The influence of silica fume on impedance behaviour of hydrating cement has been carefully investigated. The results demonstrate that impedance measurement is very sensitive to changes in hydration kinetics and microstructure development due to the presence of silica fume. A modified equivalent circuit containing a frequency dependent resistance element is proposed to model hydration of these microporous systems. A.C. impedance spectroscopy is demonstrated to be a useful tool for studying factors that influence pore structure in cement systems containing finely divided silica.

Application of A.C. impedance techniques in studies of porous cementitious materials

Abstract The influence of ionic concentration of pore solution and matrix porosity on high frequency resistance (HFR) of hydrated cement paste were carefully studied. A new relation between HFR and porosity has been derived and its validity demonstrated by a series of experiments. A better description of the intrinsic value and physical meaning of HFR is presented. A convenient means of characterizing microstructural development in hydrating cement or concrete systems is provided by the A.C. impedance technique.

Pretreatment of hardened hydrated cement pastes for mercury intrusion measurements

Abstract Porosity is one of the major factors controlling durability and strength of hydrated cement products. A measure of pore size distribution of these materials is more definitive and can lead to a basic understanding of many phenomena occuring within the material. An accurate measurement of this is, however, difficult to obtain. Hg intrusion porosimetry to 414 MPa was used in this work to measure the pore size distribution of cement pastes prepared at water/cement ratio of 0.8, 0.6 and 0.45. Specimens were predried before intrusion measurements by several techniques including solvent replacement with methanol or isopropanol, evacuation and/or heating for various periods and conditioning at 11% RH. Second Hg intrusions were also performed to investigate the effects of first intrusion. It was concluded that it is not possible to obtain an actual pore size distribution of cement paste by Hg intrusion because of its sensitivity to stress.

Mechanism of sulphate expansion I. Thermodynamic principle of crystallization pressure

Abstract A new theory concerning sulphate expansion phenomena in cement and concrete systems is developed based on the principles of chemical-thermodynamics. It suggests that sulphate expansion is a process in which chemical energy is being converted into mechanical work to overcome the cohesion of system. The expansive force results from “crystallization pressure” which is a result of the interaction between the solid product of a chemical reaction, e.g. ettringite, and the cement paste. Two conditions are necessary for the occurrence of crystallization pressure: (1). Confined crystal growth of the solid product. (2). “Activity product” of reactants in the pore solution is greater than the “solubility product” of the solid product under atmospheric pressure.

Microstructure and strength of hydrated cement

Abstract Several hydrated portland cement systems have been studied at DBR in a wide range of porosities. These systems include room-temperature hydrated paste, autoclaved paste, autoclaved with addition of sulfur and silica, hot-pressed samples and compacts of synthetic 14A tobermorite. Measurements included compressive strength, Youngs modulus, product density, porosity and helium inflow, by which the various systems were characterized. It was concluded that, at a given porosity, an optimum proportion of higher density crystalline material and poorly aligned and ill-crystallized material yields the best strength but the quantity of poorly-crystallized material required decreases with porosity.