The blast furnace slag (BFS) needs to be processed before it added as cement replacement. The processing of BFS, starting from
slag granulation, moisture reduction and
finally grinding (comminution), leads to the production of ground granulated
blast furnace slag (GGBS). GGBS can be added to cement or concrete mix as a
partial replacement for cement, as stipulated by the national standards. The
BFS obtained from steel plants contains moisture. The moisture reduction is
generally integrated with the processing of BFS.
The size reduction of BFS granules
can be carried out in the ball mill, roller press, or vertical roller mill
(VRM) or by a suitable combination of these equipment. In comparison to the
cement clinker, the BFS is moist, fine grained, more abrasive, and harder to
grind. The moisture content should be less than 4%, if grinding is done in a
ball mill. In the roller press, moisture content up to 1%–2% is desirable for
satisfactory performance. The VRM can dry and grind BFS up to 10% moisture
content. It is desirable to dry BFS in a separate unit, for higher moisture
contents.
The activity of BFS is proportional
to the glass content. The activity of inferior quality BFS (lower glass) can be
improved by increasing its fineness but at a higher expense of the grinding
energy, which increases exponentially with fineness, at higher fineness. On the
other hand, BFS with higher glass can be ground relatively coarser (to save
energy); that will give the advantage of lower water demand with good activity.
The fineness expressed in terms of
unit surface area (cm2/g, Blaine) is not a unique property, as the cements that
have identical unit surface areas can have different particle size
distributions, represented by RRSB (Rosin-Ramler-Sperling-Bennett) parameters.
The fineness of cement can be unambiguously characterized by defining the
surface area (cm2/g, Blaine) and one of the two parameters related to size
distribution, namely, the slope (n) or the position parameter (x′), in the RRSB
diagram. German Standard, DIN 66145: “Graphical representation of particle size
distributions, RRSB-grid,” contains the mathematical equation of the
distribution function and its application.
Thus to bring out full potential,
all aspects of GGBS production, namely granulation, storage, drying and
grinding need attention.
Ref: “Mineral Admixtures in Cement and Concrete”, CRC Press
(http://www.crcpress.com/product/isbn/9781439817926). Author: Dr J
D Bapat (http://www.drjdbapat.com)
Written for engineers, book
focuses on making more workable and durable concrete using mineral admixtures.
For each mineral admixture, book looks at manufacturing and processing,
physical characteristics, chemical and mineralogical composition, quality
control, and reported experiences. It also examines the provisions of national
standards.It encourages engineers to more effectively use these and other
wastes in cement and concrete to support more sustainable growth of
industry. Buy this
book online to obtain 20 % discount and free shipping. Download details: http://bit.ly/online_purchase
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