Architecture：Accelerating Admixtures.pdfVIP

189 5 Accelerating Admixtures 1.0 INTRODUCTION Admixtures can influence the properties of concrete from the moment water comes into contact with the concrete ingredients. In the fresh state, the water requirement, workability, bleeding, segregation, rate of hydration, setting time, air content, pumpability, etc., and in the hardened state, compressive, flexural and tensile strengths, creep, shrinkage, perme- ability, and durability are affected to different extents, depending on the type and amount of admixture added. The general types of admixtures used in concrete and their applications have already been described in Ch. 4 of this book. Thermal analysis techniques have been applied widely for the investigation of the role of admixtures, especially that related to the hydration of cement and cement components. Application of thermal analysis permits determination of the heat of reaction, mechanism of reaction, kinetics of reactions, compatibility of admixtures with cements, prediction of some properties, durability problems, material characteriza- tion and selection, development of new admixtures, quick assessment of some physical properties, etc. In some instances, they yield results that are not possible to obtain with the use of other techniques. 189 190 Chapter 5 - Accelerating Admixtures In this chapter, typical examples of the application of various thermal techniques to the study of the effect of admixtures on the hydration of cement and cement compounds is emphasized. Where relevant, the results obtained with these techniques are compared with those derived from other tools. 2.0 CALCIUM CHLORIDE Calc