Page 274 - Handbook of Thermal Analysis of Construction Materials

254 Chapter 6 - Retarding and Water Reducing Admixtures

11.0 OTHER ADMIXTURES

Many chemicals and waste products may act as retarders.
Thermoanalytical techniques are very useful to assess the effect of these
products on the hydration of cements. A few examples are described below.
Copper refining wastes containing Cr and Sb were added to
cements and the rate of hydration was followed by DTA, TG, and DTG.
There was a significant decrease in the amount of Ca(OH) in pastes
2
hydrated with the copper refining waste, suggesting that it is a good
retarder. [36]
The metal plating waste products from a zinc plating process were
added to portland cement and the hydration was followed by conduction
calorimetry. At an addition level of 3–10%, the rate of hydration was
greatly reduced. [37]
In an investigation of various polymers, it was found that sodium
carboxy methyl cellulose and versicol acted as strong retarders. For
example, with the reference portland cement pastes, the peak in the
conduction calorimetry occurred at 11 hours, and with versicol, the peak
shifted to 20, 29, and 34 hours at dosages of 0.25, 0.50, and 0.75%
respectively. [38]
Zinc oxide, in small amounts, retards the setting of cement very
effectively. Calorimteric investigation has shown that, at an addition of
0.25% ZnO, hydration is almost nil up to 12 hours, and, at 1% addition,
hydration is not initiated even up to 2 days. [39]
Thermoanalytical investigations have shown that potassium chro-
mate retards the hydration of C A. [39]
3

12.0 IDENTIFICATION OF WATER REDUCERS/
RETARDERS

Admixtures, heated in the presence or absence of air, yield peaks
that may be used to characterize them. Even within the same admixture
group, thermograms may show differences that could be applied for quality
control and identification purposes.
In a study of various water reducers, Gupta and Smith [40] applied
DTA and TG techniques. They obtained characteristic peaks depending on