Page 268 - Handbook of Thermal Analysis of Construction Materials
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248           Chapter 6 - Retarding and Water Reducing Admixtures


                                     In studies above, the time of termination of the induction period is
                              taken to evaluate the relative setting times of cements containing retarders.
                              The setting times of cement mortars are determined according to ASTM
                              C403 using the Proctor needle. It is an arbitrary method and the time at
                              which the setting occurs may involve both physical and chemical
                              phenomena. Ramachandran  [33]  obtained the DSC curves of set mortars
                              containing different types of retarders with or without a superplasticizer
                              (Fig. 19). All of them exhibited endothermal peaks at about 100° and
                              450–500°C indicating the formation of Ca(OH)  at the time of set. Thus, it
                                                                       2
                              is evident that at the time of set some hydration of C S takes place. The
                                                                             3
                              degree of hydration of C S for the samples is different, depending on the
                                                    3
                              admixture used. It, therefore, appears that the morphological and other
                              factors also play a part in the setting phenomenon. It appears that the setting
                              time occurs at some point after the induction period has terminated, but
                              before the time at which the exothermic peak appears in the conduction
                              calorimetry.




                              8.0    SLUMP LOSS



                                     Addition of a superplasticizer to concrete results in significantly
                              increased slump values. This increase is transient, however, and is gener-
                              ally not maintained beyond a period of 30–60 minutes. One method of
                              controlling this loss is to add a retarder in the superplasticized concrete.
                              Conduction calorimetric and different thermal methods may be adopted to
                              examine the relative retardation action of admixtures. Of the several
                              retarders studied, calcium gluconate was found to retard the hydration to a
                              maximum extent. In Fig. 20, [33]  the conduction calorimetric curves show
                              that by the addition of 0.1–0.2% gluconate there is retardation of hydration
                              of cements. However, in combination with 0.1–0.2% sulfonated melamine
                              formaldehyde, there is increased retardation. It was found that by the
                              addition of 0.05–0.1% sodium gluconate, the slump loss can be decreased.
                              For example, with only gluconate added to concrete, the slump value is
                              about 40 mm at 2 hours, but, in combination with SMF, the slump is as high
                              a 140 mm (Fig. 21). [33]
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