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• The use of SMS as a hydrophobic modifier for geopolymers is an economical method. • The modification method is simple and suitable for large-scale industrial applications. • High modification efficiency is at a small dosage: from 0° to 134.5° at 1.5 wt%. • The effects of SMS on geopolymers were analyzed using FT-IR, XRD, and SEM. • SMS improved the densification and erosion resistance of geopolymers. In this paper, the geopolymers were modified by simply blending with sodium methyl-silicate (SMS). Furthermore, the effects of SMS on the structure, micromorphology, physical properties and erosion resistance of geopolymers were investigated. The results showed that the addition of SMS can greatly increase the hydrophobicity of the geopolymer. The contact angle reached a maximum of 134.5°with 1.5 wt% addition. Moreover, due to the optimization of the atomic arrangement of the geopolymer caused by the addition of SMS, the property change of the geopolymer was presented as follows: (1) the denseness was enhanced; (2) the hardness, friction resistance and erosion resistance were improved. [ABSTRACT FROM AUTHOR]

Geopolymer with Si/Al ratios from 2 to 4 were prepared by adding different contents of fused silica into geopolymer matrix. Effects of Si/Al ratios on the structure, mechanical properties and chemical stability in air of the obtained geopolymer were systematically investigated. The results showed that all the geopolymer samples were XRD amorphous. Geopolymer with Si/Al ratios of 2 and 2.5 showed similar structure and property and they were classed as KGP-I; and geopolymer with Si/Al ratios of 3, 3.5 and 4 were similar and they were class as KGP-II. In alkaline solution, reactivity of fused silica were higher than that of metakaolin, resulting in higher content of both residual metakaolin and free alkaline cation in KGP-II than in KGP-I. Fused silica partially reacted with the alkaline solution in KGP-II indicating chemical interfacial bonding between silica and binder phase. With the increase in Si/Al ratios, KGP-II especially for geopolymer with Si/Al of 4 showed much higher mechanical properties than KGP-I due to the increased Si-O-Si bonds and residual silica as reinforcement. However, KGP-II showed worse chemical stability in air than KGP-I, with the presence of efflorescence on the surface, which was attributed to their higher residual free K + . [ABSTRACT FROM AUTHOR]

In this paper, synthetic metakaolin with fixed composition (Al2O3·2SiO2) was produced by a simple chemosynthetic route. The chemosynthetic metakaolin can eliminate the influence of impurities in metakaolin from natural kaolin minerals. The synthetic metakaolin together with NaOH and SiO2-sol were used to prepare Na-based geopolymer precursors with various molar ratios of Si/Al. The molar ratios of Si/Al from 1 to 2 were tailored by adding different contents of SiO2-sol. Zeolite/geopolymer composites or monolith-type zeolite were successfully fabricated from synthetic metakaolin-based geopolymer through a hydrothermal process. The effects of Si/Al ratios on the phase composition and microstructure of the produced zeolite/geopolymer composites or zeolites were studied. The results proved that the composition of synthetic metakaolin and geopolymer precursors can be facilely tuned, and the monolithic geopolymer precursors can be mostly, or even totally, transformed into zeolite after hydrothermal treatment. [ABSTRACT FROM AUTHOR]

Abstract Based on conventional ion exchange method from zeolite materials, we herein report a facile synthetic procedure to prepare monoclinic celsian ceramics through thermal treatment of ion-exchanged 3D printing geopolymer precursors. In this contribution, both ion exchange process and thermal evolution of celsian precursors were systematically investigated. The results proved 3D printing Na- and K- based geopolymer were ideal precursors with low residual Na+ content (0.10 meq/g) and K+ content (0.05 meq/g) and translated into monoclinic celsian after being heated at 1400 °C. With increasing Sr2+ doping concentration from 20% (mole ratio) to 30% (mole ratio), more significant phase transition results (hexagonal → monoclinic) were observed. All 3D printing geopolymer precursors kept fine integrity and stable 3D structure upon ion exchange process and heating, indicating the combination of geopolymer technique and 3D printing opens up a versatile and robust way to yield monoclinic celsian ceramics and related components of complex shapes. [ABSTRACT FROM AUTHOR]

Abstract: In this paper crystallization kinetics of potassium-based geopolymer (Si/Al=2.5, denoted as KG2.5) upon heating are investigated by differential thermal analysis (DTA) under non-isothermal conditions at various heating rates. From 35 to 700°C KG2.5 shows a significant weight loss due to both evaporation of free water and condensation of hydroxyl groups. Crystallization of KG2.5 starts at a low temperature of ∼960°C according to XRD and DTA analysis. KG2.5 also exhibits a low onset temperature of viscous sintering stage and on heating to 950°C the sintering is completed. In the DTA graphs, the exothermic peaks which are caused by leucite crystallization shift to higher temperatures with increasing heating rate. The activation energy value of crystallization for leucite is found to be 455.9kJ/mol and the corresponding Avarami constant is 3.89 indicating the three-dimensional crystal growth mechanism. [ABSTRACT FROM AUTHOR]

Abstract: The thermal evolution of cesium-substituted potassium-based geopolymer ((K1−x Cs x )2O·Al2O3·5SiO2·11H2O, x =0, 0.1, 0.2, 0.3, and 0.4) on heating is studied by a variety of techniques. Phase compositions and thermal expansion behaviors of the ceramics derived from the geopolymer are characterized. All of the geopolymer specimens with or without cesium substitution exhibit similar thermal evolution trends. Major weight losses before 600°C from all the geopolymer specimens are observed and are resulted from evaporation of free water and hydroxyl groups. Thermal shrinkage of these specimens can be divided into four stages, i.e. structural resilience, dehydration, dehydroxylation and sintering, according to the dilatometer results. For these specimens, significant difference is observed in the viscous sintering stage and the effect of cesium is to reduce the thermal shrinkage in this stage due to the increased matrix viscosity. In the ceramics derived from geopolymer, the amount of stabilized leucite increases with the amount of cesium and with 20% cesium substitution leucite is fully stabilized in cubic phase. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Ion-doped Cs-based geopolymer and pollucite were developed for Cs immobilization. • Li or Na doping decreased the crystallization temperature of pollucite and avoided Cs volatilization. • Optimized doping ratios and high-temperature treatments significantly enhanced the immobilization effect of Cs. • SEM/EDS analysis unveiled different leaching mechanisms of Cs from geopolymer and pollucite. Abstract Safe trapping of radioactive nuclear waste such as 137Cs has become a grim issue with the popularity of nuclear power and the growing yield of nuclear waste. To address this intractable problem, pollucite has been screened out as an ideal material for the long-term storage of 137Cs due to its low leaching rate and good chemical and thermal stability. With an effort to further enhance its immobilization capability and practical values, we herein report a robust route to prepare pollucite from ion-doped geopolymer at low temperature (≤1000 °C), achieving high Cs content (33.37 wt.%) and the lowest leaching rate of Cs (2.51 × 10−4 g m-2 d-1) to date. Meanwhile, the leaching mechanism of Cs in pollucite is revealed via leaching tests coupled with SEM-EDS analysis. Therefore, this contribution may provide an alternative route to preparing pollucite and open up new possibilities in the immobilization of 137Cs for real-world applications. [ABSTRACT FROM AUTHOR]

In this work, we report the fabrication of geopolymer materials containing different amounts of reduced graphene oxide loaded on zinc oxide (rGO@ZnO/GP) to be used as efficient photocatalysts for the degradation of methylene blue (MB). The synergistic effects of adsorption and photodegradation resulted in a 92.56% removal efficiency of MB within 30 min by a geopolymer containing 56%rGO@ZnO (56%rGO@ZnO/GP). The results showed that the geopolymer matrix provided a reusable carrier for rGO@ZnO, and the presence of rGO could promote the migration of photogenerated electrons and hinder the recombination of charges, thereby enhancing the photocatalytic activity. The best performing material (56%rGO@ZnO/GP) was also used to 3D print a scaffold using Direct Ink Writing technology. The 3D printed 56%rGO@ZnO/GP was also an efficient photocatalyst for the degradation of MB, and the use of a structured catalyst facilitates the recovery of the active material. This work can provide a way for preparing efficient and environmentally friendly photocatalysts. [Display omitted] • Novel rGO@ZnO/GP photocatalysts were designed. • The photocatalytic activity of rGO@ZnO/GP was tested and the photocatalytic mechanism was investigated. • rGO@ZnO/GP was able to decompose MB through the synergistic effect of adsorption and photocatalysis. • The 3D-printed rGO@ZnO/GP could be recycling and reuse. [ABSTRACT FROM AUTHOR]

Abstract: Unidirectional carbon fiber reinforced geopolymer composite (Cuf/geopolymer) is prepared by a simple ultrasonic-assisted slurry infiltration method, and then heat treated at elevated temperatures. Effects of high-temperature heat treatment on the microstructure and mechanical properties of the composites are studied. Mechanical properties and fracture behavior are correlated with their microstructure evolution including fiber/matrix interface change. When the composites are heat treated in a temperature range from 1100 to 1300°C, it is found that mechanical properties can be greatly improved. For the composite heat treated at 1100°C, flexural strength, work of fracture and Young''s modulus reach their highest values increasing by 76%, 15% and 75%, respectively, relative to their original state before heat treatment. The property improvement can be attributed to the densified and crystallized matrix, and the enhanced fiber/matrix interface bonding based on the fine-integrity of carbon fibers. In contrast, for composite heat treated at 1400°C, the mechanical properties lower substantially and it tends to fracture in a very brittle manner owing to the seriously degraded carbon fibers together with matrix melting and crystal phases dissolve. [Copyright &y& Elsevier]

Abstract: The crack initiation and propagation of short carbon fiber reinforced geopolymer matrix composites (Cf/geopolymer composites) during bending test were observed in situ by environmental scanning electron microscope (ESEM). Lots of micro cracks initiate, and then propagate on the side of the beam sample with the increase of the bending load. A nearly elastic response of load–displacement curve and significant deformation of the composites are observed at the initial stages. The propagation of the micro cracks ceases, and these cracks tend to close to some extent while the main crack forms. The fiber bridging effect in the micro and main cracks effectively keeps the composites integrity and makes the composites exhibit a non-catastrophic fracture behavior. A simple mode for the damage behavior of the composites during the bending test is discussed. [Copyright &y& Elsevier]

Abstract: A kind of sheet-like carbon fiber preform was developed using short fibers (2, 7 and 12mm, respectively) as starting materials and used to strengthen a geopolymer. Mechanical properties, fracture behavior, microstructure and toughening mechanisms of the as-prepared composites were investigated by three-point bending test, optical microscope and scanning electron microscopy. The results show that the short carbon fibers disperse uniformly in geopolymer matrix. The Cf/geopolymer composites exhibit apparently improved mechanical properties and an obvious noncatastrophic failure behavior. The composite reinforced by the carbon fibers of 7mm in length shows a maximum flexural strength as well as the highest work of facture, which are nearly 5 times and more than 2 orders higher than that of the geopolymer matrix, respectively. The predominant strengthening and toughening mechanisms are attributed to the apparent fiber bridging and pulling-out effect based on the weak fiber/matrix interface as well as the sheet-like carbon fiber preform. [Copyright &y& Elsevier]