Alkali activation of metakaolins: parameters affecting mechanical,structural and microstructural properties

The composition, structure and properties of the reaction product resulting from the alkali activation of metakaolin (MK) are directly impacted by the specific surface and composition of the initial kaolin and the type, concentration and relative amount of alkali activator used. This study aimed to analyze the effect of these parameters on the flexural strength, degree of reaction, porosity and chemical and mineralogical composition of alkali-activated metakaolin pastes. Two types of metakaolin with different specific surfaces were activated under hydrothermal conditions (85 °C, 2 h) using solutions consisting of waterglass and Na(OH) as activators (Na concentrations = 6, 8, 10, 12, 15, 18, 20 M) and two metakaolin/solution ratios. Regression analysis was used to quantify the effect of the parameters tested (activator concentration and MK/solution ratio) on flexural strength. Mathematical models were likewise built to relate the degree of reaction and amount of sodium fixed in the polymer structure to the synthesis parameters. According to ²⁹Si and ²⁷Al MAS NMR, XRD, FTIR, DTA/TG data and chemical analysis, the material obtained by activating two MKs with waterglass plus NaOH was an amorphous hydrated sodium aluminosilicate in which a Q⁴ Si (3Al) type three-dimensional structure predominated, i.e., a structure where three Al atoms are connected to SiO₄ tetrahedra. The alkaline ions in the structure maintain the electrical balance. The general formula obtained for this inorganic polymer was Na₂O·3SiO₂·Al₂O₃·3H₂O.     

Modelling of slaked lime–metakaolin mortar engineering characteristics in terms of process variables

The purpose of the present study is to determine the effect of factors such as dosage, curing conditions and use of a superplasticiser admixture on the porosity, mechanical strength and composition of slaked lime (SL)–metakaolin (MK) mortars. Statistical correlations have been established to describe the mechanical properties as well as porosity and composition of the slaked lime–metakaolin mortars.The SL/MK ratio has a moderate effect on mortar flexural and compressive strengths. The SL + MK/sand ratio is the factor with the highest impact on all the properties studied: strength, porosity and mortar composition. As this ratio increases, strength, porosity and amount of hydration and carbonation products formed in the samples also rise. The next factor by order of importance is the presence of a superplasticiser admixture, which affects porosity, strength and the amount of calcite in the sample. The presence of this superplasticiser admixture increases strength, raises the percentage of calcite in the mortars and reduces porosity. It is particularly striking that neither curing nor open air carbonation time (in the range studied) has a significant effect on the composition or porosity of the SL–MK mortars studied, although they do have a moderate effect on mechanical strength.     

Rheology of alkali-activated slag pastes. Effect of the nature and concentration of the activating solution

An understanding of the rheological behaviour of OPC-based products has been widely studied, for it is essential to determining and predicting the fresh and hardened characteristics and properties of pastes, mortars and concretes. The rheology of alkali-activated material (AAM) systems has been much less intensely researched, however.The present study aimed to ascertain the effect of factors such as the nature and concentration of the alkaline activator on the rheological behaviour of alkali-activated slag (AAS) pastes, with a comparison between the rheological parameters and fluidity of these pastes to the same parameters in OPC. More specifically, the study explored how paste rheology was affected by the nature of the alkaline activator (NaOH, 50/50 wt% NaOH/Na₂CO₃ or waterglass – Wg), its concentration (3–5% Na₂CO₃ of slag weight) and, in the waterglass solution, the SiO₂/Na₂O ratio.The findings showed that AAS paste rheology is affected by the nature of the activator. The rheological behaviour in AAS pastes activated with NaOH alone or combined with Na₂CO₃ was similar to the rheology observed in OPC pastes, and fit the Bingham model. Conversely, the AAS pastes activated with waterglass fit the Herschel–Bulkley model and their rheology proved to depend on both the SiO₂/Na₂O ratio and the Na₂O concentration. Moreover, regardless of the activator used (NaOH, Na₂CO₃ or waterglass), an increase in Na₂O concentration implies a raise of shear stress.The formation of primary C–S–H gel in Wg–AAS and its effect on paste rheology were confirmed. Gel formation was likewise shown to be related to the SiO₂/Na₂O ratio and activator concentration.     

Alkali silicate binders: effect of SiO2/Na2O ratio and alkali metal ion type on the structure and mechanical properties

The influence of SiO₂:Na₂O molar ratio and the nature of an alkali metal (Na vs. K) in commercial aqueous alkali silicate on the microstructure, textural properties, phase composition, and hydrolytic stability of an alkali silicate binder have been investigated using scanning electron microscopy, nitrogen adsorption/desorption technique, X-ray diffractometry, thermal analysis, and dissolution tests. It has been found that microstructure and textural properties of the alkali silicate binder depend both on silica to alkali molar ratio and type of alkali metal (Na vs. K). Sodium silicate binder obtained from commercial silicate solution with lower SiO₂:Na₂O molar ratio (2.2) exhibits a globular microstructure of silica xerogel with high content of micropores, whereas the binder formulated with SiO₂:Na₂O molar ratio 3.2 is characterized by more open cluster structure with lower content of micropores. It is observed that surface specific area estimated by Brunauer, Emmett, and Teller method and mesopore volume obtained by the Barrett–Joyner–Halenda method for sodium silicate binder are substantially higher than those for potassium silicate binder. The ultimate hydrolytic stability of the sodium silicate binder increases slightly with increase in the silica to alkali molar ratio within the studied range. Decreasing in SiO₂:Na₂O molar ratio and replacement of sodium silicate solution by potassium silicate solution in the corresponding filled composition lead to the improvement of mechanical properties and decrease in open porosity.     

Comparative performance of alkali activated slag/metakaolin cement pastes exposed to high temperatures

This paper presents a study on the effect of temperature exposure of binders of blast furnace slag (BFS) and metakaolin (MK) in BFS-MK weight ratios of 100-0, 50-50, and 0-100 activated with sodium silicate of modulus Ms = SiO₂/Na₂O = 1 and 5, 10 and 15% Na₂O. A blended ordinary CPC-30R Portland cement reference was used. Pastes were subjected to exposure up to 1200 °C and the performance was evaluated in terms of compressive strength, residual strength, volumetric shrinkage, physical appearance and microstructural changes at different temperatures. All the binders retained more than 30 MPa after exposure to 800 °C for 4 h; specimens of MK and CPC-30R experienced the highest strength losses of 42 and 56% respectively, while those of 100-0 and 50-50 showed minor losses of ∼20%. After heating at 1200 °C the samples showed microstructural damage and more than 65% of strength losses. XRD indicated that the 100-0 and 50/50 binders are prone to form crystalline phases as akermanite, nepheline and nosean at temperatures greater than 1000 °C, while 0-100 geopolymeric binders preserved mostly an amorphous structure even at 1200 °C with some traces of mullite. The dehydration of C-A-S-H and N-A-S-H altogether with the crystallization of the binder gel induced the formation of highly porous microstructures.     

Relationships between composition,structure and strength of inorganic polymers

This article is the second in a two-part series and discusses inorganic polymers derived from fly ash. Part 1 [1] concerns inorganic polymers derived from a metakaolin precursor. For this study, 15 fly ash-derived inorganic polymers were produced with various compositions. The effect of the concentration of each of the four component oxides (Na₂O, SiO₂, Al₂O₃ and H₂O) and two alkali cations (Na and K) on the microstructure and compressive strengths were assessed. Similar to metakaolin-derived inorganic polymers, it was observed that high-strength fly ash inorganic polymers were related to low porosity and a dense, fine-grained microstructure. Such structures were characteristic of formulations with high silica mole fractions (SiO₂/Al₂O₃ ∼ 3.9) and low water contents, as well as those with high alkali and low alumina contents. For the latter, not only was a characteristic slower strength development with increasing alkali content observed, but there was also a limit of alkali concentration (Na₂O/Al₂O₃ ∼1) beyond which the strength deteriorated. Furthermore, SEM micrographs disclose that the fly ash precursor dissolves more readily in the sodium-based system compared to the potassium equivalent. The interrelation between microstructures of the respective formulations and their strength development are discussed. It is observed that the charge-balancing role of the alkali cations in the fly ash formulations may be dominant compared to initial alkali dissolution reaction of the aluminosilicate fly ash particles, which is partly responsible for initial strength development.     

Mechanical properties of metakaolin-based geopolymers with molar ratios of Si/Al ≈ 2 and Na/Al ≈ 1

The mechanical properties of four different types of geopolymers, but of the same composition (Na/Al ≈ 1, Si/Al ≈ 2 molar ratio), made using a combination of precursors, were determined. The four types were: (i) sodium aluminate (NaAlO₂/NaOH solution), Ludox (colloidal SiO₂ solution) and metakaolin (MK), (SAGP), (ii) NaOH, fumed silica and MK (FSGP), (iii) Ludox, NaOH and MK (LGP) and (iv) commercial sodium silicate and MK (SGP). The highest crushing strength (CCS) value obtained was for SGP (70 MPa) and the lowest value was for SAGP (16 MPa). The highest modulus of rupture (MOR) value obtained was for LGP (9 MPa) and the lowest value was for SAGP (3 MPa). The fracture toughness (K_1c) and Young’s modulus (E) showed the same trend. The effect of adding sand (40 wt%) on their mechanical properties was also determined. The K_1c values increased up to 65% and E values increased up to 80% compared to samples free of sand. However, CCS and MOR values did not change much and gave mixed results. Overall, porosity is found to be the chief microstructural variable limiting the mechanical properties of the geopolymers. The properties of the geopolymers are compared with those of ordinary Portland cement.     

Mechanical and thermal characterisation of geopolymers based on silicate-activated metakaolin/slag blends

This article assesses the effect of mix design parameters on the compressive strength and thermal performance of alkali silicate-activated blends of metakaolin (MK) and granulated blast furnace slag (GBFS). A strong interrelationship between the effects of activator composition and the GBFS/(GBFS + MK) ratio is identified through statistical analysis of compressive strength data. Pastes formulated with higher SiO₂/Al₂O₃ molar ratios show improvements in mechanical strength with increasing GBFS addition, associated with the formation of a structure comprising coexisting aluminosilicate ‘geopolymer’ gel and Ca-rich Al-substituted silicate hydrate (C-(A)-S-H) reaction products. The inclusion of GBFS in MK-based geopolymers seems also to improve their performance when exposed to high temperatures, as higher residual compressive strengths are reported for these mixtures compared to solely MK-based systems. Only slight differences in shrinkage behaviour are observed at temperatures of up to 600 °C with the inclusion of GBFS; however, slag-blended pastes exhibit enhanced stability at temperatures exceeding 800 °C, as no variation in the compressive strength and no additional shrinkage are identified. These results suggest that nanostructural modifications are induced in the gel by the inclusion of GBFS into MK-based geopolymers, improving the overall performance of these materials.     

Production and characterization of geopolymers based on mixed compositions of metakaolin and coal ashes

Mixtures of coal ashes from pulverized coal combustion (PCC) or fluidized bed combustion (FBC) and metakaolin were used to synthetize geopolymers. Upon full characterization of the raw powders (chemical and mineralogical composition), geopolymerization tests were conducted using an alkali aqueous solution of NaOH/Na₂SiO₃ at different dilutions. The produced geopolymers were subjected to SEM analysis, as well as to leaching, thermal and mechanical tests. The final microstructure and the properties of the geopolymers indicate that FBC ash can be conveniently used as a partially reactive filler in combination with the metakaolin powder. The composite material has good thermal performance and compressive strength (∼30 MPa) suitable for the building sector.     

Carbonation process of alkali-activated slag mortars

This study analyzes the behaviour of waterglass- or NaOH-activated slag mortars after carbonation. The effect of a superplasticizer based on vinyl copolymer and shrinkage reducing polypropylenglycol derivative admixtures on that process was also examined. The same tests were run on cement mortars for reference purposes. The mortars were carbonated in a chamber ensuring CO₂ saturation for four and eight months, after which ages the samples were tested for mechanical strength; mercury porosimetry and mineralogical (XRD, FTIR) and microstructural characterization (SEM/EDX) were also conducted. The results obtained indicate that alkali-activated slag mortars were more intensely and deeply carbonated than Portland cement mortars. Carbonation took place directly on the gel, causing decalcification. When waterglass was the alkaline activator used, carbonation caused a loss of cohesion in the matrix and an important increase in porosity and decrease in mechanical strength. When a NaOH solution was used as the alkali activator, carbonation enhanced mortar compaction and increased mechanical strength. Finally, in waterglass-activated slag mortars, the inclusion of organic admixtures had no effect either on their behaviour after carbonation or the nature of the reaction products.     

Geopolymers based on a coarse low-purity kaolin mineral: Mechanical strength as a function of the chemical composition and temperature

A coarse mineral with 70% kaolinite and 30% quartz was calcined and chemically activated by alkaline solutions of Na₂SiO₃ and NaOH. The compressive strength evolution was investigated as a function of the curing temperature at 20 and 80 °C, and the molar ratios of SiO₂/Al₂O₃ (2.64-4.04) and Na₂O/Al₂O₃ (0.62-1.54). For curing at 20 °C, the best composition was SiO₂/Al₂O₃ = 2.96 and NaO/Al₂O₃ = 0.62, reaching 85 MPa at 28 days. Curing at 80 °C had a positive effect on the strength development only in the first 3 days. X-ray diffraction of the geopolymeric formulations showed the formation of amorphous silicoaluminates of similar nature. The microstructure consisted of unreacted quartz and metakaolinite particles in a matrix of silicoaluminate polymer and condensed silica gel from the unreacted sodium silicate.     

Effect of different superplasticizers and activator combinations on workability and strength of fly ash based geopolymer

This paper evaluates the effect of different commercial superplasticizers (SPs) such as naphthalene, melamine and modified Polycarboxylate based on the workability and strength of a class F fly ash geopolymer paste activated by two different activator combinations, i.e. 8 M sodium hydroxide solution and a multi-compound activator composed of 8 M NaOH solution (28.6%) + Na₂SiO₃ (71.4%) with a SiO₂/Na₂O ratio of 2.0. These SPs at a dosage of 1% by mass of fly ash were added to the fresh paste and flowability of the activated fly ash paste was measured via mini slump test and compared with that of the paste without using any SP. The experimental results indicated that the effect of different SPs on the workability and strength of fly ash based geopolymer directly depends on the type of activator and the SP. In the case of using 8 M NaOH solution as the activator, naphthalene based SP was an effective type; whereas modified Polycarboxylate based SP was the most efficient type when the multi-compound activator was used.     

Investigation of a synthetic aluminosilicate inorganic polymer

A melt-quenched of mixture of alumina and silica (46 wt% Al₂O₃ or Al₂O₃(SiO₂)₂) was found to react with an alkaline silicate solution (Na₂O)(SiO₂)_1.2(H₂O)_9.5) at low-temperatures to form a synthetic aluminosilicate inorganic polymer. The as-quenched material consisted of a mixture of amorphous and crystalline phases with a range of aluminium coordination environments. Upon reaction with the alkaline silicate solution, solid-state aluminium and silicon magic-angle spinning nuclear magnetic resonance (SS ²⁷Al and ²⁹Si MAS NMR) indicated that a conversion to four-fold aluminium coordination environments occurred, consistent with the formation of a three-dimensional cross-linked inorganic polymer comprised of NaAlO₄ and SiO₄ tetrahedra. Mechanical testing showed the compressive strength of the inorganic polymer increased as the Na₂O/Al₂O₃ molar ratio decreased. Solution studies indicated that 73% of the aluminosilicate starting material was reactive. Scanning electron microscopy (SEM) showed the inorganic polymers had a porous nanoscale grain structure. Open porosity was confirmed by relatively high specific surface area values. Energy dispersive spectroscopy (EDS) and elemental x-ray composition mapping showed that the high-strength specimens had a composite microstructure consisting of 40% unreacted Al₂O₃(SiO₂)₂ and an inorganic polymer binder Na₂O · Al₂O₃(SiO₂)_3.4. The high compressive strengths have been rationalized by this in-situ particle reinforced composite structure, consisting of 10 m agglomerates of unreacted starting material bonded within a sub-micron aluminosilicate/inorganic polymer matrix.     

Effect of pozzolans on the performance of fiber-reinforced mortars

Randomly oriented short fibers have been shown to increase tensile strength and retard crack propagation of cement based materials such as fiber-reinforced mortars for diverse applications, especially in aggressive environments. In the case of reinforced concrete, it is very important to produce a “high quality” cover in order to prevent corrosion of the rebars. In order to obtain a high performance material the use of a pozzolan is advisable because low permeability is achieved. The objective of this research was to determine the effect of pozzolans such as silica fume (SF), fly ash (FA), and metakaolin (MK) on the properties of fiber-reinforced mortars. Different types of natural and synthetic fibers were used. A superplasticizer was used to keep the same workability as that of the control mortar. Results of the mechanical and durability properties of the fiber-reinforced mortars are reported. The results show that a loss of resistance due to embedding fibers in mortar is compensated for by the increase in strength caused by silica fume or metakaolin additions to the mortar. The addition of 15% of SF or MK produces an improvement of up to 20% and 68%, respectively, when compared with those mortars without addition. There is a significant decrease in the coefficient of capillary absorption and chloride penetration when a highly pozzolanic material is incorporated into the matrix. In general, these materials, especially SF and MK, improve the mechanical performance and the durability of fiber-reinforced materials, especially those reinforced with steel, glass or sisal fibers. The fly ash addition had a different performance, which could be attributed to its low degree of pozzolanicity.     

Mechanical properties of sodium and potassium activated metakaolin-based geopolymers

In this study, a set of mechanical properties of geopolymers, synthesized by alkali (NaOH or KOH) activation of metakaolin and SiO₂ mixture, were characterized at ambient temperature. Samples with K/Al or Na/Al atomic ratios equal to 1, Si/Al atomic ratios in the 1.25–2.5 range and H₂O/Al₂O₃ molar ratios of 11 or 13 are cured at 80 °C for 24 and 48 h before characterization, to determine effect of Si/Al ratio and curing time on the structure and mechanical properties of geopolymers. The structure of synthesized geopolymers characterized using XRD, NMR, SEM, and density measurements was correlated to their mechanical properties, including compressive strength, Young’s modulus, hardness, and fracture toughness. The results of this study suggest a strong effect of Si/Al ratios (in the 1.5–2 range), density, and microstructure on the maximum strength, Young’s modulus, and hardness of geopolymers. There were also notable differences in strength between samples cured for 24 and 48 h, suggesting that the degree of geopolymerization reaction also plays important role in mechanical properties of this new class of inorganic polymers.     

Effects of H2O/Na2O molar ratio on the strength of alkaline activated ground blast furnace slag-ultrafine palm oil fuel ash based concrete

Effects of H₂O/Na₂O molar ratios (MRs) on the developed alkaline activated pozzolanic solid wastes (PMs)-ultrafine palm oil fuel ash (UPOFA) and ground blast furnace slag (GBFS)-were studied by using the constant mass of combined activators (10 M NaOH_aq + Na₂SiO_3aq of silica-modulus (Ms = SiO₂/Na₂O) of 3.3).The free water content (FWC) expressed as FWC/(PMs) varied from 0.02 to 0.1 by mass while the total H₂O/Na₂O MRs ranged from 18.9 to 23.1 The findings revealed that increase in H₂O/Na₂O MRs negatively affects the strength but positively impact the mixture workability (consistency). The microstructural morphology examination using Scanning Electron Microscope coupled with Energy dispersive spectroscopy (SEM + EDS) reveals the contribution of H₂O/Na₂O MRs to the product nature, compactness, and the reactivity of Ca²⁺ and Al³⁺ while Fourier transform infra-red (FTIR) spectroscopy indicates that H₂O/Na₂O ratios contributed to the product amorphousity and carbonation process but sparingly affected its formed polymerized structural units (SiQⁿ(mAl), n = 2 and 3).     

The effect of activating solution on the mechanical strength, reaction rate, mineralogy, and microstructure of alkali-activated fly ash

Alkali-activated fly ash (AAF) is a promising material that exhibits comparable material properties as cement-based materials but with much less CO₂ emission. In the present work, the effect of activating solution (SiO₂ and Na₂O content) on the performance of AAF was studied by means of isothermal calorimetry and X-ray diffraction analysis. Meanwhile, the pore structure of AAF was examined by mercury intrusion porosimetry combined with environmental scanning electron microscope. The results indicate that increasing the sodium oxide content leads to a higher extent of reaction, denser matrix and higher possibility of crystallization, corresponding to a higher compressive strength of AAF. The addition of silica in the alkaline solution retards the reaction rate and zeolite formation, while improves the microstructure of the matrix. Therefore, there is an optimal value for SiO₂ with respect to the Na₂O content for the AAF in this study.     

Composition design and performance of alkali-activated cements

In the present work, the relationship between the composition of the SiO₂–Al₂O₃–CaO precursor system and the setting time, microstructure and mechanical properties of the resulting alkali-activated cement (AAC) were investigated. The results showed that with the increase of metakaolin content and the modulus of activator solution, setting time of alkali-activated cements was prolonged. The compressive strength increased with the increase of CaO content to a certain extent, but had different trends as Si/Al ratio varied. Microstructural analyses revealed that CaO content had remarkable effects on the microstructure of AAC. In calcium-free system, the strength was dependent on the three-dimensional structure of N–A–S–H gels. As the CaO content increased gradually, the main activation product changed from N–A–S–H to C–(A)–S–H gel, resulting in a more compact structure. This investigation helps to build up a practical approach for the composition design of alkali-activated cements.     

Sisal fiber-reinforced cement composite with Portland cement substitution by a combination of metakaolin and nanoclay

This paper reports the partial replacement of Portland cement (PC) by combination of metakaolin (MK) and nanoclay (NC) in sisal fiber-reinforced cement composites by studying the microstructure, mechanical behavior, and the interfacial properties between fiber and cement matrices. The mechanical properties of cement matrix and natural fiber-reinforced composites are studied using compressive strength development and flexural behavior, respectively. The tensile behavior of the natural fiber was also investigated and analyzed by Weibull distribution model. The characteristics of hydration products were analyzed by scanning electron microscope, X-ray diffraction, and thermogravimetry analysis. Our results show that the combination of MK and NC can improve the hydration of cement more effectively, with better microstructure and enhanced mechanical properties, than mixes without them. The calcium hydroxide (CH) contents of matrixes with 50 wt% combined substitutions, containing 1, 3, and 5 wt% of nanoclay, were 58.12, 60.16, and 64.25 % less than that of PC, respectively. The ettringite phase is also effectively removed due to the substitution of MK and NC, which improve both Al/Ca and Si/Ca ratios of calcium silicate hydrates (C–S–H) due to the high content of SiO₂ and Al₂O₃. The interfacial bond between fiber and cement matrix and flexural properties of sisal fiber-reinforced cement composites are also significantly improved. The optimum interface adhesion between sisal fiber and matrix was achieved by replacing cement by 27 % MK and 3 % NC, which increased the bond strength and pull-out energy by 131.46 and 196.35 %, respectively.     

Microstructure and mechanical properties of fluorcanasite glass-ceramics for biomedical applications

Four fluorcanasite glass-ceramics were fabricated by controlled heat-treatment of as-cast Glasses A–D. These compositions have been reported previously but essentially, Glass A had the stoichiometric composition (Ca₅Na₄K₂Si₁₂O₃₀F₄) and Glasses B–D were modified by reducing the Na₂O concentration (B), adding excess CaO (C) and P₂O₅ (D). The latter two compositions have been shown to have promising bioactive response in cell culture and simulated body fluid experiments. Devitrification of the stoichiometric composition resulted in poor mechanical properties with crumbling often observed on machining. As a result, no mechanical data could be obtained. In all modified compositions, heat-treatment between 780 °C and 900 °C resulted in measurable indentation fracture toughness (IFT) and biaxial flexural strength (BFS). IFT was optimised in Glass C at 800 °C (2.53 ± 0.02 MPa m^½), but the biaxial flexural strength (BFS) was low, 167 ± 17 MPa, compared to other compositions. For heat- treated Glass D optimum mechanical properties were obtained at 800 °C with BFS and IFT, 249 ± 23 MPa and 1.95 ± 0.01 MPa m^½, respectively. The relationship between the mechanical properties and microstructure is discussed.