CONDUCTION CALORIMETRIC ASSESSMENT OF RETARDERS

The relative effects of phosphonates can be evaluated by comparing the time at which the induction period is terminated at different dosages of a chemical. In Fig. 17, the time of termination of the induction period is plotted as a function of dosage.Among the acids, the most efficient retarder is DTPMP which requires 0.05% for an induction period of 21 hours, compared to periods of 13 and 10 hours respectively for acids ATMP and HEDP at the same dosage. Among the salts, Na DTMP is the most effective retarder. 

Several techniques have been applied to obtain an understanding of the rate of hydration, types of products, complex formation, inter-conversions. 

Ramachandran and LoweryP l applied conduction calorimetry to the study of the relative effects of various retarders on the hydration of cement. The retarders used were calcium gluconate, glucose, glycolic acid, molasses, sodium borate, sodium citrate, sodium heptonate, sodium hexametaphosphate, sodium pyrophosphate, sugar-free lignosulfonate, and sucrose. The dosage of the chemicals varied between 0.025 and 1.2%. 

Gluconate was found to be a good retarder. Its action is possibly related to its poisoning action on the hydration products of cement. It has also been advocated for controlling slump loss in concrete. Conduction calorimetric curves of cement with gluconate show large hump effects for the hydration of the silicate phase (Fig. 18a). At a dosage of 0.15%, the 

Conduction calorimetry has been used to obtain the induction periods of cement containing different amounts of retarders. Table provides data on the minimum concentration of a retarder required to achieve different induction periods. 

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