Metakaolin structure

The first is metakaolin. This is a partially calcined product that forms above about 500 °C. Only about 10% of the original hydroxyl groups of the kaohnite are retained and much of the crystalline nature of the structure is destroyed. Metakaolin is considerably more reactive than the original kaolin and appears to have an especially reactive surface. It is generally used uncoated and finds most use in plasticised PVC cable insulation, where it is reported as giving uniquely useful electrical properties [86]. 

Barrer and Mainwaring (20) report the use of metakaolin as the aluminosilicate raw material for reaction with the hydroxides of K and Ba as well as the binary base systems Ba-K and Ba-TMA to form zeolites. Zeolite phases previously synthesized in the analogous hydrous aluminosilicate gel systems were crystallized with KOH, including phillipsite-, chabazite-, K-F-, and L-type structures. The barium system yielded two unidentified zeolite phases (Ba-T and Ba-N) and a species Ba-G,L with a structural resemblance to Linde zeolite L. Ba-G,L was reported previously by Barrer and Marshall (21) as Ba-G. Similar phases were formed in the Ba-K system and in the TMA-Ba system where, in addition, erionite-type phases were formed. The L-type structures are said to represent aluminous analogs of the zeolite L previously reported (22). 

A gel is delined as a hydrous metal aluminosilicate prepared from either aqueous solutions, reactive solids, colloidal sols, or reactive aluminosilicates such as the residue structure of metakaolin and glasses. 

This evidence suggests that not all Na species are mobile. Some Na species must in fact have reacted irreversibly with components on the catalyst, leaving it unavailable to poison the acid sites. It is likely that these reactions occur during the early stages of hydrothermal deactivation. The exact mechanism is unclear, but may involve reactions with extraffamework alumina. As the zeolite dealuminates from 24.55 to 24.25A unit cell size, approximately 65% of the initial framework alumina (about 15 wt% of the zeolite) comes out of the zeolite structure. Sodium, which also must leave the exchange sites as the zeolite dealuminates may react with this very reactive form of alumina. The other possibility is that as kaolin undergoes its transition to metakaolin at 800K... 

Figure 2 shows the FT-IR spectra of kaolin and MK-600 metakaolin and the samples treated at 90 C (the treatment at room temperature did not modify the metakaolin). The spectrum of the metakaolin shows the structural changes produced during the calcination. The four Al-OH stretching bands, from 3700 to 3600 cm [21-24], disappear due to the dehydroxylation, the water bands being observed at 3470 and 1630 cm. ... 

The nitrogen gas adsorption-desorption isotherms of the metakaolins are classified as type II (BDDT classification [27]). They are almost reversible with a closed hysteresis cycle, indicating the absence of micropores. The samples obtained at room temperature show N2 isotherms similar to those from metakaolins. This treatment did not significantly modify the structure of the metakaolin and the porous properties of these samples are very close to those of the parent metakaolin. The samples obtained under reflux conditions for 6h show nitrogen gas adsorption-desorption isotherms different from those of the parent metakaolins. They show an increase of adsorption at low relative pressures and reach a plateau at intermediate values of P/Po. This kind of isotherm is classified as type I (BDDT classification, [27]) and it is characteristic of microporous materials. For treatment times higher than 6 h, the isotherms are analogous to those of metakaolins, classified as type II [27], which indicates the loss of the microporosity formed at lower times. 

Attempts to accelerate LSX formation by increasing the reaction temperature enhanced the formation of A zeolite but diminished the yield of LSX. However, additional studies on powdered metakaolin showed the formation of LSX can be significantly accelerated by chemical means. Ciric has shown that A zeolite, which has a Si/Al ratio of 1.0 and the same structural components as faujasite, has a rate of formation proportional to [OH-]2 at a given temperature.(15)... 

Kaolin or clay, its more common name, is a naturally occurring mineral with the chemical formula Al Si, 0, (OH)5. It has a plate-like structure, and is refined and treated for specific uses, the largest of which is the paper industry. Metakaolin, produced by the dehydroxylation of kaolin, is often used to improve the electrical properties of PVC wire and cable compounds. At loadings of about 30 weight percent of the resin in a wire and cable PVC formulation, a doubling of the volume resistivity can be achieved. 

In addition to strength, the addition of pozzolanic materials, and in particular metakaolin, leads to a refinement of the pore structure (Khatib and Wild, 1996) and significantly reduces the permeability of the resulting concrete (Caldarone and Gmber, 1995). This, in due course, will improve the overall durability and resistance against chemical attack, including sulfates (Wild et al, 1997). 

Reaction (1) shows the calcination of kaolin, which can also be described as a thermal dehydration process that destroys the original layered structure of the system. Reactions (2) and (4) describe the alkahne dissolution of metakaolin resulting in primary condensation structures (aluminosilicate oligomers), which, by further poly condensation reactions (3) and (5) in the alkaline environment, give rise to an infinite number of repeating units, thus resulting in a geopolymeric network [10]. The formation of the aluminosilicate structure was initially simply described as a series of destructive and condensation reactions ... 

P. Rovnanik, Effect of curing temperature on the development of hard structure of metakaolin-based geopolymer, Constr. Build. Mater. 24 (2010) 1176-1183. 

M. Luz Granizo, M.T. Blanco-Varela, S. Martinez-Ramirez, Alkali activation of metakaolins parameters affecting mechanical, structural and microstructural properties, J. Mater. Sci. 42 (2007) 2934-2943. 

The design of adhesive mortars was based on binders of either hydrate lime-metakaolin or natural hydraulie lime, with the aim of formulating a complex system characterized by the highest compatibility. Nowadays, both hydrate lime-metakaolin and natural hydraulic lime mortars are widely used in the field of restoration and conservation of architectural monuments, due to their capability to enhance the chemical, physical, structural and mechanical compatibility with historical building materials (stones, bricks and mortars) (Rosario 2009). This compatibility is a very critical prerequisite for the optimum performance of conservation mortars, considering the damage caused to historic monuments dming the past decades, due to the extensive use of cement-based composites. 

Binders of either lime (L by CaO Hellas) with metakaolin (M Metastar 501 by Imerys), or natural hydraulic lime (NHL NHL3.5z by Lafarge), as well as nano-titanium dioxide (T nano-structured nano-titania by NanoPhos), used as filler due to its photocatalytic activity, were employed for the design of the adhesive mortars. The already established improvement of hydration and carbonation process due to the photocatalytic activity of nano-titania in anatase form (Hyeon-Cheol 2010) added to cement mortars, was taken into consideration to assess whether the adhesion performance of the studied... 

Metakaolin. An intermediate product formed when kaoiinite is heated at temperatures between about 500 and 850°C the layer structure of the parent kaolinite persists in modified form but collapse of the layers destroys any periodicity normal to the layers. At higher temperatures (925 C) metakaolin transforms to a cubic phase with a spinel-type structure at 1050-1100 C mullite is formed. 

Above 500 °C kaolinite starts to lose its water of crystallisation and, by 650 °C, approximately 90% of this dehydroxylation is complete, leaving residual OH groups randomly distributed but isolated so that condensation will not occur readily. The product formed is known as metakaolin. Some crystalline structure is retained [22] but X-ray diffraction patterns are so diffuse and weak that no better, more recent, identification has been made. After these structural changes the aluminium, which was originally in six-fold octahedral sites, occupies four- and five-fold sites almost equally [23]. 

Metakaolin is stable up to 980 °C, although between 900 °C and 950 °C the remaining OH groups become mobile and condense. This causes the rearrangement of the poorly crystalline metakaolinite structure to an amorphous defect spinel, which is commonly called calcined clay. This may also be regarded as formation of poorly crystalline mullite and a spinel phase with the separation of amorphous silica [25]. However, experimentally, the material produced up to 1100 °C is X-ray amorphous and the various phases suggested above are deduced from other techniques, such as nuclear magnetic resonance (NMR). 

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. 

Partial replacement of Portland cement by natural or artificial pozzolans reduces the heat of hydration and also the unit price of concrete it slows down the process of hardening and the early strength is lower. Nevertheless, the majority of the volume of structural concrete used in technically advanced regions of the world are based on rational compositions of Portland cement and so called secondary binding materials fly ash, ground granulated blastfurnace slag, metakaolin, SF, and others. 

Steveson, and K. Sagoe-Crentsil, "Relationships Between Composition, Structure and Strength of Inorganic Polymers - Part I - Metakaolin-Derived Inorganic Polymers," 7. Mater. Set., 40 [8], 2023-36... 

As a rather straightforward example. Figure 6.25 illustrates the Si MAS NMR spectrum of an alkali-activated metakaolin-silica blend (Si/Al = 2.0 and Na/Al = 1.5 molar ratios) cured for 1 week at 80°C. The formation of a zeolitic phase in this system is anticipated, which will result in up to five distinct resonances from the Q ( A1) ( = 0,1, 2, 3, 4) environments in the framework structure. Thus, the five resonances from -87 to -107 ppm... 

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