Search

Recently, techniques for acquiring, analyzing, and using 3-D shape information for aggregates (sand, gravel) have been developed and demonstrated. These mathematical techniques, based on spherical harmonic coefficient analysis, have been applied to X-ray computed tomography images. X-ray microtomographic images of cement particles are required to be able to apply these same mathematical techniques to analyze Portland cement particle shape. Such images are available in the Visible Cement Database for a single reference cement. Actual cement images, as well as some quantitative results on the nonsphericity of the particles, are given here. Three-dimensional cement particle shapes have never been available before. A demonstration of how real cement particle shapes will change the CEMHYD3D cement hydration model is also given. [Copyright &y& Elsevier]

Abstract: The linear elastic moduli of cement paste are key parameters, along with the cement paste compressive and tensile strengths, for characterizing the mechanical response of mortar and concrete. Predicting these moduli is difficult, as these materials are random, complex, multi-scale composites. This paper describes how finite element procedures combined with knowledge of individual phase moduli are used, in combination with a cement paste microstructure development model, to quantitatively predict elastic moduli as a function of degree of hydration, as measured by loss on ignition. Comparison between model predictions and experimental results are good for degrees of hydration of 50% or greater, for a range of water:cement ratios. At early ages, the resolution of the typical 1003 digital microstructure is inadequate to give accurate results for the tenuous cement paste microstructure that exists at low degrees of hydration. Elastic computations were made on higher resolution microstructures, up to 4003, and compared to early age elastic moduli data. Increasing agreement with experiment was seen as the resolution increased, even when ignoring possible viscoelastic effects. [Copyright &y& Elsevier]

Finite element procedures combined with microstructure development modeling are integrated to quantitatively predict the viscoelastic/viscoplastic relaxation of cement paste due to intrinsic calcium silicate hydrate viscoelasticity and microstructure evolution associated with the hydration process. The combined models are implemented in a computational routine to predict time-dependent stress and strain fields in cement paste. The model simulations suggest that inherent viscoelastic deformation caused by calcium silicate hydrate is not necessarily the primary mechanism leading to the overall early-age viscoelastic/viscoplastic behavior of cement paste. The effect of time-dependent dissolution of cement grains occurring during the hydration process is substantial and should be considered as a significant mechanism for the apparent viscoelasticity/viscoplasticity of cement paste. [ABSTRACT FROM AUTHOR]