Page 238 - Handbook of Thermal Analysis of Construction Materials

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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.

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