The utilisation of calcined clay, in the form of metakaolin (MK), as a pozzolanic material for mortar and concrete has received considerable attention in recent years. This interest is part of the widely spread attention directed towards the utilisation of wastes and industrial by-products in order to minimise Portland cement (PC) consumption, the manufacture of which being environmentally damaging. Another reason is that mortar and concrete, which contain pozzolanic materials, exhibit considerable enhancement in durability properties. This paper reviews work carried out on the use of MK as a partial pozzolanic replacement for cement in mortar and concrete and in the containment of hazardous wastes. The literature demonstrates that MK is an effective pozzolan which causes great improvement in the pore structure and hence the resistance of the concrete to the action of harmful solutions.

Metakaolin and calcined clays as pozzolans for concrete: a review

The development of new, sustainable, low-CO2 construction materials is essential if the global construction industry is to reduce the environmental footprint of its activities, which is incurred particularly through the production of Portland cement. One type of non-Portland cement that is attracting particular attention is based on alkali-aluminosilicate chemistry, including the class of binders that have become known as geopolymers. These materials offer technical properties comparable to those of Portland cement, but with a much lower CO2 footprint and with the potential for performance advantages over traditional cements in certain niche applications. This review discusses the synthesis of alkali-activated binders from blast furnace slag, calcined clay (metakaolin), and fly ash, including analysis of the chemical reaction mechanisms and binder phase assemblages that control the early-age and hardened properties of these materials, in particular initial setting and long-term durability. Perspectives fo...

Geopolymers and Related Alkali-Activated Materials

Properties and applications of halloysite nanotubes: recent research advances and future prospects

Mullite (3AI2O3. 2Si02) is becoming increasingly important in electronic, optical, and high-temperature structural applications. This paper reviews the current state of mullite-related research at a fundamental level, within the framework of phase equilibria, crystal structure, synthesis, processing, and properties. Phase equilibria are discussed in terms of the problems associated with the nucleation kinetics of mullite and the large variations observed in the solid-solution range. The incongruent melting behavior of mullite is now widely accepted. Large variations in the solid solubility from 58 to 76 mot% alumina are related to the ordering/disordering of oxygen vacancies and are strongly coupled with the method of synthesis used to form mullite. Similarly, reaction sequences which lead to the formation of mullite upon heating depend on the spatial scale at which the components are mixed. Mixing at the atomic level is useful for lowtemperature (<lOOO"C) synthesis of mullite but not for low-temperature sintering. In contrast, precursors that are segregated are better suited for lowtemperature (1250" to 1500°C) densification through viscous deformation. Flexural strength and creep resistS M Wiederhorn-contributing editor

Mullite for Structural, Electronic, and Optical Applications

Porcelain represents the foundation of the ceramics discipline and one of the most complex ceramic materials. Composed primarily of clay, feldspar, and quartz, porcelains are heat-treated to form a mixture of glass and crystalline phases. This review focuses on raw materials, processing, heat treatment, and mechanical behavior. Because of the complexities of the porcelain system and despite the substantial amount of research already conducted within the field, there remain significant opportunities for research and study, particularly in the areas of raw material understanding, processing science, and phase and microstructure evolution.

Porcelain—Raw Materials, Processing, Phase Evolution, and Mechanical Behavior

This reaction series has been a subject of extensive investigation since the work of Le Chatelier in 1887. Nevertheless, major problems have remained concerned with the nature of metakaolin, the manner in which it transforms into a spinel-type phase and mullite, and the relation of this spinel-type phase to mullite. The present survey brings these problems into sharp focus and provides the necessary historical background fora new approach to their solution.

The KaoIinite-Mullite Reaction Series: I, A Survey of Outstanding Problems

The Kaolinite‐Mullite Reaction Series: II, Metakaolin

X-ray diffraction data for the high-temperature phases show that a spinel-type structure develops with marked orientation at about 925°C. This phase is considered to be an aluminum silicon spinel with vacant cation sites. Mullite is thought to be formed by the decomposition of the spinel. Silica is eliminated progressively as metakaolin transforms to the spinel phase and thence to mullite. The X-ray data show variation in the mullite parameters between 1200° and 1400°C.; at 1400°C., the composition probably approximates 3A12O3·2SiO2. The nature of the intermediate spinel-type phase is discussed in relation to the crystal chemistry of spinels.

The KaoIinite‐MuIIite Reaction Series: Ill, The High‐Temperature Phases

Previous studies of the kinetics of dehydroxylation of kaolinite and halloysite point to first-order reactions, in approximate conformity with the Arrhenius relation. Isothermal weight-loss measurements have shown that the rate constants are markedly dependent on factors such as specimen size, shape, and compaction. A technique has been developed for determining the reaction kinetics of infinitely thin disk-type specimens. The reactions are then strictly first order and the Arrhenius relation is obeyed. Activation energies of 65 and 55 kcal. per mole are obtained for kaolinite and halloysite, respectively. Comparison is made between the behavior of kaolinite and halloysite on the one hand and of macrocrystalline anauxite on the other. For anauxite, nucleation and growth of nuclei produce a sigmoid-type reaction curve, but for the fine-grained minerals it is believed that nucleation alone is the rate-controlling process. The dependence of the reaction rate on geometrical factors is attributed to the retention of water vapor within the powder specimen. The influence of water vapor on these reactions is discussed generally.

Kinetics of Dehydroxylation of Kaolinite and Halloysite

The thermal reactions taking place when kaolinite is heated to high temperatures, to 1400 °C or higher, have been the subject of investigation for almost a century. One recalls the early work of Le Chatelier (1887), De l'Action de la Chaleur sur les Argiles , and of Mellor and Holdcroft (1911), The Chemical Constitution of the Kaolinite Molecule . Among the difficulties which arise in studying these reactions, the following may be mentioned : (i) The poor crystallinity of metakaolin and the long uncertainty whether this is a mixture of oxides or an alumino-silicate. (ii) The uncertainty as to the role of the spinel-type phase which forms at about 925-950°C and which has been regarded as γ-Al 2 O 3 since 1928. (iii) The structural relation, if any, of mullite to the parent kaolinite. In a series of papers to be published in the Journal of the American Ceramic Society, the present authors discuss a new concept of this reaction series based on an X-ray single crystal study of kaolinite and its reaction products. This approach to the problem shows conclusively that a series of ordered reactions takes place from kaolinite, through metakaolin, to a cubic spinel-type phase (the socalled γ-Al 2 O 3 ) and eventually to mullite. The key to the understanding of this sequence is the character of the orientation of the spinel phase. The close-packed planes of the spinel lie parallel to the oxygen and hydroxyl sheets of kaolinite, and the close-packed direction, the cube-face diagonal of the spinel, lies parallel to the baxis of kaolinite. Thus crystallographic order is preserved to a considerable extent in these transformations. The most reasonable interpretation is that the tetrahedral sites in the spinel are occupied by Si atoms and the octahedral by A atoms. The probable composition of the spinel is Si 3 Al 4 O 12 which, at first sight, seems considerably different from the usual spinel-type formula AB 2 O 4 . A comparison between these formulae and also that of γ-Al 2 O 3 is best made on the basis of 32 oxygen atoms, which leads to the following : normal spinel : A 8 B 16 O 32 proposed Si-Al spinel : Si 8 Al 2 O 3 Al 2 O 3 Al 10 2/3 O 32 γ~Al 2 O 3 : 2 2/3 Al 21 Al 2 O 3 OAl 2 O 3 One sees that 8 Si atoms occupy the 8A tetrahedral positions and that 5 1/3 vacancies occur among the 16B octahedral sites. By comparison, γ~Al 2 O 3 has 2 2/ 3 vacancies which may be distributed over all cation sites. This concept makes it easy to understand the structural continuity from kaolinite, through metakaolin, to the spinel-type phase. The proportion of silica discarded at this stage of the reaction is small in accordance with the observation that little cristobalite is formed at this temperature. Ideally the equation can be written : Si 4 A l4 O 14 -- Si 3 A l4 O 12 + SiO 2 The final stage of the reaction involves further discard of silica and the formation of mullite, and additional cristobalite. The ordered formation of mullite, which is a chain-type silicate, proceeds from the chains of Al-octahedra already existing in the spinel-type structure. There are many points of detail to be considered in relation to this new concept of the kaolinite reaction series, and these will be discussed by the authors in the forthcoming publications. Here it must suffice to emphasise that this new concept is based on a crystal-lographic interpretation of the reactions. Single crystal studies alone will establish that solid state reactions take place in an orderly sequence, with rearrangement of cations within an oxygen framework that remains largely (not wholly) continuous. One sees that 8 Si atoms occupy the 8A tetrahedral positions and that 5 1/3 vacancies occur among the 16B octahedral sites. By comparison, γ~Al 2 O 3 has 2 2/ 3 vacancies which may be distributed over all cation sites. This concept makes it easy to understand the structural continuity from kaolinite, through metakaolin, to the spinel-type phase. The proportion of silica discarded at this stage of the reaction is small in accordance with the observation that little cristobalite is formed at this temperature. Ideally the equation can be written : Si 4 A l4 O 14 -- Si 3 A l4 O 12 + SiO 2 The final stage of the reaction involves further discard of silica and the formation of mullite, and additional cristobalite. The ordered formation of mullite, which is a chain-type silicate, proceeds from the chains of Al-octahedra already existing in the spinel-type structure. There are many points of detail to be considered in relation to this new concept of the kaolinite reaction series, and these will be discussed by the authors in the forthcoming publications. Here it must suffice to emphasise that this new concept is based on a crystal-lographic interpretation of the reactions. Single crystal studies alone will establish that solid state reactions take place in an orderly sequence, with rearrangement of cations within an oxygen framework that remains largely (not wholly) continuous.

M. Nakahira

Papers

The KaoIinite-Mullite Reaction Series: I, A Survey of Outstanding Problems

The Kaolinite‐Mullite Reaction Series: II, Metakaolin

The KaoIinite‐MuIIite Reaction Series: Ill, The High‐Temperature Phases

Kinetics of Dehydroxylation of Kaolinite and Halloysite

Nouveau concept sur les réactions de passage : kaolinite-mullite