PORTLAND CEMENT - Materials for Civil and Construction Engineers - page 228

Ground Granulated Blast Furnace Slag Ground granulated blast furnace slag
(GGBF slag) is made from iron blast furnace slag. It is a nonmetallic hydraulic
cement consisting basically of silicates and aluminosilicates of calcium,
which is developed in a molten condition simultaneously with iron in a blast
furnace. The molten slag is rapidly chilled by quenching in water to form a
glassy sandlike granulated material. The material is then ground to less than
45 microns. The specific gravity of GGBF slag is in the range of 2.85 to 2.95.
The rough and angular-shaped ground slag in the presence of water and
an activator, NaOH or CaOH, both supplied by portland cement, hydrates
and sets in a manner similar to portland cement.
Ground slag has been used as a cementitious material in concrete since
the beginning of the 1900s. Ground granulated blast furnace slag commonly
constitutes between 30% and 45% of the cementing material in the mix.
Some slag concretes have a slag component of 70% or more of the cementi-
tious material. ASTM C 989 (AASHTO M 302) classifies slag by its increasing
level of reactivity as Grade 80, 100, or 120.

Silica Fume Silica fume is a byproduct of the production of silicon metal or
ferrosilicon alloys. One of the most beneficial uses for silica fume is as a min-
eral admixture in concrete. Because of its chemical and physical properties, it
is a very reactive pozzolan. Concrete containing silica fume can have very
high strength and can be very durable. Silica fume is available from suppliers
of concrete admixtures and, when specified, is simply added during concrete
production either in wet or dry forms. Placing, finishing, and curing silica
fume concrete require special attention on the part of the concrete contractor.
Silicon metal and alloys are produced in electric furnaces. The raw ma-
terials are quartz, coal, and woodchips. The smoke that results from furnace
operation is collected and sold as silica fume.
Silica fume consists primarily of amorphous (noncrystalline) silicon
dioxide The individual particles are extremely small, approximately
1/100th the size of an average cement particle. Because of its fine particles,
large surface area, and the high content, silica fume is a very reactive
pozzolan when used in concrete. The quality of silica fume is specified by
ASTM C 1240 and AASHTO M 307.
In addition to producing high-strength concrete, silica fume can reduce
concrete corrosion induced by deicing or marine salts. Silica fume concrete
with a low water content is highly resistant to penetration by chloride ions.
More information is available at the www.silicafume.org website.

SiO 2 .

1 2

SiO 2

Natural Pozzolans A pozzolan is a siliceous and aluminous material which,
in itself, possesses little or no cementitious value but will, in finely divided
form and in the presence of moisture, react chemically with calcium hy-
droxide at ordinary temperatures to form compounds possessing cementi-
tious properties (ASTM C595). Naturally occurring pozzolans, such as fine
volcanic ash, combined with burned lime, were used about 2000 years ago
for building construction, and pozzolan continues to be used today. As
shown in Table 6.2, calcium hydroxide is one of the products generated by
the hydration of

C 3 S and C 2 S. In fact, up to 15% of the weight of portland