In situ processing of MWCNTs/leucite composites through geopolymer precursor

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.     

Effects of Si/Al ratio on the structure and properties of metakaolin based geopolymer

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⁺.     

Thermal properties of Cf/HfC and Cf/HfC-SiC composites prepared by precursor infiltration and pyrolysis

Ultra-high temperature ceramic infiltrated carbon-fibre composites were prepared by precursor infiltration and pyrolysis (PIP) using a laboratory synthesized precursor. Microstructures and thermal properties including thermal expansion, thermal diffusivity, specific heat capacity and oxidative stability are correlated. XRD reveals the presence of C_f-HfC and C_f-HfC-SiC phases without formation of oxides. The CTE observed at 1200?°C is slightly higher for C_f-HfC (3.36?×?10^?6?K^?1) compared to C_f-HfC-SiC (2.95?×?10^?6?K^?1) composites. Lower thermal diffusivity of the C_f-HfC-SiC compared to C_f-HfC composites is attributed to a thermal barrier effect and cracks in the composites which formed due to the CTE mismatch between carbon fibre and the matrix as well as CO generated during graphitization. The thermal conductivity of C_f-HfC (4.18?±?0.14?Wm^?1?K^?1) is higher than that of C_f-HfC-SiC composite (3.33?±?0.42?Wm^?1?K^?1). Composites microstructures were coarse with some protruding particles (5?μm) with a homogeneous dense (～70%) matrix (HfC and HfC-SiC) for both composites.     

Microstructure and mechanical properties of a metakaolinite-based geopolymer nanocomposite reinforced with carbon nanotubes

The preparation and mechanical behavior of metakaolin-based geopolymer nanocomposite reinforced with multi-walled carbon nanotubes are presented in this study. In this work, Multiwall carbon nanotubes (MWCNTs) were added to the metakaolin-based geopolymer paste at 0, 0.5, or 1 wt% concentration. For each specimen, the mechanical properties were tested at the age of 7, 14 and 28 days. TEM and FESEM were employed to evaluate the dispersion quality of MWCNTs within the metakaolin geopolymer matrix and determine their strengthening mechanism. The test results showed that the addition of about 0.5 wt% MWCNTs increased the compressive and flexural strength by as much as 32% and 28%, respectively. Based on these results, the MWCNTs can act as effective bridges to minimize and limit the propagation of micro cracks through the metakaolin-based geopolymer nanocomposite under the conditions of homogenous dispersion and good bonding between the MWCNTs and the surrounding metakaolin-based geopolymer paste.     

Microstructural and mechanical characterization of fly ash cenosphere/metakaolin-based geopolymeric composites

Novel fly ash cenosphere (FAC)/metakaolin (MK)-based geopolymeric composites were prepared by adding FAC to the MK-based geopolymeric slurry. Microstructure, mechanical property, thermal conductivity, and bulk density of the FAC/MK-based geopolymeric composites were investigated. It was confirmed by the scanning electron microscope (SEM) and transmission electron microscopy (TEM) that the FAC did not dissolve in alkaline condition, but element diffusion took place around the interface between geopolymeric matrix and FAC. The compressive strength, thermal conductivity and bulk density of FAC/MK-based geopolymeric composites decreased monotonically with the increase of the FAC content from 15 vol.% to 40 vol.%, and the minimum values for the 40 vol.% FAC/MK-based geopolymeric composite reached 36.5 MPa, 0.173 W m⁻¹ K⁻¹ and 0.82 g cm⁻³, respectively, in the range of FAC content from 15 vol.% to 40 vol.%. The results showed that the FAC could lower thermal conductivity effectively and bulk density of FAC/MK-based geopolymeric composites at a cost of slight decrease of mechanical properties. The 40 vol.% FAC/MK-based geopolymeric composite was a promising candidate material for intermediate-temperature thermal insulation applications due to its low thermal conductivity and low density.     

Mechanical and microstructural evolutions of fly ash/slag-based geopolymer at high temperatures: Effect of curing conditions

Designing a building material with excellent heat resistance is crucial for protection against catastrophic fires. Geopolymer materials have been investigated as they offer better heat resistance than traditional cement owing to their ceramic-like properties. Curing temperature and conditions are crucial factors that determine the properties of geopolymers, but their impacts on the heat resistance of geopolymers remain unclear. This study produced geopolymers from fly ash and ground granulated blast furnace slag by using sodium silicate and sodium hydroxide solutions as alkaline solutions. To examine the effect of curing conditions on the high-temperature performance of geopolymer, four different curing conditions, namely, heat curing (70 °C for 24 h), ambient curing (20 °C), water curing, and the combination of heat and water curing (70 °C for 24 h followed by water curing), were applied. At 28 d, the specimens were subjected to high temperatures (500 °C, 750 °C, and 950 °C), and their mechanical and microstructural evolutions were studied. The results revealed that the curing condition significantly affects the properties of the unexposed geopolymer; the effect on its high-temperature performance is insignificant. Furthermore, all the specimens could maintain adequate compressive strength after exposure to the maximum temperature of 950 °C, promising the use of geopolymer for structural applications.     

Effect of PDMS content on waterproofing and mechanical properties of geopolymer composites

The present study mainly studies the effect of polydimethylsiloxane (PDMS) content on the waterproofing and mechanical properties of geopolymer composites. Firstly, hydrophobic modified geopolymer composites (HM-GC) were prepared by adding PDMS during the mixing process. Secondly, the surface wettability characteristics, water absorption, uniaxial compressive and tensile properties of HM-GC were investigated. The effect of PDMS content on the waterproofing and mechanical properties was further discussed. Finally, considering the waterproofing and mechanical properties, the optimal PDMS content was proposed. The results showed that with increasing PDMS content, the contact angle of geopolymer composites rapidly increase at first and then stabilizes. The geopolymer composites with 4% and 5% PDMS content exhibit overhydrophobic surface wettability. In addition, the water absorption gradually decreases with increasing PDMS content, indicating an improvement in the waterproofing ability. The incorporation of PDMS can enhance the compressive properties of geopolymer composites while reducing the tensile properties. Comprehensively considering the waterproofing and mechanical properties, it is reasonable to select 4% as the optimal PDMS content used in practical marine engineering.     

Effect of initial cure time on toughness of geopolymer matrix composites

The toughness of geopolymer matrix composites (GMC) has been identified as a limiting factor to their use in structural applications. Advanced ceramic matrix composites (CMC), which also are limited by brittle behavior, have shown gains in toughness through careful tailoring of the interface between fiber and matrix. This can create various crack dissipating mechanisms and prevent premature composite failure. Such interface modification has already been applied to a fiber reinforced geopolymer and while the resulting composite showed a reduction in brittle behavior, the modified interface produced an unacceptable loss in modulus without any other well-defined quantitative gains. Information gathered from other studies suggests the large decrease in modulus observed in the GMCs with the weakened interface may have been the result of poor matrix properties stemming from an inadequate cure. Therefore, this current study explores the effects of initial cure time on composite performance by measuring the mechanical properties GMCs with a modified interface. GMCs containing unidirectional Nextel 610 fiber were cured under two different sets of process conditions to better understand the influence of matrix properties. Additionally, specimens consisted of cleaned and carbon coated fiber surfaces, in an attempt to evaluate extremes of interfacial strength. Mechanical properties were then evaluated for comparison to determine if improved geopolymer matrix properties would allow a weakened interface to yield performance gains more in keeping with expectations based on CMC's. The results of the study indicate that specimens with carbon coating benefited from the longer initial cure time. The average increase in flexural modulus and strength over samples with one hour initial cure time was ~65% and ~170% respectively. Stress-strain behavior of the carbon-coated specimens with an extended cure time also indicated a greater degree of damage tolerance as compared to those without interphase.     

Preparation and characterization of polyaniline/multi-walled carbon nanotube composites

This study describes the synthesis of doped polyaniline in its emeraldine salt form (PANI-ES) with carboxylic groups containing multi-walled carbon nanotubes (c-MWNTs) via in situ polymerization. Both Raman and FTIR spectra indicate that carboxylic acid groups formed at both ends and on the sidewalls of the MWNTs. Based on the π-π* electron interaction between aniline monomers and MWNT and hydrogen bonding interaction between the amino groups of aniline monomers and the carboxylic acid group of c-MWNT, aniline molecules were adsorbed and polymerized on the surface of MWNTs. Structural analysis using FESEM and HRTEM showed that PANI-ES/c-MWNT composites are core (c-MWNT)-shell (doped-PANI-ES) tubular structures with diameters of several tens to hundreds of nanometers, depending on the PANI content. The conductivities of these PANI-ES/c-MWNT composites are 50-70% higher than those of PANI without MWNT.     

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.     

Preparation and characterization of ZrB2 and TaC containing Cf/SiC composites via Polymer-Infiltration-Pyrolysis process

To improve the thermo-chemical resistance of PIP–C_f/SiC composites, the SiC matrix is modified by adding ZrB₂ and Ta powder to the pre-ceramic slurry to form C_f/SiC–ZrB₂–TaC composites. Within this study the modified composites are investigated regarding their microstructure, chemical composition and physical properties (density = 2,39–2,72 g/cm³; porosity = 20,3–24,8 vol.-%; fiber volume content = 52–57 vol.-%). Mechanical properties are investigated in order to ensure that there is no negative influence by ZrB₂ and TaC matrix modification. The matrix modification is followed by an improvement in bending strength (up to 27% increase), Young’s modulus (up to 28% increase) and for interlaminar shear strength (up to 22% increase). Finally the thermo-chemical behavior of the C_f/SiC–ZrB₂–TaC composites is evaluated in a combustion chamber-like environment using the Airbus Group long-term material test facility (Environmental Relevant Burner Rig-Kerosene, ERBURIG^K). The results show that the thermo-chemical resistance of C_f/SiC–ZrB₂–TaC composites is improved and the oxygen permeability through the composite is decreased (from 5 to 1 layer).     

Effects of graphite on the mechanical and microwave absorption properties of geopolymer based composites

In this paper graphite/metakaolin was first ball-milled to get homogeneous powder, which was then mixed with potassium silicate solution through mechanically stirring. Post curing, we got graphite/geopolymer composites with graphite to geopolymer ratio from 0 to 18 and part the samples were further dealt with heat treatment at 600 °C. Effects of the graphite content on the mechanical properties and microwave absorption properties of the composites were systematically investigated. The results proved that when graphite to geopolymer ratio is not higher than 12, graphite dispersed homogeneously in the composites. However, graphite agglomeration was noted when graphite to geopolymer ratios are 15 and 18. With the increase in graphite to geopolymer ratio, flexural strength and fracture toughness of the composites first increased, reaching the peak value and then decreased. When the graphite to geopolymer ratio is 12, the composite showed the highest flexural strength and fracture toughness, which should be explained by the mixing rule of composites since the mechanical properties of graphite are much higher than geopolymer matrix. With the increase in graphite content, the dielectric constants of the composite increased gradually, but the magnetic constants nearly kept unchanged. It implied that the main microwave absorbing mechanism would be dielectric loss of the composites. The maximum wave reflection loss showed similar trend to the mechanical properties of composites. It reached the peak value when graphite to geopolymer ratio is 12 and then started to decline, which might also be related to the graphite agglomeration. After 600 °C heat treatment, slight decline of the reflect loss peak and obvious decrease on the thickness corresponding to the maximum reflection loss were observed.     

Effect of PAN-based and pitch-based carbon fibres on microstructure and properties of continuous Cf/ZrB2-SiC UHTCMCs

In this paper the microstructure and mechanical properties of two different C_f/ZrB₂-SiC composites reinforced with continuous PyC coated PAN-derived fibres or uncoated pitch-derived fibres were compared.Pitch-derived carbon fibres showed a lower degree of reaction with the matrix phase during sintering compared to PyC/PAN-derived fibres. The reason lies in the different microstructure of the carbon. The presence of a coating for PAN-derived fibres was found to be essential to limit the reaction at the fibre/matrix interface during SPS. However, coated bundles were more difficult to infiltrate, resulting in a less homogeneous microstructure.As far as the mechanical properties are concerned, specimens reinforced with coated PAN-derived fibres provided higher strengths and damage tolerance than uncoated pitch-derived fibres, due to the higher degree of fibre pull-out. On the other hand, the weaker fibre/matrix interface resulted in lower interlaminar shear, off-axis strength and ablation resistance.     

Fabrication method and mechanical properties of biomimetic Cf/ZrB2-SiC ceramic composites with bouligand structures

Herein, biomimetic C_f/ZrB₂-SiC ceramic composites with bouligand structures are fabricated by combining precursor impregnation, coating, helical assembly and hot-pressing sintering. First, C_f/ZrB₂-SiC ceramic films are achieved through a precursor impregnation method using polycarbosilane (PCS). Second, the PCS-C_f/ZrB₂-SiC ceramic films are coated with ZrB₂ and SiC ceramic layers. Finally, hot-pressing sintering is employed to densify helical assembly C_f/ceramic films with a fixed angle of 30°. The microstructures and carbon fiber content on the mechanical properties of biomimetic C_f/ZrB₂-SiC ceramic composites are analyzed in detail. The results show that the coated ceramic layer on PCS-C_f/ZrB₂-SiC films can heal the cracks formed by pyrolysis of PCS, and the mechanical properties are obviously improved. Meanwhile, the mechanical properties could be tuned by the contents of the carbon fiber. The toughening mechanisms of C_f/ZrB₂-SiC ceramic composites with bouligand structures are mainly zigzag cracks, crack deflection, multiple cracks, carbon fiber pulling out and bridging.     

High-flexural-strength of geopolymer composites with self-assembled nanofiber networks

Natural-fiber-reinforced geopolymer composites are environmentally friendly materials that have garnered considerable attention. In practice, the drying shrinkage and uneven dispersion of fibers in geopolymers result in low flexural strength, which limits the use of large-scale composites. In this study, high-flexural strength geopolymer composites with self-assembled nanofiber networks were hybridized using in situ polymerization. Composites with different fiber contents were evaluated via morphology and mechanical strength assessments. A mixed nanofiber and geopolymer solution was created to achieve uniform dispersion, and then the geopolymers were optimized at the nanoscale level using a bottom-up approach to achieve self-assembled nanofiber networks with good interfacial bonding. Due to the water retention properties of the nanofibers and subsequent gel material, the networks facilitated internal curing of the geopolymers, thus preventing drying shrinkage. This ultimately reduced the weight loss and increased the density of the composites. In addition, high flexural strength properties were observed in composites with a fiber content of 0.5%, due to the combination of fiber reinforcement and porosity. Thus, this work introduces new considerations for geopolymer applications.     

O＇－Sialon－ZrO_2复合材料的显微结构与力学性能的研究

研究了Ｏ＇－Ｓｉａｌｏｎ－ＺｒＯ２复合材料的显微结构与力学性能的关系。结果表明，Ｏ＇－Ｓｉａｌｏｎ形成连续网络编织状结构。ＺｒＯ２加入量较少时充当填充结构骨架的作用；ＺｒＯ２加入量增多时（至４０％），会有更多的ＺｒＯ２形成聚集体。随着ＺｒＯ２引入量的增加，材料的常温抗折强度提高，但高温抗折强度下降。Ｏ＇－Ｓｉａｌｏｎ的编织状结构可能阻碍晶界滑移。这种复合材料的高温抗折强度在１４００℃为１１２～１７３ＭＰａ。     

Characterisation and properties of geopolymer composite part 1: Role of mullite reinforcement

Geopolymer-mullite composite was prepared using fly ash and mullite powders with sodium silicate and sodium hydroxide as alkaline activators. Mullite was used as a replacement to fly ash in the 20–60 wt% range. Sodium silicate to sodium hydroxide (12 M) ratio was 1:1 while the liquid to solid ratio was 0.6:1. X-ray diffraction (XRD) analysis revealed that the set of geopolymer specimens without mullite replacement (control) showed the co-existence of amorphous and crystalline phases of quartz, magnesioferrite (Fe₂MgO₄), Lazurite (Na_8.56 (Al₆Si₆O₂₄) (SO₄)_1.56 S_0.44)) and calcium silicate hydrate. With an increasing amount of mullite replacement, calcium silicate hydrate and magnesioferrite diminished while the new phase of phillipsite (K, Na)₂(Si,Al)₈O₁₆·4H₂O) emerged. Microstructural analysis revealed Si-rich mullite needles possibly occurred by recrystallization of the original mullite. This suggestion was also confirmed by the change of the crystallite size as analysed using an X-ray diffraction technique. The ambient compressive strength was found to increase from 58 ± 21 MPa for the control geopolymer to 72–76 MPa, with a much smaller uncertainty, for the geopolymer-mullite composite. Modulus of rupture (MOR) was found to improve significantly from 0.7 ± 0.3 MPa to 3.7 ± 0.5 MPa in the 20% replacement and further to 7.8 ± 1.3 and 8.1 ± 1.1 MPa in the 40% and 60% replacement respectively. Improvement of fire resistance was observed in the 40–60% replacement thermal shock resistance property, however, was unchanged in these geopolymer-mullite composite.     

Investigation on the effect of fiber wettability on water absorption kinetics of geopolymer composites

Fiber-reinforced geopolymer composites receive widespread attention due to their desirable mechanical properties but the effect of fiber wettability on the water transport properties is rarely investigated. This work aims to reveal the effect and mechanism of hydrophilic and hydrophobic fibers, represented by polyvinyl alcohol fiber (PVAF) and polypropylene fiber (PPF), on the water absorption kinetics of geopolymer composites. Results indicate that both fibers can slightly weaken the water absorption capacity due to increased micron-scale voids and capillary length. Moreover, waterproof composite with a water contact angle of ～120° can be fabricated by adding PPF and polydimethylsiloxane. PPF weakens the vaporization-condensation process of moisture, thereby extending the duration of the induction stage by 16.7 h, within which a low water absorption rate is maintained. In contrast, hydrophilic PVAF promotes the water absorption process. Therefore, hydrophobic fibers are more beneficial to improve the waterproof performance of geopolymer composites. The development of waterproof fiber-reinforced geopolymer composites is of great potential to improve the corrosion resistance and durability of buildings serving in high-humidity environments.     

Preparation and characterization of tungsten tailing-based geopolymers

Using tailings to prepare constructive materials is of great significance for sustainable development of mineral processing industry. In this study, the possibility of preparing tungsten tailing-based geopolymers was explored in detail. XRD, FTIR, PLM, SEM and XPS analyses were carried out to characterize the phase composition, chemical bonding, microstructure, chemical state, and interface properties of tungsten tailing-based geopolymers. Results showed tungsten tailings presented little activity using NaOH as activator, while geopolymers with 60% non-pretreatment tungsten tailing and 40% metakaolin presented a 3-day compressive strength of 8.4 MPa and 28-day compressive strength of 9.1 MPa. The geopolymerization products of tungsten tailing-based geopolymers were N-A-S-H gels and aluminosilicate zeolite crystals, while tungsten tailings were wrapped by metakaolin-derived geopolymerization phases as aggregates with interfaces containing Si–O–Si bonding between quartz in tungsten tailings and zeolite and/or gel phase in metakaolin-derived geopolymer in the geopolymerization process. Besides, the leaching test results indicated that the immobilization efficiency of T6M4 geopolymers for Mn and Pb derived from tungsten tailings reached up to 97.28% and 99.95%, respectively. This research results provide a new idea for utilization of tungsten tailings on a large scale.     

Effects of high-temperature heat treatment on the mechanical properties of unidirectional carbon fiber reinforced geopolymer composites

Unidirectional carbon fiber reinforced geopolymer composite (C_uf/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.