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As part of a series of studies, effects of Na+ substitution on the thermal evolution of cesium-based geopolymers on heating were studied. A series of sodium-substituted cesium-based geopolymers, Cs(1− x)Na xGPs (where x=0, 0.1, 0.2, 0.3, and 0.4), were prepared and treated at 1300°C for 2 hours to obtain the corresponding ceramic products. The thermal evolution process was disclosed by virtue of a variety of technical, including TG- DTA, thermal shrinkage, XRD analysis, SEM, and TEM investigation. The results indicated that unheated Cs(1− x)Na xGPs was not completely amorphous after the substitution of Na+ and the crystallinity of Cs(1− x)Na xGPs gradually increased with the rise of sodium content. Meanwhile, the average particle sizes of Cs(1− x)Na xGPs also increased evidently with increases in sodium substitution. The final product after heat treatment mainly consisted of pollucite (CsAlSi2O6) and amorphous glass phase. The particle size of pollucite grain gradually decreased as more Cs+ were replaced maybe owing to the role of Na+ in the nucleation process of pollucite. Two forms of Na+ present in the final products: A small portion was present in the pollucite grains due to Na+ partial occupied the crystallographic sites of Cs+; and the rest were present in the amorphous glass phase among the pollucite grains. The average coefficient of thermal expansion (CTE) of resulting Cs(1− x)Na xGPs ceramics increased from 4.80×10-6 K−1 ( x=0) to 7.26×10−6 K−1 ( x=0.4) with increases in sodium substitution, which could be due to the amorphous glass phase had a relatively higher CTE than that of pollucite. [ABSTRACT FROM AUTHOR]

The purpose of this work was to study the role of lithium in cesium-based geopolymers and the thermal evolution during heat treatment together with thermal expansion behavior of the resulting geopolymer ceramic. A series of lithium-substituted cesium-based geopolymers, Cs(1 - x)LixGP (where x = 0, 0.1, 0.2, and 0.3), were prepared. All the geopolymer samples were heated at 1300°C for 2 h and thermal evolution on heating was studied by a variety of techniques. Phase composition, microstructure evolution, and thermal expansion behaviors of the ceramics derived from the geopolymers were characterized. All the geopolymer specimens exhibited similar thermal evolutionary trends. With increases in lithium content, overall mass loss increased gradually due to the higher hydration energy of Li+ than Cs+. Thermal shrinkage of these specimens can be divided into four stages, i.e., structural resilience, dehydration, dehydroxylation, and sintering, according to the dilatometer results. The introduction of Li results in two-step sintering behavior for the lithium-substituted cesium-based geopolymers. The average thermal expansion coefficient (CTE) of Cs(1 - x)LixGP ceramics decreased from 4.80 × 10-6 K-1 (x = 0) to 3.61 3 10-6 K-1 (x = 0.3) with increase in lithium substitution. The reason can be attributed to the presence of spodumene after thermal treatment, which has a relatively low thermal expansion coefficient compared with pollucite. Meanwhile, molten spodumene could serve as a buffer phase between pollucite crystals also conducive to the decline of CTE of this system. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, leucite ceramics with different shapes and fine integrity were directly developed from geopolymer precursors by the optimized heat treatment procedures. Phase compositions and microstructure, coupled with the mechanical properties, were systematically investigated. The effect of heating rates on the integrity of ceramics was also discussed. After high-temperature treatment at 800°C for 120min, the geopolymer was converted into a leucite ceramic. The relative density of the sample treated at 1000°C for 120min was 93% and the size of leucite grain was approximately 1–4μm. TEM results showed that the leucite grain was composed of lamellar and needle-shaped twins, which resulted from the displacive phase transformation as it went from cubic to tetragonal symmetry when it was cooled down. Samples with plate and bullet shapes maintained their fine integrity by the optimization of the heating procedures. Meanwhile, the flexural strength, Young''s modulus, fracture toughness and Vickers hardness of the leucite ceramic that resulted was 70.0MPa, 65.0GPa, 1.3MPam1/2 and 7.39GPa, respectively. [Copyright &y& Elsevier]

In this paper, a high temperature treatment procedure was designed to evaluate the effect of holding temperature on thermal evolution process of Li + substituted Cs-based geopolymer (Cs 0.7 Li 0.3 GP), including the thermal analysis, phase composition and microstructure evolution. With rising of holding temperature, amorphous unheated Cs 0.7 Li 0.3 GP gradually transformed into a multiphase system during the high temperature treatment process, which consisted of pollucite (CsAlSi 2 O 6 ), spodumene (LiAlSi 2 O 6 ) and amorphous glass phase. In the multiphase system, Cs + ions were in the form of pollucite grains, while Li + ions were in the form of spodumene nanocrystallines distributed in amorphous matrix. The pollucite grains gradually coarsened with rise in holding temperature, and the densification of the resulting products were also improved synchronously, which were related to the presence of amorphous glass phases. The amorphous glass phase would be in a molten state when holding temperature over 800 °C. And the presence of molten amorphous phase would make the mass transfer process easier, which could contribute to the growth of the crystal grains and the elimination of the pores. [ABSTRACT FROM AUTHOR]

A serial of multi-walled carbon nanotubes (MWCNTs) reinforced geopolymer composites were prepared, and then heated at elevated temperature to fabricate MWCNTs/leucite composites by in situ transformation. Effects of high-temperature treatment on the microstructure evolution and mechanical performance of the composites were investigated. The results indicated that the introduction of MWCNTs could improve the mechanical properties of geopolymer, and the optimum content was 3 wt%. The mechanical performance declined instead with the further increase in MWCNTs content up to 5 wt%, which could be attributed to the agglomeration of MWCNTs. Significant improvements in mechanical properties were achieved after the composites were treated in a temperature range from 950 °C to 1200 °C relative to their original state before heat treatment. The significant improvements could be described to the matrix densification, and leucite formation as well as the proper interface bonding state between carbon nanotube and leucite matrix. [ABSTRACT FROM AUTHOR]

Hybrid C f and SiC f reinforced geopolymer composites with Al 2 O 3 particles were successfully fabricated, and the effects of high-temperature treatment on the microstructure evolution and mechanical properties of C f -SiC f -(Al 2 O 3p )/KGP composites were investigated. Results showed that the amorphous matrix of C f -SiC f -(Al 2 O 3p )/KGP composites transformed into kalsilite and leucite phase after treated at 1000 and 1200 °C, respectively, due to the addition of fibers hindering crystallization of the geopolymer matrix. The addition of Al 2 O 3p was helpful to increase the thermal stability of composites after high-temperature treatment. Flexural strength and Young's modulus of the composites decreased after high temperature treatment and showed the maximum values at 1000 °C. All the composites showed a non-catastrophic fracture behavior after heat treated, which was attributed to fiber pulling-out and debonding. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this study, the effect of curing temperature and SiO 2 /K 2 O 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 SiO 2 /K 2 O 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 SiO 2 /K 2 O 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 106.2 MPa and elastic modulus of 8.3 GPa, respectively. With increasing SiO 2 /K 2 O 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. [ABSTRACT FROM AUTHOR]

Reduced graphene oxide/leucite nanocomposites (rGO/leucite) were prepared from reduced graphene oxide/geopolymer composites (rGO/geopolymer) followed by post high-temperature treatment process. The crystallization kinetics and effects of isothermal soaking times on microstructure evolution and mechanical properties of the final rGO/leucite composites were investigated systematically. The results show that the activation energy value for rGO/leucite crystallization was slightly lower than that of pure geopolymer which controlled by a three-dimensional growth mechanism. The scrolled and fold degree of rGO sheets in the rGO/leucite composites increased with the isothermal soaking times. The flexural strength and fracture toughness of the rGO/leucite after treated at 950 °C for 0.5 h reached their maximum values 75.4 ± 5.6 MPa and 1.40 ± 0.03 MPa m 1/2 , decreased with the isothermal soaking times, respectively. [ABSTRACT FROM AUTHOR]

Reduced graphene oxide/cubic-leucite (rGO/cubic-leucite) composites were prepared from reduced graphene oxide/geopolymer (rGO/CsKGP) composites by simultaneous in situ crystallization process and thermal reduction of GO. The effects of both rGO addition and treatment temperature on the microstructure and mechanical properties of the rGO/CsKGP composites were investigated systematically. The results showed that rGO sheets in the rGO/CsKGP composites could be further in-situ reduced through high temperature treatment. The rGO/CsKGP composites fully transform to rGO/cubic-leucite composites after treating above 1000 °C for 30 min. Compared with the pure cubic-leucite matrix, the flexural strength and fracture toughness of rGO/cubic-leucite composites were 65.0 MPa and 0.64 MPa m 1/2 , increased by ~ 37% and ~ 14% respectively. These improvements are attributed to a combination of rGO reinforcement and grain refinement. [ABSTRACT FROM AUTHOR]

Carbon fiber felt reinforced geopolymer composites were easily and simply prepared by infiltrating geopolymer slurry into carbon fiber felt. The effects of fiber contents on the microstructure, mechanical properties and toughening mechanisms of the final composites were investigated systematically. Results showed that the mechanical properties of the composites improved remarkably due to the addition of carbon fiber felt. The flexural strength, work of fracture and fracture toughness of the composites reached the maximum values of 51.5±0.6 MPa, 1817.8±133.9 J m −2 and 1.27±0.12 MPa m 1/2 with increasing fiber contents of 5 vol%, respectively. The composites showed a typical pseudoplasticity behavior of fracture. The improvements of the work of fracture and fracture toughness for composites are attributed to the cracks deflection and carbon fiber pulling-outs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Geopolymer nanocomposites with different rGO contents were synthesized by in situ reduction of graphene oxide under alkaline solutions. The effects of rGO contents on the microstructure and the mechanical properties of the rGO/geopolymer composites were characterized systematically. Results showed that GO could be partially reduced in alkaline geopolymeric solutions using relatively short synthesis periods of 15 min at loadings of 0–0.5 wt% at room temperature. The resulting rGO was evenly dispersed in the final amorphous composite structures. Measured fracture toughness values increased 61.5% with increasing rGO contents to 0.5 wt%, reaching 0.21 MPa m 1/2 . The flexural strength of the rGO/GP reached a maximum value of 17.9 MPa with increasing rGO contents to 0.3 wt%. Strong interface bonding, crack deflection and propagation, rGO pull-out, wrapping and anchoring around geopolymer particles contributed to the improved fracture toughness and flexural strength. [ABSTRACT FROM AUTHOR]

Homogeneously dispersed graphene reinforced geopolymer composites were easily and simply prepared through in-situ reduction of graphene oxide (GO) in alkaline geopolymeric solutions. The in-situ GO reduction degree, microstructure and mechanical properties of reduced graphene oxide (rGO)/geopolymer composites were investigated systematically. The results revealed that the rGO C/O ratio increased dramatically after in situ reaction at 60 °C for 72 h in geopolymeric solutions. The reduction is mainly attributed to conversion of C=O to C–O bonds and then elimination of residual oxygen-function groups under the alkaline conditions. The as-obtained rGO was uniformly dispersed and exhibited good long-term stability in the geopolymeric solutions. The final amorphous geopolymer matrix structure was unaffected by the homogeneously dispersed rGO yet strong interfacial bonding was observed. As a result, the fracture toughness of the composite with 1 wt% rGO was improved by ~17% compared with the pristine geopolymer. [ABSTRACT FROM AUTHOR]