Analytical and Numerical Models of Transport in Porous Cementitious Materials
- PDF / 401,643 Bytes
- 7 Pages / 420.48 x 639 pts Page_size
- 67 Downloads / 238 Views
ANALYTICAL AND NUMERICAL MODELS OF TRANSPORT IN POROUS CEMENTITIOUS MATERIALS
EDWARD J. GARBOCZI AND DALE P. BENTZ National Institute of Standards and Technology and Center for Advanced Cement-Based Materials Building Materials Division, 226/B348 Gaithersburg, Maryland 20899 ABSTRACT Fluid flow under applied pressure gradients and ionic diffusion under applied concentration gradients are important transport mechanisms that take place in the pore space of cementitious materials. This paper describes: 1) a new analytical percolation-theory-based equation for calculating the permeability of porous materials, 2) new computational methods for computing effective diffusivities of microstructural models or digitized images of actual porous materials, and 3) a new digitized-image mercury intrusion simulation technique. INTRODUCTION Most chemical and physical processes that degrade cementitious materials are dependent on an external source of either water or ions or both. Understanding the rates of these processes at the microstructural level is necessary in order to develop a sound scientific basis for the prediction and control of the service life of cement-based materials, especially for radioactive-waste containment materials that are required to have service lives on the order of hundreds of years. An important step in developing this knowledge is to understand how transport coefficients, such as diffusivity and permeability, depend on the pore structure. KATZ-THOMPSON
PERMEABILITY THEORY
The Katz-Thompson permeability theory (1,2] uses a pore diameter measurement and a single measurement of the electrical conductivity or ionic diffusivity of a fluid-saturated porous sample to predict permeability, using percolation theory. The fundamental parameter of the theory is the critical pore diameter, d . The definition of dc can be understood by considering the following thought experiment. Consider a porous piece of hardened cement paste. Fill each pore with a liquid, in descending order of diameter, beginning with the largest pores first. When enough pores have been filled so that there is a connected pathway of liquid from one side of the sample to the other, stop. The diameter of the last pore filled is defined as dc. This thought experiment is approximately realized in a mercury intrusion experiment. Because mercury is non-wetting, in an intrusion experiment the largest
Mat. Res. Soc. Symp. Proc. Vol. 176. @1990 Materials Research Society
676
starting at the sample surface, followed pores are filled first, by increasingly smaller pores as the intrusion pressure is Katz and Thompson experimentally verified that the increased. inflection point in the cumulative intrusion curve, where total intruded volume is plotted versus intrusion pressure, defines dc (1,2]. The importance of dc for flow becomes obvious when one considers that a pore channel can only contribute significantly to flow when it is relatively large and part of the connected pathway The subset of pores with d > dc, being the set across the sample. of th
Data Loading...
