Metakaolin application

Acid activation has long been known as a preparation method for porous materials. In general, the acid solution dissolves the octahedral sheets of the clay and produces a significant modification over the tetrahedral sheets [11]. The solids obtained have higher surface area and better adsorptive and catalytic properties than the original clays, depending on the starting material and the conditions of the treatment [12-15]. The activation of metakaolin has been reported only in a few articles, and the application of the solids obtained to different catalytic reactions has been described [3,7,16-18]. 

This study focuses on a systematic analysis of the chemical activation of kaolin with HCl solutions under different conditions of time and temperature, with a complete discussion of the phenomena governing this process and a wide characterization of the solids obtained. Such a systematic study lacks in the literature and the interest in looking for new applications of natural clays justifies it. By this reason, we have carried out the acid activation of the metakaolins obtained by calcination of a kaolin between 600-900°C and a complete study of the solids prepared, where the porous properties of the prepared materials receive special attention, due to the high importance that these properties have over the industrial applications of the final solids. 

Geopolymer bonded mortar with full metakaolin as raw material for construction applications can reach a 3-day compressive strength above SOOKgf/cm by our previous research. From that point we realized geopolymer can be developed as an excellent binder to replace cement if the cost could be reduced. 

When kaolin is heated, it undergoes chemical and physical changes. Between approximately 450 °C and 700 °C, dehydroxylation occurs (often described as loss of water of crystallisation) to form a product called metakaolin. This is accompanied by approximately 13% loss of mass as water. At higher temperatures, between 950 °C and 1030 °C, metakaolin undergoes an internal rearrangement to form an amorphous product described as a defect spinel. For rubber applications, products are manufactured at temperatures just above these transitions. Metakaolin is produced primarily for use in flexible polyvinyl chloride wire and cable insulations, but is also used in EPDM cable insulations. 

The main conditions for high durability of a cement-based composite may be summarized as high impermeability and density of the matrix, chemical and thermal compatibility of all material components and the existence of an internal structure that can control microcracking (Mather 2004). Density and impermeability of the matrix may be achieved by appropriate material design and often by application of secondary and ternary binder blends with fly ash, SF, metakaolin, etc. (cf. Chapter 4). For the control of cracking, adequate material composition and appropriate curing are necessary. Moreover, excessive stresses and detrimental external actions should be avoided or reduced by adequate structural detailing. 

Because of the limited availability of SF on the world market and its increasing price, it is difficult to predict its future application for high performance concrete. It is probable that various kinds of fly ash, blastfurnace slag or metakaolin will play a similar role in high performance concrete composition, certainly after necessary verification of their quality and compatibility with other components. 

Richardson, I. G. 2004. Tobermorite/jennite- and tobermorite/calcium hydroxide-based models for the structure of C-S-H applicability to hardened pastes of tricalcium silicate, p-dicalcium silicate, Portland cement, and blends of Portland cement with blast-furnace slag, metakaolin, or silica fume . Cement and Concrete Research 34 (9) 1733-1777. 

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