Fly ash-based boroaluminosilicate geopolymers: Experimental and molecular simulations

Geopolymers are cement-free construction materials which are produced by mixing an aluminosilicate source such as fly ash with an alkali activator. Despite their eco-friendly nature, geopolymers suffer the negative impact of the sodium silicate part of the alkali activator on the environment. The use of borax, one of the eco-friendly salts of boric acid, as an activator can potentially lead to the production of more environmentally-friendly geopolymer. However, a better understanding of their theoretical properties could be a milestone to produce new generations of geopolymers with high performance. A growing interest in the prediction of the macroscale properties of geopolymer compounds was the most compelling motivation for this study. Building upon this, the current study focused on both points to apply borax as a potential replacement for silicate-based activators and model all the experiments by molecular dynamics (MD) simulation. Substituting boron with aluminium in the molecular structures of geopolymer was the core idea of the simulation. Compressive strength, density and elastic modulus tests were conducted, and the results were compared with the MD simulation outcomes. Increasing the content of borax in the mixture led to a decrease in all of the properties, although the range of 10–30% of replacement eventuated in acceptable results. A fair agreement between simulation and experimental results was achieved through which the best fitting parameters for atomistic modelling of geopolymers were found.     

Natural and synthetic hydroxyapatite/zirconia composites: A comparative study

Zirconia precursor was precipitated in a HAp particles suspension using two HAp powders of natural origin and a synthetic powder. The first natural HAp was extracted from animal bones by treatment with hot NaOH solution and the second one by treatment under hydrothermal conditions with water.Hydrous zirconia was precipitated in the HAp suspension. Pressureless sintering was performed at 1000–1300 °C and hot pressing at 1050–1300 °C.It was found that zirconia additive promotes decomposition of both HAp of natural origin as well as the synthetic one. The most stable HAp was the one extracted from bovine bones by treatment with water in an autoclave. This reaction leads to the formation of β–TCP and the CaO–ZrO₂ solid solution.The hot pressed composites show essentially higher strength and fracture toughness as compared to the pure hydroxyapatite polycrystals.     

Structure and mechanical properties of nanofibrous ZrO2 derived from alternating field electrospun precursors

Nanofibrous zirconia (ZrO₂) meshes were prepared from precursor fibers which were synthesized using the method of free-surface, high-yield alternating field electrospinning (AFES). The weight ratio of zirconyl chloride salt to polyvinylpyrrolidone (PVP) polymer in liquid precursors was investigated for its effect on the spinnability and formation of precursor fibers as well as on the resulting fibrous ZrO₂. The precursor fiber generation measured at a rate up to 5.6 g/h was achieved with a single flat 25-mm diameter alternating current (AC) electrode, which corresponded to production of up to 1.5 g/h of fibrous ZrO₂. The calcination process involved annealing the fibers at temperatures which ranged from 600 °C to 1000 °C and produced 0.1–0.2 mm thick fibrous ZrO₂ meshes. Individual nanofibers were found to have diameters between 50 and 350 nm and either a tetragonal (t-ZrO₂) or monoclinic (t-ZrO₂) structure depending on the calcination temperature. The annealed meshes with total porosity between 98.0 ± 0.2% and 94.6 ± 0.2% showed little deformation or cracking. Tensile strength and modulus of fibrous t-ZrO₂ meshes strongly depended on porosity and varied from 0.07 ± 0.03 MPa to 1.05 ± 0.3 MPa and from 90 ± 40 MPa to 388 ± 20 MPa, respectively. The m-ZrO₂ meshes resulted similar moduli, but much lower strengths due to their brittleness. A power-law relationship between the elastic modulus and porosity of AFES-derived nanofibrous t-ZrO₂ meshes, in comparison with other porous zirconia materials, was also investigated. The results of this study have demonstrated the feasibility of free-surface AFES in sizeable production of zirconia nanofibers and highly porous nanofibrous ceramic structures.     

Mine tailings-based geopolymers: Properties,applications and industrial prospects

The use of mine tailings (MTs) as aggregates or precursors of alkali-activated materials and geopolymers (GPs) seems to be a promising approach for their sustainable utilization since it allows not only reducing the dynamics of MTs accumulation in the environment and the environmental damage they cause but also it combines the advantages geopolymer technology that is associated with reducing the carbon footprint, the ability to utilize other technogenic aluminosilicate waste, the versatility of the properties of GPs as a general construction binder. Taking into account the complex material composition of mine tailings, and relatively little knowledge of the features of the geopolymerization of tailings and the influence of various factors on the properties of MTs-based geopolymers, there is now a need to generalize these aspects and assess the prospects for possible applications. This article is a generalization and a detailed analysis of the relationship between structural, mechanical, and thermal properties, durability, leaching behavior, and other important characteristics of MTs-based geopolymers. Here, in addition to the key fundamental aspects of the formation of properties of MTs based geopolymers, well-known examples of their applications in binder pastes, mortars, and concretes, as well as bricks, backfill materials, adsorbents, porous materials, and other promising applications are considered in detail. In addition, economic and production aspects are also discussed.     

Alkali-activated cement based on natural SiO2-containing material: Part I. Strength, hydration, microstructure and durability

An alkali-activated cement (AAC) based on natural SiO₂-containing materials—grounded porcellanite (Pr) and highly dispersed pure quartz sand—was examined. Sodium hydroxide was used as an alkali activator. The pressed specimens were prepared and were cured in an autoclave at a pressure of 1.6 MPa and a temperature of 205 °C. It was shown that the strength of cement as well as compound and the microstructure of its hydration products depend on the cement composition. It was distinguished that autoclave-cured cementing matter comprises secondary quartz and the mass of sodium hydrated silicates along with the initial Pr crystal phases. After a 2-year storage under water, 15% Na₂SO₄, and Dead Sea water, the strength of specimens decreased by 17.5-20%. Control specimens, prepared with Portland cement and immersed in a 15% Na₂SO₄ solution for 2 weeks, were broken up completely. Positive results of long-term durable tests suggest that an AAC based on natural raw material would be stable in other salt solutions.     

Effect of tetragonal zirconia nanoparticle content on enamel strength

The fracture failure of enamel is caused by hole-type flaws and cracks generally present in enamel. In this study, the effect of tetragonal zirconia nanoparticles (nano-t-ZrO₂) on enamel strength was investigated. The addition of nano-t-ZrO₂ improved the surface morphology and reduced the porosity of the enamel as determined by morphological and porosity distribution analyses. The strain on the enamel surface was recorded using a strain gauge attached to the enamel surface. The crack initiation force and strain were measured using the strain gauge, and the crack propagation diagrams at various stages of the tensile process were recorded. The relationship between the crack initiation energy and nano-t-ZrO₂ content was found to be non-linear. Nano-t-ZrO₂ affected the strength of the enamel. Moreover, the tensile force required for the appearance of cracks first increased and then decreased with an increase in the nano-t-ZrO₂ content. The addition of nano-t-ZrO₂ to the enamel matrix suppressed the formation of stress concentration regions and decreased the brittleness of the enamel. At a nano-t-ZrO₂ content of more than 5 wt%, the ZrO₂ particles agglomerated.     

麻纤维增强聚乙烯复合材料的制备及性能研究

通过双螺杆挤出造粒,注塑成型制备了麻纤维增强高密度聚乙烯( HDPE)复合材料,测试了复合材料的力学性能并观察其微观结构,分析了相容剂马来酸酐接枝聚丙烯( PP-g-MAH)的用量和麻纤维的含量对复合材料力学性能的影响.结果表明:PP-g-MAH的加入提高了苎麻/HDPE复合材料的力学性能,并且在PP-g-MAH含量为...     

Metakaolin-based geopolymers: Efflorescence and its effect on microstructure and mechanical properties

Efflorescence in geopolymers results from mobility of excess alkali and consequent crystallization of alkali carbonates. Efflorescence potential of various geopolymers has been reported previously but the knowledge regarding the effect of efflorescence on the microstructure and mechanical properties of geopolymers remains limited. In this work, metakaolin-based geopolymers were exposed to air, partially immersed in water, and fully immersed, to simulate different processes involved in efflorescence formation. The mechanical properties were assessed by compressive, splitting tensile and flexural strengths, and linear deformation. The microstructural features were investigated by SEM, synchrotron XRD, multinuclear MAS NMR, MIP and synchrotron X-ray microtomography. Extensive efflorescence resulted in a reduction of mechanical strength and changes in the nanostructure and microstructure, which is different from observations for Portland cement-based materials, where efflorescence is usually regarded as a surface or aesthetic problem. The understanding of the relationship between efflorescence formation, the synthesis and exposure conditions provides important insight into the manufacturing and application conditions of geopolymer related materials.     

The microstructural and mechanical properties of geopolymer composites containing glass microfibres

This paper presents the effects of microfibre contents on mechanical properties of fly ash-based geopolymer matrices containing glass microfibres at 0, 1, 2 and 3 mass%. The influence of glass microfibres on the fracture toughness, compressive strength, Young's modulus and hardness of geopolymer composites are reported, as are the microstructural properties investigated using scanning electron microscopy. Results show that the addition of 2 mass% glass microfibres was optimal, exhibiting the highest levels of fracture toughness, compressive strength, Young's modulus and hardness. The results of the microstructural analysis indicate that the glass microfibres act as a filler for voids within the matrix, making a dense geopolymer and improving the microstructure of the binder. This leads to favourable adhesion of the composites, and produces a geopolymer composite with good mechanical properties, comparable to pure geopolymer. The failure mechanisms in glass microfibre-reinforced geopolymer composites are discussed in terms of microstructure.     

Mechanical properties of interfaces within a flax bundle – Part I: Experimental analysis

In order to model the tensile behaviour of flax fibre based composites, the properties of each of the constituents need to be determined. In addition to the fibres, the matrix and the fibre/matrix interface, the fibre/fibre interface present within a bundle of flax fibres is an element which is rather hard to characterize but whose properties also need to be taken into account to understand properly the deformation and rupture modes of the derived composites. In the first part of this study, the protocol used to determine these properties is described; the results of the mechanical tests and the microscopic observations carried out on pairs of fibres are given and exploited to lead to the fibre/fibre interface properties. In the second part, various cohesive zone models will be evaluated using these interface properties and numerical simulations will be performed for the purpose of validation.     

Quasi-static compression behavior of nickel oxide,nickel oxide:zirconia,nickel:zirconia and nickel foams

An effective material for use in shock mitigation should spread the deflection of the shock wave over a longer period of time and should minimize the force felt by the object under impact. Ductile or brittle cellular materials are currently gaining importance due to their unique high energy absorption characteristics. Reticulated cellular foam structures of nickel oxide (NiO) and nickel oxide:zirconia (NiO:YSZ 60:40 percentage by wt.) were fabricated by polymeric sponge replication process. These foams are reduced under hydrogen atmosphere to produce metallic nickel (Ni) and nickel:zirconia (Ni:YSZ) cermet foams, respectively. X-ray diffraction studies on the struts confirmed the corresponding phase formation. Further, the volume fraction of the solid in foam is estimated through image analysis. All the foams are subjected to uni-axial compression and the stress–strain curves were recorded. A comparative evaluation of progressive deformation behavior at room temperature was also carried out. Stress–strain curve of the nickel foam shows distinctly three regimes under compression, a deformation regime showing a linear dependence in the strain with stress. This is followed by a second region showing a plateau corresponding to the energy absorption resulting from the permanent plastic deformation while retaining the integrity and finally densification region through the wall collapse resulting in the maximum compressive strength. Stress–strain curves of all other foams such as NiO, NiO:YSZ and Ni:YSZ has demonstrated a similar fracture behavior under compression which caused not only by unstable crack propagation originating from a single crack, but also by merging of many cracks leading to the formation of the crushed zone. Compressive strength is found to be a strong function of solid fraction supporting the load and percentage porosity of NiO foams. Estimation of relative energy absorption has exhibited higher energy absorption irrespective of the material of construction at higher strain rates.     

Tribo-mechanical characterization of spark plasma sintered chopped carbon fibre reinforced silicon carbide composites

Short carbon fibre (C_f) reinforced silicon carbide (SiC) composites with 7.5 wt% alumina (Al₂O₃) as sintering additive were fabricated using spark plasma sintering (SPS). Three different C_f concentrations i.e. 10, 20 and 30 wt% were used to fabricate the composites. With increasing C_f content from 0 to 20 wt%, micro-hardness of the composites decreased ~28% and fracture toughness (K_IC) increased significantly. The short C_f in the matrix facilitated enhanced fracture energy dissipation by the processes of crack deflection and bridging at C_f/SiC interface, fibre debonding and pullout. Thus, 20 wt% C_f/SiC composite showed >40% higher K_IC over monolithic SiC (K_IC≈4.51 MPa m^0.5). Tribological tests in dry condition against Al₂O₃ ball showed slight improvement in wear resistance but significantly reduced friction coefficient (COF, μ) with increasing C_f content in the composites. The composite containing 30 wt% C_f showed the lowest COF.     

Mine tailings-based geopolymers: A comprehensive review

The mining industry produces a large amount of stone waste and tailings, which poses a threat to the environment. Dumping is the most common means of disposing of this industrial waste, contributing to soil degradation and water pollution with the acquisition of valuable land. Fortunately, it can be recycled in a variety of technologies, including the promising geopolymerization technology, which turns waste into value. This review paper presents recent advances in the production of mine tailings-based geopolymer composites from industrial waste as a potential sustainable building material. This article also provides in-depth studies on the behaviors and characteristics of mine tailings composites utilized in geopolymer production, such as physical properties, mechanical properties, durability properties, microstructural properties, thermal properties, leaching behavior, and potential applications. Besides, study developments are moving towards a comprehensive understanding of the environmental footprints and economic benefits of mine tailings-based geopolymer composites for building applications utilizing mine tailings as suitable concrete material. This review paper also highlights knowledge gaps that must be overcome to progress mine tailings composites for geopolymers, as well as future study opportunities based on prior research and existing challenges.     

Processing,properties and applications of highly porous geopolymers: A review

Geopolymers, possessing a semi-crystalline three-dimensional inorganic network generated by the dissolution and reaction of a solid alumino-silicate source with an activating solution, have attracted increasing attention from both academia and industry because of their unique and favorable characteristics. This review deals with the synthesis, characterization and potential applications of porous geopolymers, realized through different processing routes. Firstly, the processing approaches are divided into five categories: (i) Direct foaming, (ii) Replica method, (iii) Sacrificial filler method, (iv) Additive manufacturing, and (v) Other methods. Their microstructure, porosity and properties are compared and discussed in relation also to the different processing routes. This review highlights the fact that porous geopolymers are promising low-cost candidates for technologically significant applications such as catalyst supports or membranes, filtration of liquid or gases, adsorption and insulation. This review aims at summarizing the main published results and fostering further investigations into developing innovative ways to generate components with improved properties.     

Mechanical and durable behaviour of alkaline cement mortars reinforced with polypropylene fibres

The development of new binders, alternative to traditional cements and concretes obtained by the alkaline activation of different industrial by-products (blast furnace slags and/or fly ashes), is an ongoing study and research topic of the scientific community.

The mechanical and durable behaviour of alkaline cement mortars reinforced with polypropylene fibres has been the object of the present investigation. Three different alkaline matrices were used: (a) granulated blast furnace slag activated with waterglass (Na₂SiO₃+NaOH) with a concentration of 4% Na₂O by mass of slag and cured at room temperature, (b) aluminosilicate fly ash activated with 8M NaOH and cured at 85 °C during the first 24 h and (c) 50% fly ash+50% slag activated with 8M NaOH solution at room temperature. In the mechanical tests (flexural and compressive strengths), two different dosages of fibres were used: 0.5% and 1% by mortar volume. Shrinkage tests according to ASTM C 806-87 standard with (1%) and without fibres were also carried out. The durability tests carried out were freeze/thaw and wet/dry cycles. In these tests, the dosage of fibre was 0.5% by mortar volume. The results obtained show that the nature of the matrix is the most important factor to strength development, more than fibre presence and content amount.     

Hydration reactions and physicochemical properties in a novel tricalcium-dicalcium silicate-based cement containing hydroxyapatite nanoparticles and calcite: A comparative study

The mineral trioxide aggregate (MTA) is Portland type cement whose main application in dentistry is retrograde filling. The purpose of this study was to analyze hydration reactions and physicochemical properties of a new tricalcium-dicalcium silicate-based cement containing nanocrystalline hydroxyapatite (nHAp) and calcite. The new formulation was compared with Biodentine™ and MTA-Angelus™ as control samples.Hydration reactions were monitored by Raman spectroscopy, X-ray diffraction, radiopacity, pH, setting time, and compressive strength. The compressive strength reaches its higher value at 7 days following the sequence: Biodentine™ (104.8 MPa) > Cement + 5% nHAp (59 MPa) > MTAAngelus™ (27.3 MPa), in agreement with the pH values measured at 24 h: Biodentine™, Cements + nHAp or + calcite (10.6–11.6) > MTA-Angelus™ (9.7). Mean setting times was around 30 min and no significative differences (p = 0.0001) were observed. In the Biodentine™ control samples, Ca₃SiO₅ diminishes until disappear at 28 days of hydration. On their turn, calcium silicate hydrate (CSH) increases continuously in the range of time analyzed. The present results suggest that the physicochemical properties were improved for the new cement with nanosized hydroxyapatite nanoparticles and relevant information on chemical properties is of valuable importance for testing predictive models for Biodentine™ and MTA-Angelus™.     

The coloring of geopolymers by the addition of copper compounds

The development of a new coloring technique is desirable to increase the commercial value of geopolymers. Selected copper compounds, i.e. Cu(OH)₂, CuO, Cu₂O, CuCO₃?Cu(OH)₂?H₂O, CuCl₂?2H₂O and CuSO₄?5H₂O, were added to the initial reactants in order to color the geopolymers in the same manner as naturally occurring minerals. When Cu(OH)₂, CuO and Cu₂O were used, these compounds remained in the geopolymer matrix following hardening of the material. On the contrary, CuCO₃?Cu(OH)₂?H₂O, CuCl₂?2H₂O and CuSO₄?5H₂O were not detected in the final products. XAFS analyses were performed to investigate the local structure of copper in the geopolymers produced. The results showed that the copper spectra of geopolymers incorporating Cu(OH)₂, CuO and Cu₂O correspond to those of pure Cu(OH)₂, CuO and Cu₂O, respectively. However, when CuCO₃?Cu(OH)₂?H₂O, CuCl₂?2H₂O and CuSO₄?5H₂O were added, the copper generated spectra similar to that of the mineral chrysocolla ((Cu, Al)₂H₂Si₂O₅(OH)₄?nH₂O) than the respective copper compounds.     

Compressive strength degradation of SiC fibers exposed to high temperatures due to impurity-induced internal oxidation

SiC fiber oxidation is a potential factor limiting the operating temperature of SiC_f/SiC composites owing to the strength degradation after oxidation. Herein, we fabricated 1-μm-diameter pillars at the core of the fiber cross-sectional surface after SiO₂ removal to eliminate surface effects caused by external oxidation. The fiber strength significantly decreased during the first hour of oxidation in dried air at 1400 °C, but this deterioration became less pronounced after 10 h. Simultaneously, the oxidation lowered the Young’s modulus and Weibull modulus. Oxidation considerably increased the porosity and the alterations in the mechanical behavior were primarily caused by the variations in porosity. Oxidation-induced pores were frequently detected at the fiber core and were partially filled with SiO₂. Compared with those of the as-received fibers, O impurities in the oxidized fiber core were significantly reduced. Thus, the fiber strength was potentially degraded by the internal oxidation reaction between residual C and O.     

Effect of geothermal waste on strength and microstructure of alkali-activated slag cement mortars

Mortars of blast furnace slag replaced with 10% of a geothermal silica waste were cured for 90 days. The binder was activated by 6 wt.% Na₂O equivalent of NaOH and water glass. The presence of the silica enhanced the formation of hydration products as shown by nonevaporable water (NEW) results. Backscattered electron images indicated that the microstructures of blended slag had less porosity than those of neat slag mortars and the interfacial zone between aggregate and hydration products was dense and of homogeneous composition similar to the matrix of hydration products. The main hydration products were C-S-H and for NaOH a hydrotalcite type phase was found as finely intermixed with the C-S-H.     

Structural and mechanical properties of NbN and Nb-Si-N films: Experiment and molecular dynamics simulations

The structural and mechanical properties of NbN and Nb-Si-N films have been investigated both experimentally and theoretically, in their as-deposited and annealed states. The films were deposited using magnetron sputtering at substrate bias (U_B) between 0 and −70 V. While NbN films were found to crystallize in the cubic δ-NbN structure, Nb-Si-N films with Si content of 11–13 at% consisted of a two-phases nanocomposite structure where δ-NbN nanocrystals were embedded in SiN_x amorphous matrix. Films deposited at U_B=0 V were highly (001)-textured. Application of substrate bias potential led to a depletion of light atoms, and caused a grain size refinement concomitantly with the increase of (111) preferred orientations in both films. The maximum hardness was 28 GPa and 32 GPa for NbN and Nb-Si-N films, respectively. NbN and Nb-Si-N films deposited at U_B=−70 V exhibited compressive stress of −3 and −4 GPa, respectively. After vacuum annealing, a decrease in the stress-free lattice parameter was observed for both films, and attributed to alteration of film composition. To obtain insights on interface properties and related mechanical and thermal stability of Nb-Si-N nanocomposite films, first principles molecular dynamics simulations of NbN/SiN_x heterostructures with different structures (cubic and hexagonal) and atomic configurations were carried out. All the hexagonal heterostructures were found to be dynamically stable and weakly dependent on temperature. Calculation of the tensile strain-stress curves showed that the values of ideal tensile strength for the δ-NbN(111)- and ε-NbN(001)-based heterostructures with coherent interfaces and Si₃N₄–like Si₂N₃ interfaces were the highest with values in the range 36–65 GPa, but lower than corresponding values of bulk NbN compound. This suggests that hardness enhancement is likely due to inhibition of dislocation glide at the grain boundary rather than interfacial strengthening due to Si-N chemical bonding.