Measurement and Modeling of Porosity in Drying Cement Paste
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MEASUREMENT AND MODELING OF POROSITY IN DRYING CEMENT PASTE
LESLIE PARROTT Cement and Concrete Association, Wexham Springs, UK
ABSTRACT
Preliminary models for predicting the performance of exposed concrete show that the porosity sub-model plays a pivotal role in linking properties with the developing gradients of cement paste microstructure that arise from drying and carbonation. The difficulties of porosity characterization are reviewed and some alternatives to traditional methods are considered. Property/porosity relationships are considered with particular reference to the volume, size-distribution and continuity of the larger pores. The factors influencing porosity under normal exposure conditions, such as cement hydration, drying and carbonation, are reviewed and the possibilities of modeling their effects are considered. Some comparisons between predicted and measured values of porosity are given. It is concluded that progress in modeling the performance of exposed concrete is likely to be linked with improvements in methods of pore structure characterization.
INTRODUCTION Preliminary models for predicting the service performance of concrete [1-3] have indicated that the sub-models of pore structure and moisture state of the cement paste phase play pivotal roles in linking the developing microstructure with concrete properties. The variations of cement paste porosity that develop with increasing time and distance from the exposed surface in drying concrete arise from changes in internal moisture, cement hydration and carbonation. Such factors as ambient humidity, curing times and size of a concrete member influence the moisture history at a given distance from the exposed surface and thus cause time-dependent gradients in cement hydration [4]. Variations in cement hydration will be accompanied by related variations in pore structures [5,6,7,8]. However, even in the absence of hydration gradients, the pore structure is significantly affected by drying [9,10,11,12]. Atmospheric drying is invariably associated with reactions between carbon dioxide and the cement hydrates [13,14]. This creates a carbonated surface layer that gradually increases in thickness and normally has a reduced porosity [8,15,16,17]. The assumptions necessary to quantify porosity variations due to moisture history, cement hydration and carbonation are considered herein. Two related topics are discussed. The first are the serious difficulties in pore structure characterization associated with sample preparation, the wide range of pore sizes and geometries to be measured, the choice of experimental methods and the reliable analysis of results. The second is what pore structure characteristics are required to reliably describe the properties and performance of cement paste and concrete. A detailed description of pore size distribution alone is unlikely to be sufficient for property prediction; some measure of pore connectivity will be required for prediction of diffusion and permeability properties, while the mechanical properties of the s
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