Page 272 - Handbook of Thermal Analysis of Construction Materials
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252           Chapter 6 - Retarding and Water Reducing Admixtures


                                     In another example, conduction calorimetry was use to explain a
                              flash set in an oil cement slurry containing a lignosulfonate admixture. [22]
                              Conduction calorimetric curves were obtained for systems such as portland
                              cement, C A-gypsum-lime, and C AF-lime-gypsum systems containing
                                       3                     4
                              refined or unrefined lignosulfonates. The results showed that unrefined
                              lignosulfonate destabilizes the hydration process of the C AF-gypsum-
                                                                                  4
                              lime mixture causing a flash set. In the conduction calorimetry, the C AF-
                                                                                          4
                              10% gypsum-5% lime with 1–2% unrefined lignosulfonate (UL) dramati-
                              cally increases the intensity of the very early ettringite peak and reduces the
                              peak intensity for C S hydration at about 9–10 hours (Fig. 23). The
                                                 3
                              induction period is practically absent. It was concluded that the xylose
                              present in lignosulfonate was responsible for the flash set.
























                              Figure 23. Effect of unrefined lignosulfonate on the hydration of C AF-10% gypsum- 5%
                                                                              4
                              lime system.




                              10.0 READY-MIX CONCRETE


                                     In ready-mix concrete trucks, the unused concrete is washed out
                              and this poses environmental, waste, and other problems. A novel approach
                              consists of adding a retarder-based admixture, such as carboxylic acid or
                              phosphorous-based organic acids and salts, to the concrete mix in the
                              truck. [35]  This stabilizes the concrete from further hydration. When an
                              activator is added, subsequently normal hydration occurs.
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