Page 271 - Handbook of Thermal Analysis of Construction Materials
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Section 9.0 - Abnormal Setting                               251


                              9.0    ABNORMAL SETTING


                                     Several instances of quick setting of cements have been encoun-
                              tered involving the use of admixtures. Greene [34]  found that a particular
                              white cement containing 1.54% SO  set abnormally when used in the
                                                               3
                              presence of a carbohydrate-type of water reducer. In Fig. 22, DTA of the
                              unhydrated cement shows peak doublets below 200°C for the gypsum
                              hydrate and another at about 500°C for calcium hydroxide. [34]  Comparison
                              of the samples with and without the admixture at 4 hours indicates
                              differences. The endothermal peaks at about 200°C indicate low
                              sulfoaluminate formation in the presence of the admixture. Even at 1 hour,
                              low sulfoaluminate hydrate had formed (not shown in the figure). By the
                              addition of 0.4% SO , there was no formation of low sulfoaluminate at 4
                                                3
                              hours. These results suggest that, in the presence of the admixture, SO
                                                                                              3
                              consumption was accelerated, and an insufficient amount of SO  was
                                                                                         3
                              present in the solution to control C A hydration, leading to the quick setting.
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                              Figure 22. DTA curves of cement, unhydrated and hydrated, at different times with or
                              without the water-reducing admixtures.
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