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.     

In-situ thermo-mechanical testing of fly ash geopolymer concretes made with quartz and expanded clay aggregates

The mechanical and microstructural properties of geopolymer concretes were assessed before, during and after high temperature exposure in order to better understand the engineering properties of the material. Fly ash based geopolymer concretes with either quartz aggregate or expanded clay aggregate were exposed to various temperatures up to 750 °C using a thermo-mechanical testing apparatus. Microstructural investigations were also undertaken to better understand the measured changes in the mechanical properties. It was found that dehydration of capillary water caused cracking and strength losses at temperatures ≤ 300 °C, an effect that was more severe in the quartz aggregate geopolymer due to its lower permeability. At higher temperatures (T ≥ 500 °C) sintering promoted strength increases which enabled both concrete types to yield significant strength advantages over conventional materials. Stress–mechanical strain curves, which form the basis of the fire design of concrete structures, are reported.     

Characterisation and properties of geopolymer composites. Part 2: Role of cordierite-mullite reinforcement

Our previous research paper on geopolymer-mullite composites showed promising results on compressive strength and fire resistance. However, no improvement in thermal shock resistance was observed in the afore mentioned study. In this study, further attempts to improve thermal shock resistance of the geopolymer were explored. The research was performed by compositing a fly ash-based geopolymer with cordierite-mullite at 20, 40 and 60 wt% replacement. X-ray diffraction (XRD) of the cured geopolymer composite specimens showed the existence of cordierite, mullite, quartz, cancrinite and lazurite. It was found that compressive strength and strength retention after thermal exposure at 400 °C were improved in the geopolymer composite specimens, especially those with 20–40 wt% replacement. Upon further heating to 600 °C, all geopolymer specimens showed insignificant differences in compressive strength. Fire resistance was found to improve with increasing proportion of replacement contents.     

Production of nepheline/quartz ceramics from geopolymer mortars

This research has investigated the mechanical properties and microstructure of metakaolin derived geopolymer mortars containing 50% by weight of silica sand, after exposure to temperatures up to 1200 °C. The compressive strength, porosity and microstructure of the geopolymer mortar samples were not significantly affected by temperatures up to 800 °C. Nepheline (NaAlSiO₄) and carnegieite (NaAlSiO₄) form at 900 °C in the geopolymer phase and after exposure to 1000 °C the mortar samples were transformed into polycrystalline nepheline/quartz ceramics with relatively high compressive strength (～275 MPa) and high Vickers hardness (～350 HV). Between 1000 and 1200 °C the samples soften with gas evolution causing the formation of closed porosity that reduced sample density and limited the mechanical properties.     

Thermo-mechanical characterization of a silica-alumina refractory concrete based on calcined algerian kaolin

The aim of this work is to study the thermo-mechanical behaviour (bending and compressive tests, creep and thermal shock resistance) of a refractory concrete based on local kaolin grogs and aluminous cement. Strength tests revealed a behaviour that is almost linear elastic for temperatures up to 800 °C and visco-plastic at 900 °C. A crack bridging strengthening process was observed at 800 °C. The creep tests were carried out at different temperatures between 1000 and 1150 °C using stresses in the range (0.75–2.76 MPa). The stress exponent was about 1.255. Microscopic observations suggested an intergranular creep mechanism.A water quenching test was used for estimating the thermal shock resistance of the material. The tested samples supported 80 cycles of standardized cyclic thermal shock without failure. Ultrasonic measurements were applied in order to evaluate the of ultrasonic velocity changes after these thermal shock tests. Strength degradation of the samples was evaluated using two models based on ultrasonic velocity changes during test and compared with the experimental values.     

Strength of hot pressed ZrB2–SiC composite after exposure to high temperatures (1000–1700 °C)

Residual strength (room temperature strength after exposure in air at high temperatures) of hot pressed ZrB₂–SiC composites was evaluated as function of SiC contents (10–30 vol%) as well as exposure temperatures for 5 h (1000–1700 °C). Multilayer oxide scale structures were found after exposures. The composition and thickness of these multilayered oxide scale structure was dependent on exposure temperature and SiC contents in composites. After exposure to 1000 °C for 5 h, the residual strength of ZrB₂–SiC composites improved by nearly 60% compared to the as-hot pressed composites with 20 and 30 vol% SiC. On the other hand, the residual strength of these composites remained unchanged after 1500 °C for 5 h. A drastic degradation in residual strength was observed in composites with 20 and 30 vol% SiC after exposure to 1700 °C for 5 h in ZrB₂–SiC. An attempt was made to correlate the microstructural changes and oxide scales with residual strength with respect to variation in SiC content and temperature of expsoure.     

Novel thermal insulating and lightweight composites from metakaolin geopolymer and polystyrene particles

In this work, new foamed thermal insulation geopolymer composite based on polystyrene particles (PP) and metakaolin was developed. Compressive strength, flexural strength, high temperature resistance and microstructure were evaluated. The experimental results show that compressivestrengthand flexural strength of the thermal insulation geopolymer composite decrease with increasing polystyrene particle content. However, it still exhibits considerable and sufficient strength. The dry density and thermal conductivityalso decrease as polystyrene particle content increases due to the contribution of polystyrene particles with low density. The floatation of the thermal insulation geopolymer composite on water surface indicates the relatively low density and a good quadratic function relationship can be found between thermal conductivity and dry density. Furthermore, the dense interfacial transition zone indicates the high compressive strength and flexural strength of thermal insulation geopolymer composites. The cumulative intrusion volume corresponding to the porosity decreases and the critical pore diametersshift to lower values with addition of polystyrene particles. Geopolymer composites gain strength after exposure around 400 °C, and it suffers dramatic strength loss after 800 °C temperature exposure especially for the 100% polystyrene particles addition specimen.     

Mechanical properties of geopolymer concrete exposed to dynamic compression under elevated temperatures

Through adoption of a self-designed high temperature SHPB apparatus herein, an experimental study is made on the mechanical properties of geopolymer concrete (GC) exposed to dynamic compression under elevated temperatures. As the results have turned out, the weight loss is remarkable within temperature ranges from room temperature to 200 °C as well as from 600 °C to 800 °C. The dynamic compressive strength of GC grows higher at 200 °C than at room temperature, but suffers a dramatic drop at 800 °C. The critical strain is higher at elevated temperature than that at room temperature. At 200 °C and 600 °C, respectively, its energy absorption property is superior to that at room temperature. However, at 400 °C and 800 °C, respectively, it is inferior to that at room temperature. The strain rate effect of the dynamic increase factor (DIF) obtained from test data can reflect the inherent nature of GC. The DIF assumes a linear relationship with the logarithm of strain rate.     

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.     

Aspects of the deformational behaviour of alkali activated fly ash concrete at elevated temperatures

As a result of tightening of environmental legislation, there is a renewed interest in the technology of alkali activated concrete also known as geopolymer concrete (GPC). Yet, for GPC to assume its own niche in the construction market, it must display equal or better properties at elevated temperatures than Ordinary Portland Cement (OPC) concrete. Currently there is sufficient evidence that GPC has a better resistance to fire; however not all of its behavioural aspects at high temperatures have been investigated. The aim of this research therefore is to investigate its deformational behaviour, in particular the existence or lack of transient creep, which OPC is known to undergo under first time heating when subject to compressive stresses. It was found that not only GPC undergoes transient creep, but also exhibits both expansive and contractive volume changes during heating. It was also found that GPC maintains its structural integrity at 1000 °C.     

Tensile creep behavior of three-dimensional four-step braided SiC/SiC composite at elevated temperature

This article presents experimental results for tensile creep deformation and rupture behavior of three-dimensional four-step braided SiC/SiC composites at 1100 °C and 1300 °C in air. The creep behavior at 1300 °C exhibited a long transient creep regime and the creep rate decreased continuously with time. The creep behavior at 1100 °C exhibited an apparent steady-rate regime and the creep deformation was smaller than that at 1300 °C. However, the creep rupture time at both temperatures showed little difference. The mechanisms controlling creep deformation and rupture behavior were analyzed.     

Effect of the curing temperature on the creep of a hardened cement paste

Uniaxial creep tests have been performed at ambient temperature on a saturated hardened class G cement paste hydrated at 60 °C and 90 °C. The results have shown that creep is enhanced at higher curing temperature. The visco-damage model of Challamel et al. (2005) has been extended and used to analyze the experimental results. The model parameters have been calibrated on the results of the creep tests performed at different stress levels for both curing temperatures. The model correctly reproduces the effect of the curing temperature on time dependent properties of the material. The enhanced creep at higher curing temperature is attributed to the development of more damage in cement paste and to significant weakening of the mechanical properties.     

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.     

Fracture toughness,strength and creep of transparent ceramics at high temperature

Fracture toughness, four-point bending strength of transparent spinel, Y₂O₃ and YAG ceramics in function of temperature (from room temperature up to 1500° C) were measured. Creep resistance at 1500–1550° C was studied too. Grain size distribution was determined on polished and etched surfaces of samples. Fracture surfaces after tests were examined by scanning electron microscopy. The obtained results showed that: in the case of spinel ceramics fracture toughness and strength decreased from 20 to 800° C, increased from 800 to 1200° C and decreased at higher temperature; in the case of Y₂O₃ ceramics they increased from 400 to 800° C, and next kept constant up to 1500° C; in the case of YAG ceramics they kept constant from 20 to 1200° C and then decreased. The creep strain rate was measured for spinel and YAG but not for Y₂O₃ ceramics which appeared creep resistant. The hypotheses concerning toughening and creep mechanisms were proposed.     

Low temperature depolymerization and polycondensation of a slag-based inorganic polymer

In this paper, the depolymerization and polycondensation process, compressive strength, gel content and conductance of an alkali-activated slag (AAS) (an inorganic polymer or high-calcium geopolymer) paste cured at low temperature (−25 °C~25 °C) were studied. The results showed that gel content of the AAS inorganic polymer sample first increased and then decreased with the decreasing of the curing temperature for 24 h, reaching a maximum value of 37.8 wt% in the samples cured at 0 °C. Chemical structural data are obtained by nuclear magnetic resonance (NMR) spectroscopy for 6-coordinated (octahedral) aluminum can be observed at low temperature (<0 °C) and the gel contains primarily Q₁(1Al), Q₂(₁Al) and Q₃ at 0 °C. The inductively coupled plasma emission spectrometry (ICP) analysis of the products showed depolymerization of a great amount of slag at low temperature. The impedance analysis indicated that low temperature (＜0 °C) will inhibit the polycondensation process but almost not affect the depolymerization process so as to make a distinction between the depolymerization and polycondensation process in the AAS inorganic polymer system.     

Time-dependent behaviour of hardened cement paste under isotropic loading

The experimental results of isotropic compression tests performed at 20 °C and 90 °C on a class G hardened cement paste hydrated at 90 °C (Ghabezloo et al., 2008, Cem. Conc. Res. 38, 1424–1437) have been revisited considering time-dependent response. Within the frame of a viscoplastic model, the non-linear responses of the volumetric strains as observed in drained and undrained tests and of the pore pressure in undrained tests are analysed. The calibration of model parameters based on experimental data allows to study the effect of the test temperature on the viscous response of hardened cement paste showing that the creep is more pronounced for a higher test temperature. The effect of the hydration temperature on the time dependent behaviour is also studied by evaluating the model parameters for a cement paste hydrated at 60 °C. The time-dependent deformations are more pronounced for hydration at a higher temperature.     

Effect of composition and heat treatment conditions on the tensile strength and creep resistance of SiC-based fibers

Polymer-derived SiC-based fibers with fine-diameter (∼10–15 μm) and high strength (∼3 GPa) were prepared with carbon-rich and near-stoichiometric compositions. Fiber tensile strengths were determined after heat treatments at temperatures up to 1950 °C in non-oxidizing atmospheres and up to 1250 °C in air. The creep resistance of fibers was assessed using bend stress relaxation measurements. Fibers showed excellent strength retention after heat treatments in non-oxidizing atmospheres at temperatures up to 1700 °C for the carbon-rich fibers and up to 1950 °C for the near-stoichiometric fibers. The near-stoichiometric fibers also showed considerably better strength retention after heat treatments in air. Creep resistance of the as-fabricated fibers was greatly improved by high-temperature heat treatments. Heat-treated near-stoichiometric fibers could be prepared with ∼3 GPa tensile strengths and bend stress relaxation creep behavior which was significantly better than that reported for the Hi-Nicalon™ Type S near-stoichiometric SiC fibers.     

Fabrication and properties of thermal insulating material using hollow glass microspheres bonded by aluminum–chrome–phosphate and tetraethyl orthosilicate

Thermal insulation material made by hollow glass microspheres (HGM) with different content of aluminum–chrome–phosphate solution (ACP) and tetraethyl orthosilicate (TEOS) as binders was formed, dried and sintered at 250 °C, 450 °C or 650 °C for 2 h. Properties such as density, compressive strength, thermal conductivity and microstructure of the specimens were determined. It is found that TEOS improved the distribution of ACP and increased the compressive strength of the specimens. HGM bonded by appropriate amount of ACP and TEOS achieved preferable value of density, compressive strength and thermal conductivity which were significant for thermal insulation materials. The compressive strength of specimens sintered at 450 °C and 650 °C was higher than that of the specimens sintered at 250 °C.     

Coupled strain rate and temperature effects on the tensile behavior of strain-hardening cement-based composites (SHCC) with PVA fibers

This paper reports the experimental findings on the tensile behavior of Strain-hardening cement-based composites (SHCC) subjected to elevated temperatures and different strain rates and to combinations of these parameters. Uniaxial tension tests with in-situ temperature control were performed at 22 °C, 60 °C, 100 °C and 150 °C. In addition, the effect of loading rate was investigated using the strain rates of 10^? 5 s^? 1, 3 × 10^? 4 s^? 1 and 10^? 2 s^? 1 at all four temperatures considered. It was shown that tensile strength decreases both with an increase in temperature and with a decrease in the strain rate. The strain capacity increases with decreasing strain rate at temperatures of 22 °C and 60 °C, but for the temperature of 100 °C this material property increases when the strain rate increases. At 150 °C the investigated SHCC loses its ductility and no noticeable strain rate effect can be observed. Furthermore, the residual properties of SHCC were evaluated using uniaxial tensile tests at room temperature on the specimens which were previously heated to 60 °C, 100 °C or 150 °C. The residual tests showed that the strength, strain capacity, and work-to-fracture decrease with increasing pre-treatment temperature. However, in comparison with the results of the in-situ tests with elevated in-situ temperatures, the residual tests on SHCC yielded higher tensile strength and lower ductility. These results and possible mechanisms leading to changes in mechanical performance are discussed on the basis of the observed crack patterns on the specimens' surfaces as well as the microscopic investigations of the condition of fibers on fracture surfaces.     

Capillary rise properties of porous geopolymers prepared by an extrusion method using polylactic acid (PLA) fibers as the pore formers

The geopolymers were prepared from sodium silicate, metakaolinite, NaOH and H₂O at SiO₂:Al₂O₃:Na₂O:H₂O of 3.66:1:1:x, where x = 8–17, and curing temperatures of 70–110 °C. Since the bending strength of the geopolymers was highest (36 MPa) where H₂O/Al₂O₃ = 9 and the curing temperature = 90 °C, these conditions were adopted. The porous geopolymers were prepared by kneading PLA fibers of 12, 20 and 29 μm diameter into the geopolymer paste, at fiber volumes of 13–28 vol%. The resulting paste was extruded using a domestic extruder, cured at 90 °C for 2 days then dried at the same temperature. The PLA fibers in the composites were removed by alkali treatment and/or heating. The highest capillary rise was achieved in the porous geopolymers containing 28 vol% of 29 μm fibers. The capillary rise of this sample, estimated by the equation of Fries and Dryer¹ was 1125 mm.