Metakaolin reactivity

A gel is defined as a hydrous metal aluminosihcate prepared from either aqueous solutions, reactive soflds, colloidal sols, or reactive aluminosihcates such as the residue stmcture of metakaolin and glasses. 

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]. 

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... 

However, different studies have revealed that the amorphization of kaolin produces solids more reactive under chemical treatments. This amorphization can be performed by calcination, obtaining metakaolin by heating from 550 C to 900 C. The metakaolin is an amorphous phase formed by the dehydroxylation of the octahedral layers and the deformation of the tetrahedral sheets [5-7]. Also the grinding produces the amorphization of kaolin [8], by means of delaminating [9,10]. 

The highly reactive metakaolin used 1n our work is formed during the 700°C heat treatment which is also needed to remove the organic pore former 1n the pelletized adsorbents. 

Metakaolin is much more chemically reactive than the kaolinite from which it was formed [24] and also, deduced from the properties of the polymer composites in which it is used it has a very reactive surface. This is probably a consequence of an increase in the number of Lewis sites due to the reduction in coordination numbers of the aluminium ions and to the presence of reactive hydroxyls. 

Gruber, K. A., Ramlochan, T., Boddy, A., Hooton, R. D., Thomas, M. D. A. (2001) Increased concrete durability with high-reactivity metakaolin, Cement and... 

The Al reaction remains a critical factor in the performance of these Si rich systems. Given that aluminate anions for the reaction are solely derived from the dissolution of mineral oxides under alkaline conditions, monomeric [Al(OH)4] ions are probably the only aluminate species existing under high alkaline conditions. On the other hand, silicate species come from both soluble alkaline silicates and the dissolution of mineral oxides. In the specific case of metakaolin systems, the silicate species from the dissolution of particles are difficult to predict because the hydrolysis process of amorphous silica is kinetically dependent on various factors, such as the reactivity of the particles, temperature, time, and the eoncentration and pH value of alkaline silicate solutions. 

Perera er at." added iron to an aluminosilicate based geopolymer using either ferric nitrate solution or freshly precipitated ferric hydroxide. It was determined that the iron was present in octahedral sites, either as isolated ions in the geopolymer matrix, or as unreacted oxyhydroxide aggregates. In this study, typical aluminosilicate chemistries were used (i.e. metakaolin added to sodium silicate solution) and the iron was added as a minor compoitfrtt (vt%). Under these conditions, the highly reactive aluminosilicate geopolymers set fairly quickly and may have not allowed sufficient time for iron to participate in the reaction process. 

The alkali silicates is one of the raw materials classically used in the formulation of new materials like geopolymers. Geopolymers are amorphous three-dimensional aluminosilicate binder materials which are synthesized at ambient temperature by the alkaline activation of silica solution and aluminosilicates derived from natural minerals, calcined clay or industrial byproducts. Previous study focused on the aluminosilicate sources have shown that the presence of impurities and the reactivity of the metakaolin (aluminosilicate source) can lead to the formation of one or several networks in geopolymers materials [1]. Indeed, the different sources of metakaolins conduct to the presences of various siliceous species in the solution which react with alumina. These multiple combinations lead to the formation of different networks (then various properties of geopolymers). To understand the formation of these various networks, studies relative to the neutralization of siliceous species in solution have been done. Parmentier [2] showed that the ammonium molybdate could react with silica to create silicomolybdic compounds. More recent studies demonstrated that ammonium molybdate could also react with these species in an alkaline environment [3]. These analyses showed that ammonium molybdate could react not only with monomers and dimers, but also with larger molecules. According to this, molybdate can permit to complex siliceous species and to modify polymerization reactions. The aim of the study is to study the influence of the ammonium molybdate addition on the kinetics of the polycondensation reaction as well as on the formation of several networks. 

The use of pozzol an i c f i I lers, such as the more conventional natural pozzolans and fly ash and the advanced reactive silica fume and metakaolin can reduce the alkalinity of the matrix as well as the content of CH, and thus slow down the two processes which lead to the degradation in the properties of the composite chemical attack and microstructural changes at the interface (in particular deposition of CH). 

CB = carbon black SF = silica fume HRM = high reactivity metakaolin FA = fly ash [65]. 

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