Unreacted Metakaolin Research Articles

Investigating irradiation effects on metakaolin-based geopolymer

Nuclear power facilities face a critical challenge: concrete deterioration caused by prolonged neutron irradiation. This study explores a promising alternative, a metakaolin-based potassium geopolymer, which was implanted with Ti+ ions to mimic the irradiation. We observed significant improvements in the geopolymer's mechanical properties after irradiation. Nanoindentation showed that microhardness increased by a remarkable 90 %, accompanied by a 46 % rise in reduced modulus and a 23 % reduction in contact depth. However, irradiation also induced surface cracking. Detailed characterization revealed microstructural changes, including the disappearance of unreacted metakaolin and densification of the geopolymer matrix. While X-ray diffraction showed no significant structural alterations, Fourier-transform infrared spectroscopy indicated a decrease in water content, and Raman spectroscopy revealed a reduction in intensity, peak shift, and an increase in full-width half maximum, suggesting the presence of titanium ions and potential modifications in the local chemical environment. This initial investigation paves the way for geopolymers to be environmentally friendly alternatives in nuclear reactor shielding. While our findings highlight promising mechanical property improvements after irradiation, further development is necessary to address crack mitigation. This study provides new insights to guide future optimization of geopolymer composition and microstructure for enhanced resistance to irradiation effects.

Salicylic acid–methanol extraction of aluminosilicate hydrates

AbstractThis study focuses on understanding how salicylic acid and methanol (SAM), commonly used to dissolve certain calcium silicates in portland cement, affect geopolymer gels and other sodium aluminosilicate hydrates. The technique has been used in the past to selectively dissolve hydrated calcium silicates formed through alkali activation of aluminosilicate mixtures containing calcium in the expectation that the extraction does not dissolve alkali aluminosilicate hydrates. However, the work presented here reveals that SAM solution does, in fact, dissolve aluminosilicate hydrates. Zeolite A, a three‐dimensional, low‐silica, crystalline sodium aluminosilicate hydrate, was fully dissolved upon SAM extraction, zeolite X was partially dissolved in the SAM extraction, and only zeolite Y was completely unaffected by SAM extraction. Similar behavior was observed with amorphous sodium hydroxide‐metakaolin geopolymer products—at an early age (1 day), SAM dissolved all the gel and left only unreacted metakaolin, and at a later age (7 days), SAM partially dissolved the geopolymer gel. Nuclear magnetic resonance spectroscopy indicated that dissolution requires the presence of silicon at the Q4(4Al) sites. Extraction was seen to involve some dealumination and complete removal of sodium. The mechanism of aluminosilicate dissolution in SAM appears to be acid attack followed by removal of aluminum via chelation with salicylic acid.

Effect of Sand Type and PVA Fiber Content on the Properties of Metakaolin Based Engineered Geopolymer Composites

This study investigates the mechanical and physical properties of engineered geopolymer composites (EGCs) consisting of metakaolin (MK) based geopolymer (GP) binder, microsilica sand (MS) or river sand (RS), and polyvinyl alcohol (PVA) fiber. Plain GP matrices and fiber-reinforced composites were studied. Aspects of the material composition evaluated here included binder composition, aggregate type, and fiber content. Compressive strength test results revealed that GP mortars presented greater compressive strength than pure GP binders. Moreover, the addition of PVA fibers produced an important increment in compressive strength while increases in fiber content generally enhanced compressive strength. MS composites tended to produce higher compressive strength than RS composites. While exhibiting low densities (i.e., 1.80–1.94 g/cm3), the composite materials evaluated exceeded the compressive strength of regular concrete (i.e., 30 MPa). Furthermore, the best performing composite presented a compressive strength of 57.5 MPa, thus falling in the category of high-strength concrete. The EGC-like tensile pseudo strain-hardening (PSH) behavior of MK-based GP composites was confirmed by a uniaxial tensile test with MS-composites exhibiting greater tensile strength and ductility compared with RS composites; yet, robust PSH behavior was not achieved mainly because of a lack of proper fiber dispersion. Scanning electron microscopy (SEM) revealed unreacted metakaolin particles in some areas within the geopolymer gel of both GP binders evaluated in this study, suggesting incomplete geopolymerization in the binder. Furthermore, energy dispersive X-ray spectroscopy (EDS) microchemical analysis of a geopolymer gel encountered close agreement between the target composition of the GP binders produced and the actual chemical composition encountered in the specimens evaluated.

Pore Analysis and the Behaviour of the Unreacted Metakaolin Particles in the Networks of Geopolymer Cements Using Metakaolins From Kaolinitic and Halloysitic Clays

This work aims to study the pore analysis and the behaviour of the unreacted metakaolin particles in the geopolymer cements from halloysitic and kaolinitic clays. The XRD patterns of the geopolymer cements show the unreacted metakaolin particles at about 21 °(2θ). The micrography images of the specimen from calcined kaolinitic clay confirm the presence of the unreacted metakaolin particles except the one using calcined kaolinitic clay from Bangoua. Whereas those from calcined halloysitic clay are compacts, homogeneous and denser microstructure. The compressive strength of the specimen from calcined halloysitic clay (88.50 ± 2.08 MPa) is higher compared to those from calcined kaolinitic clays (between 45.43 ± 1.84 and 57.20 ± 1.09 MPa). The pore analysis shows that the pore size diameters of the specimen from calcined halloysitic clay are in the range 7.4–16.2 nm while those from calcined kaolinitic clays are ranging from 5.5 to 12.3 nm. It was found that although the geopolymer cement from calcined halloysitic clay contains more unreacted metakaolin particles, it possesses the mesopores due to the inclusion of these fine unreacted metakaolin particles in its structure.

Control of the alumino-silico-phosphate geopolymers properties and structures by the phosphorus concentration

Acid-based geopolymers are interesting materials, as they usually present higher mechanical and thermal properties than their alkali-based counterparts do, but they are difficult to use for complex shapes due to the high viscosity of their reactive mixture. This can be overcome by the addition of water in the formulation. In this study, geopolymers have been synthesized from metakaolin and phosphoric acid with variable [P] concentration (by a modification of either the water content or Al/P ratio). Measurements of the viscosity, density and consolidation times have been performed on reactive mixtures differentiated by their water content. Furthermore, a study of the geopolymers’ microstructure and mechanical properties confirmed their evolution with [P]. Finally, a characterization by thermal and structural analyzes of their structure (Fourier transform infrared spectroscopy and X-ray diffraction), have been carried out in order to assess the influence of the water content on the different amorphous networks. It appears that the addition of water influences the structure of the material, as it leads to the formation of new amorphous networks. More specifically, for Al/P = 1 and variable water amounts, the networks are mainly based on the coexistence of Al–O–P with an hydrated silica network, while for variable Al/P, the Al–O–P network coexists with the unreacted metakaolin. In addition, it seems that all the properties are governed by the parameter nP/nAl + nH2O.

Microstructural and mechanical properties of poly(sialate-siloxo) networks obtained using metakaolins from kaolin and halloysite as aluminosilicate sources: A comparative study

This work focuses on the comparison between the mechanical and microstructural properties of poly(sialate-siloxo) networks based on metakaolins from halloysite and kaolin. Poly(sialate-siloxo) networks were prepared using three metakaolins as aluminosilicate sources. Sodium waterglass from rice husk ash and commercial sodium waterglass were used as chemical reagents. The obtained results showed that metakaolins from kaolins have plate shapes with coarse particle sizes whereas the one from halloysite has a spherical morphology and smaller particle sizes. The IR spectra of poly(sialate-siloxo) networks from calcined halloysite indicate the higher value of the wavenumber of the main band. The XRD patterns of all poly(sialate-siloxo) networks show the broad hump structure with higher intensity between 18 and 40°(2θ). The XRD patterns of poly(sialate-siloxo) networks show the band of the unreacted metakaolin at about 20.45°(2θ). This band is more pronounced on the XRD patterns of geopolymer cements from calcined halloysite. The obtained poly(sialate-siloxo) networks based on metakaolins from halloysite and kaolin have a compact, homogenous and denser microstructures. The compressive strength values of the poly(sialate-siloxo) networks using calcined kaolin are ranging from 58.43 to 66.52 MPa whereas those using calcined halloysite are between 72.29 and 88.50 MPa. The compressive strength values of poly(sialate-siloxo) networks using calcined halloysite are higher compared to those from calcined kaolin. The higher compressive strength values of the geopolymer cements from calcined halloysite could be attributed to the fine and spherical particle sizes of calcined halloysite. This implies that the shape and the fine particle sizes of the raw materials influence the properties of the poly(sialate-siloxo) networks. Metakaolin from halloysite can be used as an aluminosilicate source for producing poly(sialate-siloxo) network with higher mechanical properties.

New selective dissolution process to quantify reaction extent and product stability in metakaolin-based geopolymers

A selective dissolution process is developed that can quantify the amount of soluble material, geopolymer gel and remnant unreacted precursor in metakaolin-based geopolymer systems and determine the nanostructural features of the raw materials and geopolymer gel components. The susceptibility of alkalis leachability from the alkaline aluminosilicate hydrate-type gel (N-A-S-H) produced during the geopolymerization is not fully understood. This phenomenon led to deleterious processes from a microstructural, aesthetic and performance point of view. Geopolymers were synthesised using different contents and types of alkalis (M/Al = 0.50–0.83, where M represents Na or K), different contents of soluble silica in the activator (expressed as SiO2/M2O ratio of 1.0, 0.5 and 0.0), and curing temperatures (25 and 50 °C). The selective dissolution process is based on neutral dissolution at pH 7 to extract the soluble materials and acid dissolution using a strong acid at pH 0 to dissolve the geopolymer gel, which provides for the first time a method to quantify the (i) soluble material, (ii) geopolymer gel and (iii) unreacted material in geopolymers. The soluble material provides a reliable indication of the materials that can be removed from the geopolymers in a neutral pH environment and hence the potential for leaching and efflorescence, which is useful for durability prediction and service life. Quantification of remnant unreacted metakaolin determines the reactivity of the precursor and assesses the suitability of different synthesis conditions for varied applications. This work therefore provides a novel and widely applicable approach to determine the susceptibility of geopolymer materials to leaching.

Relation between working properties and structural properties from 27Al, 29Si and 31P NMR and XRD of acid-based geopolymers from 25 to 1000°C

Highlights: (maximum 85 characters, including spaces, per bullet point).  Acid-based geopolymers display different processing-dependent working properties.  Acid-based geopolymers are mostly composed of Al-O-P and Si(OH)4 networks.  The quantities of Al-O-P and unreacted metakaolin govern the thermal resistance.  The increase of the Young's modulus is related to a higher thermal resistance.  Ordered structures and low amounts of physisobed water lead to water resistance.

Slag‐fly ash and slag‐metakaolin binders: Part II—Properties of precursors and NMR study of poorly ordered phases

AbstractSodium silicate‐activated slag‐fly ash binders (SFB) and slag‐metakaolin binders (SMKB) are room‐temperature hardening binders that have excellent mechanical properties and a significantly lower carbon footprint than ordinary Portland cement (OPC). The aim of this study was to use nuclear magnetic resonance (NMR) spectroscopy to study the nanostructure of poorly ordered phases in SFB by varying slag/fly ash ratio, curing time, and curing temperature. Fly ash was completely substituted with metakaolin and the effect of this substitution on the poorly ordered phases was studied. It was observed that the proportion of geopolymer was generally higher in SMKB when compared to SFB. Although C–N–A–S–H and geopolymer coexisted in SFB and SMKB, C–N–A–S–H was the major product phase formed. The mean chain length (MCL) and the structure of C–N–A–S–H gel were estimated as a function of time, temperature, and slag/fly ash ratio. The MCL was found to have a negative correlation with slag/fly ash ratio and Ca/(Si+Al) ratio, but positive correlation with curing temperature. The average Si/Al atom ratios for geopolymers were also estimated. Lastly, the increased proportion of five‐coordinated aluminum (Al(V)) in metakaolin resulted in the decreased unreacted metakaolin in the hardened binder but did not increase the geopolymer content.

Optimization of Slag Content and Properties Improvement of Metakaolin-slag Geopolymer Mixes

Object:Geopolymers mixes were fabricated from Metakaolin (MK) and Blast Furnace Slag (BFS) waste material in the presence of sodium hydroxide and sodium silicate which were used as alkali activators. To optimize the suitable amount of slag, eleven batches were designed, mixed and homogenized for 30 min.Method:To determine the suitable amount of liquid required for pasting, normal consistency and setting time were determined. The physico-mechanical properties at different curing agesi.e. 3, 7, 28 and 90 days, were determined. X-ray diffraction and scanning electron microscope were used to investigate phase composition and microstructure.Result:To guarantee forming geopolymer gel and to check its amount, HCl extraction test was performed. Salicylic acid/methanol extraction was also performed to verify the presence and amount of Calcium Silicate Hydrate (CSH). The results revealed that calcium-rich slag (BFS) accelerated the hardening process and decreased the alkaline liquid consistency. For geopolymer without BFS, two phases, namely; un-reacted metakaolin and geopolymer gel were formed. For geopolymers with BFS, three phases were formed, namely; un-reacted metakaolin, geopolymer gel and CSH with aluminum substitution (CASH) gel. The bulk density was increased with increasing BFS and curing time. The strength was increased with increasing of BFS, reaching its maximum (about 120 MPa) for the specimen containing 70% slag, cured for 28 days.

Compressive Strength and Setting Time Modification of Class C Fly Ash-Based Geopolymer Partially Replaced with Kaolin and Metakaolin

Class C fly ash is widely used as a geopolymer raw material. It contains high calcium oxide and iron oxide resulting in a fast setting property. The influence of using kaolin and metakaolin replaced class C fly ash in geopolymer processing were investigated in term of compressive strength and setting time. Kaolin and metakaolin which calcined at different temperature of 600°C and 700°C replaced Class C fly ash between 0 to 50 wt. %. The geopolymers were prepared at constant KOH concentration as 6.0 M, K2SiO3/KOH ratio as 1.0, solid/liquid ratio as 1.5 and 7 days of curing. The compressive strength is obviously increased when fly ash was replaced with both kinds of metakaolin, although, it is inclined to decrease when replaced by kaolin. The compressive strength is increased up to 13% and 47% with the replacement by 50 wt. % of metakaolin calcined at 600°C and 700°C, respectively. On the other hand, the replacement by 50 wt. % of kaolin give the initial setting time prolonging from about 6 min to 80 min. However, the initial setting time of metakaolin calcined at 600°C and 700°C replacement is also improved to about 37 min and 20 min, respectively. These results from their difference of amorphous phase and reactivity of the replacement materials. XRD analysis reveals the combination phases of amorphous geopolymer gel that is the broad hump centered at 28 – 30° 2θ, and products from the reaction such as calcium oxide, arcanite, calcium silicate hydrate, quartz and muscovite which the two latter are from unreacted metakaolin. They are confirmed by FT-IT result and microstructure evaluation by SEM. Therefore, the partially replacement of fly ash with kaolin and metakaolin in fly ash-based geopolymer production are affected to the compressive strength and resulted to modify the setting time.

Synthesis of granulated binder-free LTA zeolite from metakaolin using ultrasonic treatment

It was studied the influence ultrasonic treatment (22 kHz) of an aqueous suspension of metakaolin on the synthesis of the granulated binder-free LTA zeolite. Mixtures with a molar ratio of the solid ingredients of 6Al2Si2O7:12NaOH (stoichiometric) and 6Al2Si2O7:12NaOH:2Al2O3 have been investigated. XRD, SEM, FT-IR and rotational viscosimetry have been used for testing. It was established that in the mixture with an Al2O3 excess after the ultrasonic treatment, sodium aluminates of cubic syngony were synthesized. Moulding pastes have been prepared from the sonicated suspensions and pellets of 3 mm diameter have been formed by extrusion. It is shown that sodium aluminates increased both the strength of coagulation structure of pastes and the mechanical strength of the pellets. It was found that after the thermal treatment of the granules at 650 °C, Na6Al4Si4O17 was formed. In the mixture with an Al2O3 excess, Na8Al4Si4O18 was also detected. The zeolitization of sodium aluminosilicates and unreacted metakaolin during hydrothermal crystallization in the NaOH solution with concentration of 2 mol l−1 allowed synthesizing the LTA zeolite. The pellets from the stoichiometric mixture contained 91 wt% zeolite and the remainder was unreacted Na6Al4Si4O17 and other byproducts. The pellets from the mixture with an Al2O3 excess contained 97 wt% LTA.

Effect of curing temperature and SiO2/K2O molar ratio on the performance of metakaolin-based geopolymers

In this study, the effect of curing temperature and SiO2/K2O molar ratio on the mechanical properties of metakaolin-based geopolymers were investigated. Different curing temperatures (from 50° to 90°C) were used for curing process. A series of geopolymers with different SiO2/K2O molar ratio (N) were prepared, with N=2.4, 2.0, 1.6 and 1.2, respectively. The phase composition, microstructure and mechanical properties of the resulting geopolymers were studied. The results indicate that curing temperature and SiO2/K2O molar ratio impacts final microstructures and mechanical properties of the resulting geopolymers. Microstructures become denser with increases in curing temperature, which was beneficial to the improvement of geopolymer's mechanical properties. The optimum curing temperature was ≈ 80°C giving the best mechanical properties with the compressive strength of 106.2MPa and elastic modulus of 8.3GPa, respectively. With increasing SiO2/K2O molar ratios, the microstructure was quite porous and contained unreacted metakaolin particles, and the corresponding mechanical properties declined sharply, attributed to low degrees of geopolymerization caused by high alkali content.

Effect of cordierite addition on compressive strength and thermal stability of metakaolin based geopolymer

Metakaolin based geopolymer composites have been synthesized at room temperature by mixing metakaolin with NaOH and sodium silicate solutions, then adding cordierite powder from 0 to 50wt% as a weight fraction of geopolymer matrix. Compressive strength was measured after aged 28days. The thermal stability was analyzed by placing cube specimens in the furnace at 800°C for 2h. The compressive strength of cordierite-metakaolin based geopolymer composites was studied in comparison with those without cordierite addition. In this study, the compressive strength showed an increase with cordierite addition (from 0 to 30wt%), whereas the molar ratio SiO2/Al2O3=3.5 and H2O/Na2O=13, the strength increased from 45MPa to 57MPa. The microstructure revealed the unreacted metakaolin phase dispersed in the dense geopolymer matrix corresponding with the XRD results. It is evident that cordierite content in metakaolin based geopolymers can provide the good thermal stability at high temperature. Thermal shock resistance of 50wt% cordierite added to geopolymers at 800°C attained 15 cycles without cracking, although, the residual compressive strength was declined. Cordierite-metakaolin based geopolymer composites could be considered as a good candidate for high temperature refractory applications.

Geopolymer binders from metakaolin using sodium waterglass from waste glass and rice husk ash as alternative activators: A comparative study

Rice husk and waste glass from sources in Cameroon were used for producing sodium waterglass (NWG) solution denoted S1 and S2 respectively as alternative activators to prepare metakaolin-based geopolymer binder. Metakaolin-based geopolymer binders (G1, G2) were obtained using freshly prepared NWG with a mass ratio NWG/MK=0.83. The IR spectra of S1, S2 using ATR and KBr methods show the presence of SiQ0, SiQ1, SiQ2 units, and S1 contained also SiQ3 and SiQ4 units. The mechanical testing, environmental scanning electron microscopy, mercury intrusion porosimetry, X-ray diffractometry, infrared spectroscopy, amount of binders and thermogravimetric analysis are investigated to study the properties of the geopolymer binders. The results show that the compressive strength values of geopolymer G2 (22.9, 27.6, 32.6, 36 and 39.7MPa) are higher than that of G1 (17.7, 19.1, 21.2, 29.9 and 32.8MPa) at 7, 14, 21, 28 and 56days respectively. The microstructure of G2 is more compact with fewer unreacted metakaolin particles. It can be concluded that sodium waterglass from waste glass and rice husk ash are suitable alternative activators for the production of metakaolin-based geopolymers. In addition, they constitute a better ecological choice when compared to commercial sodium silicate from melting process.

Comparison of metakaolin-based geopolymer cements from commercial sodium waterglass and sodium waterglass from rice husk ash

Three sodium waterglass (NWG) such as commercial NWG (S1), NWG from pure rice husk ash (S2) and NWG from raw rice husk ash (S3) were applied for producing geopolymer cements using metakaolin (MK) as aluminosilicate source. Geopolymers (Geo1, Geo2 and Geo3) were prepared using each NWG with the molar ratios SiO2/Na2O and H2O/Na2O kept constant at 1.5 and 12, respectively. It could be observed that the water absorption of Geo1, Geo2 and Geo3 is 7, 9 and 13.2 % and the mass loss is 15.8, 14.7 and 12.4 %, respectively. Their compressive strength at 20 days (37.5/34.3/29.6 MPa) and 28 days (43.3/40.3/33.2 MPa) increases with increasing the aging and decreases in the course Geo1/Geo2/Geo3. Their average pore radius (6/8/20 nm) and cumulative pore volumes (155/205/245 mm3/g) increase in the course Geo1/Geo2/Geo3. It is discussed that the presence of phosphate known as corrosion inhibitors in raw rice husk ash hinders the dissolution of SiO2. It entails the formation of NaH2PO4 in S3 which reduces the soluble Si atoms. Therefore, less amount of metakaolin could be dissolved leaving thus a higher amount of unreacted metakaolin particles in Geo3. The reacted volumes and compositions of the geopolymers are different in the three cases, too. A content of approximately 20, 25 and 35 % of unreacted metakaolin was proved for Geo1, Geo2 and Geo3, respectively.

Influence of gibbsite and quartz in kaolin on the properties of metakaolin-based geopolymer cements

Three different kaolins (K1, K2, K3) from sources in Cameroon were applied for producing geopolymer cements. The kaolins differ significantly in their gibbsite and quartz contents. Thermal transformation (700°C, 4h) into their metakaolins MK1, MK2, MK3 shows the total loss of crystalline kaolinite and reveal the typically rather broad bump in the X-ray pattern. Gibbsite becomes dehydrated into γ and χ-Al2O3. Geopolymer cements (GP1, GP2, GP3) were obtained using freshly prepared sodium silicate solutions (NWG) with a ratio NWG/MK=0.87. It could be observed that the initial (60/80/90min) and final (90/140/160min) setting time increases and their 28day compressive strength (49/39/30MPa) decreases in the course GP1/GP2/GP3. It is discussed that the higher content of quartz in K1 (up to 22wt.%), compared to K2 (10wt.%) and K3 (8wt.%) promotes higher strength values and decreased setting times. Gibbsite was not present in K1, but up to 11wt.% in K2 and 28 wt.% in K3, transformed in its dehydrated forms remains unreacted during geopolymerization. Therefore, the higher content of gibbsite in the kaolinite could be related to a lower strength. The reacted volumes and compositions of the geopolymer become almost the same in all three cases. A content of 30–50% of unreacted metakaolin was proved in all cases.

The effect of limestone on sodium hydroxide-activated metakaolin-based geopolymers

Blends of metakaolin and limestone can be alkali-activated with NaOH to form solid binders, which show relatively low strength but offer potential as a model system by which the reaction processes of more complex systems can be better understood. The effects of curing procedure, limestone content and alkalinity of the activator are able to be related to the mineralogy, mechanical properties and microstructure of hardened pastes. The presence of limestone enhances the release of Al and Si ions from metakaolin, with the Al released in the early stages of the reaction being bound into AFm-type phases. Dissolution of LS is slightly higher when a lower alkalinity sodium hydroxide activator is used. The heat treatment of pastes activated with 3M NaOH solution resulted in a lower extent of reaction of limestone, while with 5M solution, heat-curing at early age resulted in more reaction. The main alkali-activation product in metakaolin–limestone–NaOH pastes is a geopolymer gel with inclusions of unreacted metakaolin, limestone particles, zeolite A, and AFm phases, with different zeolites such as faujasite-like and hydrosodalite phases also identified at higher reaction temperatures.

Firing transformations of an argentinean calcareous commercial clay

Mineralogical transformations caused by firing are usually studied by XRD methods only semi-quantitatively. In this work the original mineral disappearance and the neo-mineralization were evaluated quantitatively. Furthermore an indirect non crystalline phase quantification was performed under 1100 ºC was also carried out using the quartz content as internal standard. This study specifically discusses the behavior of an Argentinean white calcareous earthenware commercial when subjected to traditional ceramic firing, besides the technological importance of this particular material, it acts as a model for other clay based ceramic materials. Materials were subjected to thermal treatments between 700 ºC and 1100 ºC. A preliminary sintering characterization was carried out by contraction and porosity evolution. Simultaneous thermogravimetric and differential thermal analysis (TG-DTA) was carried out to elucidate the actual temperature at which the chemical changes occur. Finally, a quantitative analysis based on the Rietveld refinement of the X-ray diffraction patterns was performed to characterize the crystalline phases present in both the clay and in the materials obtained after different thermal treatments. The actual chemical reactions are proposed. The phases identified after firing at traditional working temperature (1040 ºC) are quartz, plagioclase, and the Spinel type alumino-silicate, accompanied by the non-diffracting un-reacted metakaolin and some amount of amorphous glassy phase. At intermediate temperatures (900 ºC) the presence of gehlenite was also detected. The carbonates (calcite and dolomite) presence and decomposition were also evaluated and demonstrated to determine the sintering characteristics of this material.

Evolution of Mineralogical Phases by27Aland29SiNMRinMK‐Ca(OH)2System Cured at 60°C

The evolution of the metastable phases in metakaolin/Ca(OH)2systems cured at high temperatures, remains mostly unknown, newer techniques may now help to establish both the kinetic mechanism of the pozzolanic reaction and the thermodynamic stability of the main hydrated hexagonal phases: Stratlingite (C2ASH8) and tetra calcium aluminate hydrate (C4AH13). For this reason this work examines the kinetics of the pozzolanic reaction in theMK/Ca(OH)2system over 123 d at 60°C using nuclear magnetic resonance spectroscopy (27Aland29SiNMR). The results obtained by27Aland29SiNMRshow that during the first 30 h, the metastable phasesC2ASH8andC4AH13, coexist with the cubic phase (C3ASH6) obtained directly from the pozzolanic reaction. The gel C–S–H is clearly identified after 21 h of reaction, whereas at shorter times the C–S–H bands overlap those with the unreacted metakaolin ones. After 123 d of pozzolanic reaction, the first signs of the cubic phase are detected, a consequence of the conversion reaction of the metastable phases, and a phenomenon not previously identified.