- Author
-
Bentz, D. P.
|
Garboczi, E. J.
- Title
- Computer Model for the Diffusion and Binding of Chloride Ions in Portland Cement Paste.
- Coporate
- National Institute of Standards and Technology, Gaithersburg, MD
- Report
-
NISTIR 5125,
February 1993,
24 p.
- Distribution
- Available from National Technical Information Service
- Keywords
-
cements
|
computer models
|
diffusion
|
adsorption
|
binding
|
building technology
|
chloride ions
|
concentration profiles
|
hydration
|
microstructure
|
random walk
|
simulation
- Abstract
- A two-dimensional computer model has been developed to simulate the diffusion and binding of chloride ions in cement paste. The model is based on a random walk algorithm in which chloride species randomly diffuse throughout the cement paste microstructure and interact with various phases of the paste. Reaction with unhydrated C₃A and C₄AF and adsorption by the C-S-H gel phase are the two binding processes included in the model. Input to the model is a digital image of cement paste microstructure which can be obtained from a real cement sample or from a digital-image-based microstructure model. The operation of the diffusion and binding model is demonstrated on pastes made from two cements whose differing compositions are captured by combining backscattered electron and x-ray images obtained using a scanning electron microscope. These initial images are "hydrated" using the microstructure model to produce final images to be utilized as input into the diffusion model. Chloride concentration profiles are generated for both the binding and no-binding cases for both microstructures for times of up to several hours after exposure to the chloride. Binding is seen to have a significant effect on the early-time penetration of chlorides into cement paste. Results for the two cements indicate that a higher chloride diffusion coefficient may be offset, at least initially, by a greater chloride binding capacity.