Setting and nanostructural evolution of metakaolin geopolymer

The nanostructural evolution during formation of geopolymers and its correlation with setting have not been well understood. In this study, penetration resistance and ultrasonic wave reflection tests were conducted to measure setting, and solid‐state ²⁷Al NMR and liquid‐state ²⁹Si NMR were used to examine nanostructural changes in a metakaolin geopolymer as a function of time. Aluminum was released rapidly during the first 10 hour after mixing and immediately condensed with silicate species in solution to form larger sized aluminosilicate oligomers, which then condensed to form large structural units. Our evidence suggests these units form near metakaolin particle surface. Smaller sized silicate ions in the sol phase then attach to these units to form a gel with a more interconnected network structure. The initial stage of this attaching process was seen to be associated with set, which in this mixture occurred at 15 hour.     

Effects of treatment temperature on the reduction of GO under alkaline solution during the preparation of graphene/geopolymer composites

Homogeneously dispersed reduced-graphene-oxide (rGO) reinforced geopolymer composites were successfully prepared through in-situ reduction of graphene oxide (GO) under alkaline geopolymeric condition. The effects of treatment temperatures on the reduction of GO under the alkaline solution during the rGO/geopolymer preparation process were characterized systematically. The results showed that GO could be in situ reduced under alkaline geopolymer solution at various temperatures (25–80 °C) for 3 h. The reduction degree of rGO was improved with increasing the reaction temperature. The rGO was well dispersed, and the rGO/geopolymer composites showed amorphous structure.     

Reaction kinetics and mechanical properties of a mineral-micropowder/metakaolin-based geopolymer

In this study, metakaolin was partially replaced with mineral micropowder to prepare a mineral-micropowder/metakaolin-based geopolymer was prepared under alkali activation, and the compressive and flexural strengths of various geopolymer specimens were determined. Geopolymer reaction kinetics were examined using the Johnson-Mehl-Avrami-Kolmogrov model, and the effects of the mineral-micropowder content on the properties and structure of the metakaolin-based geopolymer were investigated. Results revealed that micropowder addition significantly influenced the mechanical properties, microstructure, and reaction heat of the geopolymer. At a powder content of 30 wt%, the polymer exhibited superior mechanical properties; furthermore, the compressive and flexural strengths of the specimens cured for 28 d were 58.3 MPa and 12.6 MPa, which were 24.1% and 40% higher than those of the control group, respectively. Meanwhile, the geopolymer setting time was significantly reduced because the presence of calcium in mineral micropowder promoted the geopolymerisation reaction. Therefore, the formation of a multi-gel phase considerably enhanced the geopolymer structure.     

Mechanical properties and compositional heterogeneities of fresh geopolymer pastes

This work explains the elastic properties of the reactive suspensions of metakaolin in sodium silicate solutions immediately after mixing. The flow properties of the interstitial fluid were obtained by mimicking it with synthetic aluminosilicate gels of different Si/Al molar ratios. By comparing these results with the rheological properties of fresh geopolymer pastes and with the ones of unreactive suspensions, we showed that the early age mechanical properties of geopolymer mixes cannot be explained by the colloidal interactions between metakaolin grains but rather by the formation of a gel with a molar ratio Si/Al < 4.5. In addition, we measured the evolution of the total concentration of tetrahedral Al in the interstitial liquid by static NMR. It was thus evidenced that the afore-mentioned aluminosilicate gel is formed at a very early stage of the geopolymerisation reaction resulting in a heterogeneous suspension with an Al-rich gel formed at the grain boundaries.     

Interface evolution of the Cf/leucite composites derived from Cf/geopolymer composites

In this paper, effect of heat treatment temperature on the interface structure of the carbon fiber reinforced geopolymer composites was investigated by transmission electron microscopy (TEM), selected-area diffraction (SAD) analysis and high-resolution transmission electron microscopy (HRTEM). In the composite treated at 1100 °C, carbon fiber showed a good bond with the leucite matrix and no interface reaction layer was observed, while due to the thermal mismatch between fiber and matrix, microcrack which was perpendicular to the fiber axial direction can be seen in the matrix. With increase in heat treatment temperature to 1200, 1300 and 1400 °C, interface reaction occurred and reaction layers with thickness of 50, 100 and 1000 nm, respectively, were detected. The interface layer was formed by the reaction between Si–O groups in the matrix and C element in the fiber. Especially for the sample treated at 1400 °C, serious interface reaction led to the formation of β-SiC and property of carbon fiber was greatly degraded.     

Geopolymer with Hierarchically Meso‐/Macroporous Structures from Reactive Emulsion Templating

We present a simple synthetic route to hierarchically porous geopolymers using triglyceride oil for a reactive emulsion template. In the new synthetic method, highly alkaline geopolymer resin was first mixed with canola oil to form a homogeneous viscous emulsion which was then cured at 60°C to give a hard monolithic material. During the process, the oil in the alkaline emulsion undergoes a saponification reaction to be decomposed to water‐soluble soap and glycerol molecules which were then extracted with hot water to finally yield porous geopolymers. Nitrogen adsorption studies indicated the presence of mesopores, whereas the SEM studies revealed that the mesoporous geopolymer matrix are dotted with spherical macropores (10–50 μm) which are due to oil droplet template in the emulsion. Various synthetic parameters including the precursor compositions were examined to control the porosity. BET surface area and BJH pore volume of the materials were up to 124 m²/g and 0.7 cm³/g, respectively, and the total pore volumes up to 2.1 cm³/g from pycnometry.     

Differentiation of the solid-state NMR signals of gel,zeolite phases and water species in geopolymer-zeolite composites

Geopolymer-zeolite composites were synthesized using a silica-rich industrial byproduct from chlorosilane production and sodium aluminate. Pastes were cured at 80 °C and 80% RH, and subsequently dried in two different climates: at 23 °C and 50% RH, and under vacuum. ¹H MAS, ²³Na MAS and ²⁹Si MAS NMR and XRD measurements were performed after the drying procedures as well as after subsequent aging. Zeolite Na-A was found beside traces of faujasite-type zeolite and zeolite EMT as major crystalline phases in the cured composites; the fraction of geopolymeric gel in the reaction products was determined to be ~18% on a molar basis. Various water species could be distinguished using ¹H MAS and ¹H-²⁹Si CP MAS NMR, applying rotor-synchronized echo experiments. The largest fraction of the pore water resides in the α-cages of the zeolite Na-A and in the geopolymeric gel; in addition, water exists in the β-cages of the zeolites and adsorbed at sodium ions. The water species in α-cages and in the pores of the geopolymeric gel exhibit slightly different chemical shifts of 4.7 ppm and 4.9 ppm, respectively, in the ¹H MAS NMR spectra. Changes of the water content in the geopolymer pores of differently dried samples were observed and led to slightly varied chemical shifts in the ²⁹Si MAS NMR spectra too. Measurements after more than 500 days revealed no significant aging effects of the composites, which confirm their chemical stability.     

Evolution of the microstructure of unconsolidated geopolymers by thermoporometry

This paper studies the evolution of the pore size distribution of a fresh unconsolidated geopolymer paste between one day and a week, using thermoporometry. This was made possible by following a careful protocol for sample preparation and for analysis by differential scanning calorimetry. In contrast with nitrogen gas adsorption, this method quantifies directly the amount of water in pores. It also does not require heat and vacuum drying, thus maintaining the fragile pore structure of the unconsolidated paste. Moreover, it was found that, in a typical metakaolin-based sodium geopolymer with a 10 to 20 hours workability period, the porosity gradually refines during the first week while the mesoporous volume is cut in half. This is probably due to the fact that the geopolymer network was still actively condensing from the activation solution. Part of the pore water never froze and, from mass balance, this residual water was attributed to the water bound in the hydration shell of the sodium counter ions. Only a minor occurrence of covalently bound protons as silanol groups was observed. The results presented here usefully complement data obtained by conventional techniques at later ages on consolidated geopolymers. It supports the growing body of literature on the structural evolution of geopolymers with time.     

Immobilization of strontium-loaded zeolite A by metakaolin based-geopolymer

Zeolites are the preferred inorganic ion exchange materials for purifying radioactive waste liquid. Radionuclide-loaded zeolites, which are considered to be radioactive waste, are strictly required to be encapsulated within a solid matrix. In this paper, we investigate the feasibility of immobilizing exhausted zeolite A, loaded with ⁹⁰Sr radionuclide, in metakaolin based-geopolymer. The geopolymer solidification blocks had better mechanical performance and leaching resistance in deionized water, sulfuric acid, magnesium sulfuric and acetic acid buffer solutions than the cemented blocks. While the compressive strength of the geopolymer solidification product was 37.62 MPa after curing for 28 days, the equivalent value for the cement block was only 11.32 MPa. The geopolymer solidification blocks also exhibited even lower compressive strength loss after high-temperature and freeze-thaw cycles tests. XRD and EDS analysis indicated that most of the strontium radionuclide in the geopolymer solidification blocks was incorporated in the zeolite structure as the charge balancing cation. The microscopic analysis revealed that geopolymer matrix appeared more compact and dense, and encapsulated the Sr-loaded zeolite A more tightly than did the cement. Therefore, it could be concluded that metakaolin based-geopolymer are more compatible with exhausted zeolite A and present a remarkable advantage for radioactive waste immobilization.     

Bond strength between blended slag and Class F fly ash geopolymer concrete with steel reinforcement

In this paper, geopolymer concrete bond with both deformed and smooth reinforcing steel bars is investigated using the standard RILEM pull-out test. The geopolymer binder is composed of 85.2% of low calcium fly ash and 14.8% of ground granulated blast furnace slag (GGBFS). The tests were aimed to assess the development of the bond strength from 24 h to 28 days after casting, with different heat curing conditions. The results show that 48 h of heat curing at 80 °C is required in order to obtain similar or better performances to those of the reference 45 MPa OPC concrete. The 28-day bond strength and the overall bond stress–slip behaviour of the geopolymer concrete were similar to those previously reported for OPC-based concretes. Providing intensive heat curing, high early bond strength can be achieved showing that Class F fly ash geopolymer concrete is well suited for precast applications.     

Mechanical and chemical properties of metakaolin-based geopolymer exposed to elevated temperature

A method is presented to fabricate metakaolin-based geopolymers that are structurally and mechanically stable up to 600°C. The chemical environment of the geopolymers is characterized using thermogravimetric analysis and Fourier-transform infrared spectroscopy. Residual free water turned into steam and caused damage to the geopolymer when exposed to elevated temperatures. The curing temperature was increased from 80 to 120°C to remove water during the curing process. A correlation was drawn between the amount of Si-O-Al linkage formed and the position of fingerprint peaks in infrared spectra, providing a tool to evaluate the level of geopolymerization. Flexural and tensile properties of geopolymers fabricated using the optimized method were measured for no heat treatment and for exposure to elevated temperatures of 200, 400, and 600°C. The flexural strength was measured to be 10.80 ± 2.99 MPa at room temperature, 10.36 ± 0.64 MPa at 400°C, and 8.04 ± 1.60 MPa at 600°C. The flexural modulus is reported to be 13.09 ± 3.40 GPa at room temperature and 11.03 ± 0.53 GPa at 600°C. The flexural toughness decreased with increasing temperature. The tensile properties of the geopolymer were measured with direct tensile tests paired with an extensometer. The tensile strength decreased from 4.16 ± 2.08 MPa at room temperature to 3.13 ± 0.97 MPa at 400°C, and 2.75 ± 0.86 MPa at 600°C. The Young's modulus decreased from 45.38 ± 30.30 GPa at room temperature to 26.88 ± 6.65 GPa at 600°C. Both flexural and tensile tests have shown that the metakaolin-based geopolymers cured at 120°C is mechanically stable at temperatures up to 600°C.     

Hydroxyl Terminated Hydrophilic Silanes

Hydroxyakyl terminated silanes were prepared by reacting 3-aminopropyl alkoxysilanes with ethylene carbonate. Unlike most silanes, these hydrophilic silanes are partially hydrolyzed in water and the siloxane oligomers thus obtained remain soluble in the water. Further condensation leads to water soluble linear or branched polysiloxanes depending on the structure of the hydroxylalkyl terminated silane. The water solubility is not limited to a narrow pH range and does not require alcohols (or other additives). The preparation method of these silanes was found to proceed smoothly at RT with no need for a solvent or a catalyst. The reactions followed second order reaction kinetics irrespective of the number of alkoxy groups attached to the silicon atom. Some condensation was unavoidable even in the absence of any catalyst but the viscosity remained low (0.10–0.16 Pa·sec) and was primarily dependent on the density of branching and the extent of the hydrogen bonding. The structures of these silanes were confirmed by FTIR, ¹H NMR and ²⁹Si NMR.     

Development of fly ash and iron ore tailing based porous geopolymer for removal of Cu(II) from wastewater

This work aims to verify the feasibility of utilizing iron ore tailing (IOT) in porous geopolymer and intends to broaden the application of porous geopolymer in heavy metal removal aspect. Porous geopolymer was prepared using fly ash as resource material, which was partially replaced by IOT at level of 30%, by weight, with H₂O₂ as foaming agent and removal efficiency, adsorption affecting factors, adsorption isotherms and thermodynamics of Cu²⁺ by the developed porous geopolymer were investigated.The experimental results uncover that the porous amorphous geopolymer was successful synthesized with total porosity of 74.6%. The transformation of fly ash and IOT into foaming geopolymer leads to the formation of porous structure encouraging Cu²⁺ sorption. Batch sorption tests were carried out and geopolymer dosage, Cu²⁺ initial concentration, pH, contact time and temperature were the main concern. Both Langmuir and Freundlich models could explain the adsorption of Cu²⁺ on the porous geopolymer due to the high fitting coefficients. The uptake capacity reaches the highest value of 113.41 mg/g at 40 °C with pH value of 6.0. The thermodynamic parameters ΔHº, ΔSº and ΔGº suggests the spontaneous nature of Cu²⁺ adsorption on porous geopolymer and the endothermic behavior of sorption process.     

Fully reacted high strength geopolymer made with diatomite as a fumed silica alternative

Geopolymers are formed by mixing of aluminosilicate sources with alkaline meta-silicate solution at room temperature. In the current study, diatomite of Turkish origin was fully utilized as a fumed silica alternative for the preparation of geopolymer, having a typical formula of K₂O•Al₂O₃•4SiO₂•11H₂O. From XRD of this sample, a broad peak centered at 28° 2θ indicated the well-known formation of amorphous geopolymer, as well as a fully reacted microstructure of geopolymer as seen by scanning electron microscopy. Additionally, geopolymer having the same formula was made by using fumed silica, in order to compare with geopolymers prepared from diatomite. The Weibull modulus was calculated from four-point bending and compressive strength testing of both geopolymer composites. The use of diatomite as a fumed silica substitute in geopolymer production resulted in a very close flexure strength 9.2 (± 4.2 MPa) when compared to geopolymer made from fumed silica 10.2 (± 3.3 MPa). There was a significantly higher compressive strength 71 (± 13.9 MPa) and Weibull modulus (5.4), than comparable properties of geopolymer made from fumed silica, which had a compressive strength 54 (± 25.8 MPa) and Weibull modulus of 2.0. The discrepancy was attributed to some self-reinforcement of the geopolymer matrix due to unreacted diatomite.     

SiC fiber reinforced geopolymer composites,part 1: Short SiC fiber

In this paper, short SiC fiber (SiC_sf) reinforced geopolymer composites (SiC_sf/geopolymer) were prepared and effects of fiber contents and lengths on the microstructure and mechanical properties of the composites were investigated. In-situ crack growth was carried out to study the fracture behavior and toughening mechanism of the composites. The results showed that SiC_sf/geopolymer composite developed weak interfacial bonding state through mechanical interlocking rather than chemical interfacial reaction. The presence of SiC_sf not only enhanced both flexural strength and work of fracture, but also prevented the catastrophic failure as seen in neat geopolymer. When fiber content was 2.0 vol% with length of 5 mm, the composite obtained the highest flexural strength and work of fracture, which were 5.6 and 63 times as high as those of neat geopolymer, respectively. In-situ crack growth together with fractographs showed that toughening mechanisms of the composite included formation and propagation of microcracks, crack deflection, fiber debonding and significant pulling-out.     

Porous alkali-activated material from hypergolic coal gangue by microwave foaming for methylene blue removal

Overall, 100% hypergolic coal gangue (HCG)-based geopolymer foams were produced by a novel saponification-microwave foaming combined route. Microwave foaming with and without expired vegetable oil was first used to produce CG-based geopolymer foams. Macropores were mainly generated by microwave foaming, and mesopores were mainly obtained by the addition of expired soybean oil that underwent a saponification reaction. The effects of the oil content on the density, porosity, pore morphology, compression strength, and methylene blue adsorption properties were studied. High total porosity (85.9–89.0 vol%) and acceptable compression strength (0.46–1.1 MPa) HCG-based geopolymer foams were produced. Foams with 12.59 wt% oil exhibited the best adsorption properties, with an adsorption capacity up to 9.4 mg/g and high removal efficiency of about 95.3%. These solid-waste-based porous components are promising monolithic adsorbents for wastewater treatment.     

Structural and chemical properties of thermally treated geopolymer samples

This article presents the results of the compositional, structural and morphological study of geopolymers synthesized from metakaolin and an alkali activator. The study involved the investigation of the structural and chemical properties of the geopolymer, in addition to thermally treated geopolymers up to 600 and 900 °C. The precursor of the geopolymer, and the geopolymer samples before and after the thermal treatment, were investigated by Fourier transformation infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and SEM analysis. The corrected average value of the ratio of silicon and aluminum in the geopolymer samples (Si_GP:Al) is about 1.46, which suggests that the obtained geopolymer samples represent a mixture of roughly equal amounts of sialate and sialate-siloxo units. Annealing the geopolymer samples at 600 °C decreases the amount of Si-ONa bonds and induces the cross-linking of polymer changes. At the same time, other sodium containing alumino-silicate phases are created. The thermal treatment at 900 °C leads to a considerable reduction of oxygen and particularly sodium, followed by significant morphological changes i.e. formation of a complex porous structure. Additionally, a new semicrystaline phase appears. Both XRD and XPS results imply that this new phase may be nepheline and it is plausible that this phase begins to nucleate at temperatures below 900 °C.     

Pozzolanic activity of mechanochemically and thermally activated kaolins in cement

This paper reports on the mechanochemical activation by intensive grinding as an alternative way to produce pozzolanically reactive metakaolin. The physicochemical properties, pozzolanic reactivity and impact on cement hydration of mechanochemically and thermally activated kaolinitic clays were compared. Mechanochemical activation of kaolin led to an amorphous hydrous material with increased specific surface area and high water content. Unlike in thermally activated kaolins, the Al coordination environment was partially retained during mechanochemical treatment. At low clinker replacement levels of 10%, the use of mechanochemically activated kaolins accelerated the hydration of the blended cement. The pozzolanic reaction reduced portlandite contents strongly and from early on. The combined effect of enhanced cement hydration and pozzolanic reaction at early ages was reflected in a higher early age strength development (up to 7 days). The differences between mechanochemically and thermally activated kaolins largely disappeared at later ages.     

Celsian formation from barium-exchanged geopolymer precursor: Thermal evolution

In this paper, thermal evolution, including element & phase composition and microstructure, of Ba²⁺ exchanged K-based geopolymer precursor (BaGP) were systematically investigated during high-temperature treatment. The results proved that celsian precursor with lower residual alkaline cation content were obtained through amorphous geopolymer than traditional ion-exchanged celsian through crystallized zeolite. With the increase in temperature, weight loss of BaGP was due to evaporation of OH groups and decomposition of BaCO₃. Similar to K-based geopolymer, BaGP showed amorphous structure, and nanometer-sized celsian nucleuses first crystallized from the amorphous BaGP matrix after it was treated at 900 °C. In the treatment temperature range from 1000 to 1400 °C, hexagonal celsian became the main phase. After being treated at 1400 °C, hexagonal celsian grains were clearly noticeable with extra SiO₂ locating between celsian grains. It was therefore concluded that geopolymer precursor technique provides an alternative route for the preparation of celsian ceramics.     

One‐Part Geopolymers Based on Thermally Treated Red Mud/NaOH Blends

In this study, one‐part “just add water” geopolymer binders are synthesized through the alkali‐thermal activation of the red mud which is relatively rich in both alumina and calcium. Calcination of the red mud with sodium hydroxide pellets at 800°C leads to decomposition of the original silicate and aluminosilicate phases present in the red mud, which promotes the formation of new compounds with hydraulic character, including a partially ordered peralkaline aluminosilicate phase and the calcium‐rich phases C₃A and α‐C₂S. The hydration of the “one‐part geopolymer” leads to the formation of zeolites and a disordered binder gel as the main reaction products, and the consequent development of compressive strengths of up to 10 MPa after 7 d of curing. These results demonstrate that red mud is an effective precursor to produce one‐part geopolymer binders, via thermal and alkali‐activation processes.