This work was done by a collaboration through the Industrial Affiliate Program of the National Science Foundation Science and Technology Center for Advanced Cement−Based Materials (ACBM). We thank the Center for partial funding of this work. A detailed critique of some parts of this work by Karen Scrivener of Imperial College, London, has been very helpful. Special thanks go to Tony Crossley of the Schlumberger Laboratory for Computer Science in Austin, Texas, for help in producing the white-noise-based images. We are grateful to Chris Hall and Tim Jones at Schlumberger Cambridge Research, for helpful comments and discussion at various stages of this work.