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Friday, January 31, 2014

Blended Cement with Blast Furnace Slag

          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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