Page 193 - Handbook of Thermal Analysis of Construction Materials

176 Chapter 4 - Introduction to Concrete Admixtures

6.2 Slag

Granulated slag is a by-product obtained as a finely divided
material from ferrous and nonferrous metal industries. It is used as a
component of blended cement or as a supplementary cementing material.
Slags have similar chemical composition to that of normal cement. They are
essentially glassy with glass contents around 75%. The chemical composi-
tion corresponds to melilite phase, a solid solution phase between gehlenite
(C AS) and akermanite (C MS ). It may take about three days before
2 3 2
cementing properties of slags to be noticeable. Some hydration takes place
immediately after water addition and a protective layer is formed on the
surface of the slag. Alkalis, gypsum, and lime serve as activators or
accelerators for slag hydration. Alkali-activated slags have found commer-
cial uses in some countries. Up to 65–80% of slag replacement in slag
concretes is possible.
The resistance of slag concrete to sulfate is increased if more than
65% cement is replaced by slag at a constant gypsum addition of 5%. The
high resistance to sulfate attack can be attributed partly to lower permeabil-
ity of concrete.

6.3 Silica Fume
Silica fume is a by-product from the silicon metal or ferro-silicon
industry. Its particle size ranges from 0.1 µm to 0.2 µm, the surface area
2
being 20–23 m /g. It is highly pozzolanic and improves the properties of
concrete in fresh and hardened states.
Addition of silica fume accelerates the hydration of C S. The
3
CaO/SiO ratio of the C-S-H product is also changed. For example, at
2
a w/s ratio of 10, C S yields a C-S-H product with CaO/SiO of 0.8 at 24
2
3
hours, and this ratio decreases to 0.33 and 0.36 at C S/SiO ratios of 1 and
3 2
0.4, respectively. In the hydration of cement, the calcium hydroxide formed
as a product is consumed by silica fume. Silica fume in cement increases the
overall rate of reaction. [87] Figure 13 gives the strength results of concrete
containing silica fume. Silica fume was added as an addition. The increase
in strength depends on the amount of silica fume added. [87]
Durability of silica fume concrete to freeze thaw action is generally
satisfactory especially at silica fume content of less than 20%. [88] Incorpo-
ration of silica fume in concrete increases its resistance to sulfate attack.
Reduced availability of lime and increased impermeability are the main
reasons for sulfate resistance.