Architecture：Retarding and Water Reducing Admixtures.pdfVIP

221 6 Retarding and Water Reducing Admixtures 1.0 INTRODUCTION Lignosulfonate-based admixtures form a major percentage of the retarder/water reducer type of admixtures used in the concrete industry. Several techniques have been used to obtain an understanding of the action of retarders/water reducers, such as the mechanism of their action, rate of hydration, setting times, microstructure, etc. The techniques that have yielded important results include DTA, TG, DSC, scanning electron microscopy, XRD, chemical shrinkage measurements, isotherms, loss on ignition, IR, etc. These techniques are generally applied on samples that are hydrated for certain periods of time. The conduction calorimetry, however, follows the instantaneous evolution of heat as a function of time. It provides a method of quickly assessing the relative rates of hydration in the presence of different amounts/types of admixtures. The time of termination of the induction period gives information on the relative retarding action of various types of retarders. 221 222 Chapter 6 - Retarding and Water Reducing Admixtures 2.0 LIGNOSULFONATES The lignosulfonate-based admixtures have been used more widely than other water reducers. They are capable of reducing water requirements and retarding the setting times of concrete. They influence the dispersion and the hydration rate of the individual cement compounds, and, thus, the cement itself. Techniques such as XRD, DTA, DSC, TG, DTG, and conduction calorimetry have been used extensively to follow the hydration of cement and cement compounds containing different types and amounts of lignosulfonates (LS). 2.1 Tricalcium Aluminate Tricalcium aluminate hydrates to form, initially, hexagonal pha