IN PREPARING THIS CHAPTER ON SLAG AS A CEmentitious material, the contents of the 4^th edition were drawn upon. The author acknowledges the authors of the previous editions, where this subject was included in the chapter on Mineral Admixtures. The authors were C. J. Cain, L. H. Tuthill, and E. C. Higginson, respectively, of each previous edition. This current edition will review and update the topic as addressed by the previous authors, introduce new technology that has been developed, and include up-to-date references.