Page 331 - Handbook of Thermal Analysis of Construction Materials

308 Chapter 8 - Supplementary Cementing Materials

4.0 SLAGS

Blast furnace slag (referred to as slag in this chapter) is formed as
a liquid in the manufacture of iron, and when cooled slowly, crystallizes and
has virtually no cementing properties. If cooled rapidly below 800°C, it
forms a glass that has hydraulic properties and is called granulated slag. It
is blended or mixed with portland cement in amounts up to 80%. The
reactivity of slag depends on its bulk composition, glass content, and
fineness.
The glass content in slag can be determined by light microscopy or
from the difference shown in XRD determination of crystalline phases.
Some success has been achieved in the estimation of glass content by
DTA. [35]
The principle hydration products of slag cements are essentially
similar to those found in portland cement pastes. The microstructure of slag
cement pastes is also similar to that of portland cement pastes. X-ray
microanalysis has, however, shown that the C/S ratio of C-S-H product in
hydrated slag cement is lower than that found in portland cement paste.
Slags can be activated by calcium sulfate and other compounds
such as NaOH, lime, water glass, phosphogypsum, etc. When calcium
2+
sulfate is added, the precipitation of ettringite provides a sink for Ca and
-
Al(OH) ions released from the slag. The nature of products resulting from
4
the activation of slags is similar to that in normal portland cement pastes
(Table 5). [1]
Supersulfated cement is made by grinding together a mixture of
80% slag, 5% portland cement, and about 15% gypsum or anhydrite.
Supersulfated cement does not contain lime and is capable of resisting
sulfate attack. In the supersulfated cement, the main products are C-S-H and
ettringite. The amount of ettringite reaches a limit in about 3 days, the period
during which all sulfate will have reacted.
Thermal techniques such as DTA and conduction calorimetry have
been applied extensively for a study of the mechanism of hydration of slag,
divitrification in slags, the rate of hydration of slags under different
conditions, the identification of compounds, and the effect of various
activators. [37]
Estimation of the glassy phase in slags and slag-cements is needed
for control purposes. The glass content of a slag has a strong influence on
the strength of cement-slag systems. Schwiete and Dolbar [36][37] determined
the glass contents of several slags and found a good correlation between the
glass contents and compressive strengths (Fig. 12).