Hydration of mechanically activated granulated blast furnace slag
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I. INTRODUCTION
GROUND granulated blast furnace slag (GGBFS) is a glassy, granular material consisting essentially of silicates and aluminosilicates of calcium and other oxides. It is formed when molten blast furnace (BF) slag produced as a byproduct in the making of iron is rapidly cooled, usually by immersion in water, and then ground, to improve its reactivity. The major oxides SiO2, CaO, Al2O3, and MgO constitute the bulk of the slag. The slag shows primarily cementitious behavior (latent hydraulic activity), but may show some pozzuolanic character (reaction with lime) as well. The hydration of the slag is of considerable interest in research on Portland slag cement and alkali-activated slag cements. Although the slag without an activator does react with water, the rate of hydration is very slow.[1–6] No significant hydration product can be found, even after several months. Mehta[3] reported that an impermeable coating of aluminosilicate forms on the surfaces of slag grains within a few minutes of exposure to water. Unless a chemical activator, such as Portland cement, gypsum, sodium silicate, etc., is present,[4–22] further hydration is inhibited.[4] Song and Jennings[5] reported enhancement of the hydration of neat slag due to prolonged ball milling; however, no details were presented. Recently, we made the striking observation
RAKESH KUMAR and SANJAY KUMAR, Scientists, S. BADJENA, Research Fellow, and S.P. MEHROTRA, Director, are with the National Metallurgical Laboratory, Jamshedpur – 831 007, India. Contact e-mail: [email protected] Manuscript submitted February 10, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
that complete hydration of the slag is possible without a chemical activator, if the slag is mechanically activated in an attrition mill (also referred to as agitator bead mill, stirred media ball mill, stirred bead mill).[23–26] The slag that was attrition milled in our laboratory for about 30 minutes or more was found to hydrate completely after 28 days. This was an interesting finding, since the presence of a chemical activator is generally considered essential to the slag hydration. This finding led to a systematic study on the hydration of mechanically activated slag by attrition milling, and the results are presented in this article. The term “mechanical activation” in this article refers to breakage and to any other bulk or surface change(s) induced in the solid phase due to the milling process. The focus of this article is on the characterization of mechanically activated slag and on understanding the nature of the slag-hydration product in terms of its microstructure and the phases formed. The results presented in this article are not only of relevance in the context of hydraulic cements, but may open up possibilities for developing newer materials based on the hydrated slag. II. MATERIALS AND METHODS A. The Blast Furnace Slag A granulated BF slag sample from one of the integrated steel plants in the state of Chattisgarh (India) was used. The chemical composition of the slag
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