Importance of Metakaolin Impurities for Geopolymer Based Synthesis

Nanostructural Evolution of Calcium-Containing Alkali-Activated Metakaolin During High Temperature Exposure

“…Induced phase transformations as a result of amorphous materials being exposed to high temperatures are of interest to a wide range of materials-focused research domains, including the development of fire tolerant sustainable cements, − crystallization of amorphous metals, , perovskite synthesis, − and structural transformations in chalcogenide glasses. , In particular, it is known that alkali-activated metakaolin (AAMK), a type of sustainable cement, possesses ideal chemical phase transformation behavior when exposed to high temperatures (up to ∼1000 °C), − where the sodium-alumino-silicate-hydrate (N–A–S­(−H)) gel formed at ambient temperature is seen to undergo a local atomic reconstruction process above ∼600–800 °C, forming crystalline nepheline . At ambient temperature, this N–A–S­(−H) gel is characterized by a three-dimensional alumino-silicate structure with silica as Q 4 units and alumina in q 4 (Q n and q n denote connectivity of tetrahedral silica and alumina units, respectively), where the negative charge associated with the alumina sites is charge-balanced by the positive alkali ions. − The absence of detrimental decomposition reactions on heating of AAMK, such as those involving hydroxide- and carbonate-based phases in calcium-rich cements (e.g., Ca­(OH) 2 and CaCO 3 ), − and minimal amount of bound water contained within the N–A–S­(−H) gel − make AAMK (i) an attractive option for fire resistant mortars and concretes ,,− and (ii) a low temperature precursor for aluminosilicate-based ceramics. − One current drawback of AAMK is the need for large amounts of water during synthesis due to the plate-like particles of metakaolin (MK), which leads to an overall high porosity and high degree of physical shrinkage on heating . However, with the emergence of flash-calcined MK that can have a lower water demand than conventional rotary-calcined MK, such physical changes may be suppressed in the future.…”

Preparation of Geopolymeric Materials from Industrial Kaolins, with Variable Kaolinite Content and Alkali Silicates Precursors

“…In addition to some applications already mentioned [12][13][14][15][16][17], the use of geopolymers as coatings on metallic substrates as thermal barriers [37], with an adhesion strength of more than 3.5 MPa on steel depending on their chemical composition, as well as their application as refractory adhesive material for metals and gaskets [40] and material resistant to chemical attack by acids [77], are of particular interest.…”

“…In recent decades, geopolymers have been seen as an environmentally viable alternative to Portland cement because of their performance. They have attracted attention for properties such as mechanical compressive strength, low permeability, good chemical resistance and durability against acid and sulfate attack, as well as excellent fire performance [5,13,15,[33][34][35][36][37][38]40,[77][78][79][80][81][82]. It has also been reported that obtaining geopolymers as alkali-activated inorganic materials reduces greenhouse gas (GHG) emissions [81,83].…”

“…The preparation of geopolymers has been achieved from different raw materials among them, and kaolin is the most common [5][6][7][8][9][11][12][13][14][15]17,[36][37][38]81,[86][87][88], a raw material in which kaolinite is found in varying proportions [89], as well as clays [10,39], silica fume [16], fly ash [33,36,78], red mud from alumina production [87,90], slag and other industrial byproducts and wastes [34,36,79,91], various sources of silica [16,77,92,93], coal ash [94] and others [36,95,96]. In essence, the kaolinite (1:1 aluminosilicate with structural OH groups) [89] found in kaolin, heat-treated until it loses structural OH and transforms into metakaolinite, is the main precursor, either as a single raw material or mixed with others, in the formation of geopolymeric materials [5,9,15,[36][37]…”