Reference: Modelling and Simulation in Materials Science and Engineering, 2, 783-808, (1994).Go back to Part I Chapter 4 Sec (4b) Go back to Table of Contents
Dale P. Bentz*, Peter V. Coveney+, Edward J. Garboczi*,
Michael F. Kleyn+ and Paul E. Stutzman*
* National Institute of Standards and Technology, Gaithersburg, Maryland 20899 USA 1
+ Schlumberger Cambridge Research, High Cross, Madingley Road, Cambridge, CB3 0HG UK
Cellular automaton algorithms, which operate on a starting digital image of a watercement suspension, are described. The algorithms simulate the microstructure development process due to hydration reactions that occur between cement and water. This paper describes the evolution of the cement model from a simple model, which treated the cement particles as single phase materials, with a greatly simplified hydration chemistry, into a model which has many more chemical species and includes numerous reactions which eventually convert the viscous watercement suspension into a rigid porous solid.
Methods are presented for generating two and three dimensional images representing suspension initial conditions; these are derived both from micrographs of real cements and computerbased algorithms. The 2D initial images are based on the processing of backscattered electron and Xray images of real cement suspensions. The 3D images employ either spheres to represent cement particles, or more realistic randomly shaped particles via an algorithm which smooths and thresholds a 3D lattice whose sites are initially populated with random white noise.
A convenient measure of the point at which the initial paste turns into a solid material is the percolation threshold of the solids. Consideration of these models has already led to the prediction and subsequent experimental observation of a sharplydefined onset of shear wave propagation, from ultrasonic measurements through hydrating cement slurries. The amount of hydration needed to reach the percolation threshold can be determined in the present simulations, and our results are compared with time of shear wave onset in actual cement slurries.
Variants of the basic model provide insight into both early time behaviour that is of primary interest to oilwell cementing and the later time microstructural properties that are of interest in the construction industry.