Synthesis and characterization of new inorganic polymeric composites based on kaolin or white clay and on ground-granula
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. Maseri Construction Steel Design Center (ARCELOR INNOVATION), Boulevard de Colonster B57, Sart-Tilman B-4000 Liège, Belgium (Received 2 June 2003; accepted 21 July 2003)
Alkali activation of dehydroxylated kaolin or clay yielded high-strength polymeric materials, so-called geopolymers. They were synthesized by mixing the aluminosilicate with solutions of sodium metasilicate and KOH followed by adding 45 wt.% of ground-granulated blast furnace slag. The influence of the aluminosilicate source, its activation temperature, and the order of mixing raw materials were studied on the workability of the blending paste, the microstructure, and the Vickers hardness of the geopolymer samples. The polymeric material is completely amorphous according to x-ray diffraction. Solid-state 27Al and 29Si magic-angle-spinning nuclear magnetic resonance showed that the geopolymer consists of AlO4 and SiO4 tetrahedra linked together through a polymeric network constituted by branched entities SiQ4(4Al) and SiQ4(3Al), but also by less-polymerized silicates SiQ1 and SiQ2. Scanning electron microscopy showed a homogeneous polymeric gel matrix containing unreacted slag (and quartz) grains; thermogravimetric analysis and differential scanning calorimetry exhibited a high content of water and an elevated melting point (1260 °C). Vickers hardness values are in the range of 200 MPa. I. INTRODUCTION
Concurrently to research on opportunities to improve durability and mechanical properties of hydraulic cements such as Portland cement, new types of binders, named geopolymers, were developed during the last two decades. The formation, texture, and structure of these geopolymers differ significantly from those of classical hydraulic cements. Geopolymers are synthetic mineral products that combine properties of polymers, ceramics, and cements. They are characterized by high mechanical properties such as hard surfaces and high compressive strength, by good thermal and chemical stabilities, by high surface smoothness, and precise moldability.1 Therefore, they have a lot of potential applications such as versatile solidification and immobilization technology2,3 as well as a durable construction material. These materials result from a rapid polymerization of aluminosilicates in a concentrated aqueous medium, either in alkaline hydroxide or in alkaline silicate, at temperatures ranging from 20 °C to 100 °C. The system polycondenses very rapidly, in a few minutes, and yields a three-dimensional polymeric network with Al in fourfold coordination. a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 11, Nov 2003
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In this study, two aluminosilicate precursors were used: kaolin and white clay, both of them containing kaolinite. They were calcined to favor the fourfold coordination of aluminum atoms prior to the reaction. Kaolinite is an aluminosilicate with a 1:1 uncharged dioctahedral layer structure. Each layer consists of a sheet of SiO4 tetrahedra for
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