SiC fines effects on the microstructure and properties of bauxite-based low-cement refractory castables

In the present paper, the effect of silicon carbide (SiC) fines was studied on the phase and microstructural properties of bauxite-based low-cement refractory castables (LCC) at different firing temperatures. XRD and SEM techniques were employed to evaluate phase and microstructure analysis, and physical/mechanical properties were measured according to standard methods. The results showed that the oxidation of SiC particles related to the oxygen partial pressure and active/passive oxidation behavior of SiC fines could markedly affect the microstructure and properties of bauxite-based LCC fired at high temperatures. Despite the negative influence of addition of SiC fines on cold mechanical strength, its usage up to 6 wt% could greatly improve the thermo-mechanical properties of LCC refractories due to increasing of matrix refractoriness.     

Mechanical properties of refractory castables bonded with hydratable magnesium carboxylate-boric acid

Hydratable magnesium carboxylate (HMC), which is similar to the properties of cement, can be used as a potential binder for refractory castables. However, its decomposition may lead to poor mechanical properties at medium temperatures (300 °C–1100 °C). This work investigated the effects of boric acid on the mechanical properties and microstructural evolution of castables bonded with hydratable magnesium carboxylate. The mechanical strength, bulk density, apparent porosity, thermal shock resistance, and sintering properties of the castables were evaluated. The results showed that the mechanical properties of HMC-bonded castables (HMCC) at various temperatures can be improved by adding boric acid. Boric acid reacts with HMC to form magnesium carboxylate borate ester (MCBE), which improves the bonding strength between HMC molecules. Thus, the cold modulus of rupture of HMCC containing boric acid dried at 110 °C are higher than that of calcium aluminate cement-bonded refractory castables (CACC). The decomposition temperature of MCBE is 77 °C higher than that of HMC, so MCBE can endow castables with better mechanical properties at 110 °C–500 °C. The B₄C obtained by MCBE pyrolysis could form a boron-rich liquid phase, which can accelerate the structural densification of castables via transient liquid phase sintering, thus improving the mechanical properties of castables at 500 °C–1100 °C. Moreover, boric acid can improve the thermal shock resistance of HMCC. The residual strength rate first increases and then decreases with an increasing boric acid, and reaches a maximum value of 29.7% (1 wt% boric acid is added), which is 2.3 times that of the CACC. The nanoindentation test showed that the microcracks in the matrix of 1 wt% boric acid castables are easy to initiate but difficult to propagate, so the microcracks are many and wavy.     

Effect of micro-spinel powders addition on the microstructure and properties of alumina refractory castables

The microstructural evolution and comprehensive properties of alumina refractory bonded with calcium aluminate cement and silica sol have been studied. Results have been correlated with the microstructural and phase evolutions using X-ray diffraction and scanning electron microscopy, as function of the pre-formed spinel powders. Matrix samples were obtained for phase and microstructural characterization in details, and the results were compared with those corresponding to the refractory castables. The room temperature and high temperature properties, including permanent of linear change, mechanical properties, hot modulus of rupture (HMOR) and slag resistance were measured. The castables exhibited a microstructural optimization and properties enhancement due to the addition of pre-formed spinel. A lot of secondary spinel and CA₆ (CaO·6Al₂O₃) with small size were produced in the castables, and the contents of micro pores were greatly increased. As a result, the permanent of linear change of castables was decreased by 61%, while cold modulus of rupture (CMOR) and HMOR were increased more than 45% and 100%, respectively. The penetration indexes in the static slag resistance were decreased from 28.8% to 12.2%.     

Room temperature mechanical properties of high alumina refractory castables with spinel, periclase and dolomite additions

The mechanical properties of refractory castables at room temperature are critical parameters for selecting suitable operating conditions for the structural design of refractory components. In this work, high alumina refractory castables based on the alumina-rich zone of the Al₂O₃–MgO–CaO ternary phase equilibrium diagram were prepared by adding synthetic spinel, periclase and dolomite via three processing routes. Bending strength studies at room temperature under several thermal treatments and the analysis of the elastic modulus of the refractories and their matrices point to two different mechanical behaviours. From room temperature to 1000 °C the refractory castables present a pronounced non-linear stress–strain behaviour both in the uniaxial tensile and compressive modes, as a result of damage to the microcrack network. Above 1000 °C the refractory castables begin to sinter owing to a transitory liquid phase, the crystallization of calcium aluminate cement phases (such as CA₂ and CA₆, for example) and the self-forming spinel phase (refractory castables with periclase or dolomite additions). At higher firing temperatures the sintering process leads a strengthening of the mechanical properties.     

Microstructure and refractory properties of spinel containing castables

The bauxite-based and kaolin-based refractory castables investigated were carefully prepared. They are composed of 90 wt.% well-graded (coarse, medium, and fine) bauxite or kaolin aggregates, 10 wt.% binding matrix and adequate amount of distilled water. The binder mixture was calcium aluminate cement (CAC) containing 80% alumina and magnesium-aluminate spinel (MA-spinel) either preformed or in situ. The castable batches were cast into cubes (25 mm × 25 mm × 25 mm), left at 100% relative humidity for 24 h cured for 7 days under water, and dried at 110 °C for 24 h. The samples were then subjected to firing at 1550 °C for a soaking time of 1 h.     

Comparison of natural and manufactured fine aggregates in cement mortars

The performance of cement mortars using manufactured fine aggregates produced by cone crushing or impact crushing has been compared to that of mortars prepared from a natural sand control-sample. Samples from both crusher products have been additionally subjected to classification for partial removal of fines, being also used in preparing mortars. Particle shape analyses indicated that material produced by impact crushing presented intermediate sphericity and aspect ratio, between those found in natural fine aggregate and cone-crushed material, and that the aspect ratio of the cone-crushed material increased for finer particle sizes. The unclassified impact crusher product presented the highest packing density, and mortars produced from it had comparatively low porosity and low absorptivity and the highest unconfined compressive strength. The classified product from cone crushing presented low packing density and mortars were characterized by the highest porosities, absorptivities and lowest unconfined compressive strength, probably mostly due to its poor particle shapes. Modeling of the stress-strain response with scalar damage mechanics showed that manufactured aggregate produced from classification of the cone crusher yielded a mortar with highly inelastic deformation response, whereas mortars produced from unclassified product of impact crushing showed more elastic deformation response. Results were also analyzed in light of de Larrard's Compressible Packing Model.     

On the relationship between ceramic strength and the requirements for mechanical design

During the last fifty years the mechanical properties of ceramic materials have been greatly improved, their toughness and strength have been increased and the scatter of strength decreased. Adequate statistical design procedures for brittle materials exist but cracking and brittle fracture of ceramic components still occur very often.In this review the theory of brittle fracture and the underlying assumptions are critically discussed and the measurement procedures of strength are reviewed. It is shown that the strength of materials, the strength of specimens and the strength of components are often quite different properties. Three main factors are identified which – in order to avoid unexpected failure of components – have to be considered much more than in the past: (i) hidden stresses, i.e. stresses caused by thermal strain mismatch, by contact (for example in joints) and internal stresses, (ii) the quality of the component's surfaces and edges and (iii) proper handling of ceramic materials and components.It can clearly be stated that the mechanical properties of many ceramic materials are appropriate even for applications under severe loading conditions but bad or incomplete mechanical design, insufficient surface finish and mishandling are the main reasons for unexpected failure of ceramic components.     

焦炭强度指标间关系探讨

焦炭的冷态强度测定周期较短?成本低廉,而热态性能测定周期相对较长?成本昂贵。因此,利用焦炭冷态强度预测热态性能,不但对指导焦化生产具有一定的意义,而且利用各强度指标间的关系可以检验实际测量数据是否存在异常值。     

Microstructure and phase evolution of alumina-spinel self-flowing refractory castables containing nano-alumina particles

The microstructure and phase composition of alumina-spinel self-flowing refractory castables added with nano-alumina particles at different temperatures are investigated. The physical and mechanical properties of these refractory castables are studied. The results show that the addition of nano-alumina has a great effect on the physical and mechanical properties of these refractory castables. With the increase of nano-alumina content in the castable composition, the mechanical strength is considerably increased at various temperatures. It is shown that nano-alumina particles can affect formed phases after firing. The platy crystals of CA₆ are detected inside the grain boundaries of tabular alumina and spinel grains in samples fired at 1500 °C. CA₆ phase can be formed at lower temperatures (1300 °C) with the addition of nano-alumina particles. As a result of using nanometer-sized alumina particles with high surface area, the solid phase sintering of the nano-sized particles and CA₆ formation can occur at lower temperatures.     

The effect of recycled concrete aggregate properties on the bond strength between RCA concrete and steel reinforcement

The purpose of this study was to investigate the influence that replacing natural coarse aggregate with recycled concrete aggregate (RCA) has on concrete bond strength with reinforcing steel. Two sources of RCA were used along with one natural aggregate source. Numerous aggregate properties were measured for all aggregate sources. Two types of concrete mixture proportions were developed replacing 100% of the natural aggregate with RCA. The first type maintained the same water–cement ratios while the second type was designed to achieve the same compressive strengths. Beam-end specimens were tested to determine the relative bond strength of RCA and natural aggregate concrete. On average, natural aggregate concrete specimens had bond strengths that were 9 to 19% higher than the equivalent RCA specimens. Bond strength and the aggregate crushing value seemed to correlate well for all concrete types.     

Experimental study on the failure mechanism of recycled concrete

In this study, Young's modulus, strength, and peak strain of recycled concrete under both compressive and tensile loading were experimentally studied to understand its failure mechanism. Due to the different colors of natural aggregates, old hardened mortar, new hardened mortar, and interfacial transition zone (ITZ), the quantity and the distribution of each phase were analyzed by images processing and analysis of cut sections. With the tests, the failure processes and crack situation of the recycled concrete under tensile and compressive loadings were illustrated. It was found that some mechanical properties of recycled concrete are similar to those of mortar, for instance, lower Young's modulus, higher peak strain and more brittleness, due to a larger volume content of both new and old hardened mortar. When compared with old hardened mortar, new hardened mortar has more significant influence on both the strength and the Young's modulus of recycled concrete.     

刚玉质浇注料孔径分布及其与强度的定量关系

以板状刚玉骨料(6～3,3～1和≤1 mm)和细粉(≤0.088和≤0.045 mm)、铝酸钙水泥和α-Al2O3微粉(d50=1.754μm)为原料,制备了α-Al2O3微粉质量分数分别为0、2%、4%、6%和8%的刚玉质浇注料。测定了浇注料分别经110、1 100和1 500℃热处理后的冷态强度和气孔孔径分布。利用Atzeni方程验证浇注料强度与其显气孔率和气孔中位径之间的定量关系,并借助灰色关联理论分析了不同气孔孔径区间与浇注料强度的相关性。研究发现:浇注料气孔孔径分布受其内部微粉填充效应和基质烧结效应的共同影响,微粉的引入使其气孔中位径下降,而烧结则使其气孔中位径增大;Atzeni方程经添加常数修正项后能较好地描述在相同温度下热处理后浇注料的强度与其气孔中位径之间的关系;不同孔径区间与浇注料强度的灰色关联度有明显差异,且同一孔径区间与其强度灰色关联度的大小也随热处理温度的变化而变化。在整个热处理温度范围内,<0.5μm的气孔孔径区间与浇注料强度的灰色关联度最大。     

Stress versus strain relationship of high strength concrete under high lateral confinement

A common application of high strength concrete (HSC) is in columns subjected to large compressive forces. However, a major problem is the insufficient ductility available in HSC columns. To determine the required lateral reinforcement to maintain sufficient ductility, a good understanding of the stress-strain behaviour of confined concrete needs to be established. This paper describes a testing program carried out to obtain experimental data of complete (ascending and descending) stress-strain relationships between axial stress, axial strain and lateral strain for HSC. Compressive strengths of concrete tested were 100 MPa and 60 MPa. The confining pressures used were 4 MPa, 8 MPa and 12 MPa. A total of 18 stress-strain curves are presented. The experimental results obtained seem to indicate that, for high confining pressures, the lateral strain at peak stress for 100 MPa concrete was 20% less than that of the 60 MPa concrete.     

Modelling the relationship between tensile strength and porosity in bioceramic scaffolds

A model describing the relationship between tensile strength and total porosity in brittle open-cell macroporous foams is developed and applied to silicate ceramic scaffolds produced by sponge replication and subsequent sinter-crystallization. The tensile strength of the scaffolds decreased from 7.4 to 2.3 MPa as the total porosity increased from 0.40 to 0.79. The results of the model, which is based on the concepts of fracture mechanics, were in good agreement with the experimental data (R² = 0.88), which supports the good predictive capability of the approach presented. In principle, this model could help biomaterials scientists not only to estimate the tensile strength of highly-porous bioactive glass and ceramic scaffolds, which is often difficult to determine experimentally, but also to improve the rational design of porous bioceramics with customized properties.     

Coarse-grained refractory composites based on Nb-Al2O3 and Ta-Al2O3 castables

Coarse-grained refractory composites, with grain sizes between 20 and 5000?µm, based on Nb-Al₂O₃ and Ta-Al₂O₃ castables were produced for the first time and characterised in terms of shrinkage after sintering, splitting tensile strength, compressive strength, porosity and the measurement of the elastic properties (E, G and $\nu$). After sintering at a temperature of 1600?°C, the shrinkage of the composites was 1.5% and 0.3%. Measured values of splitting tensile strength were between 5.5 and 15?MPa and the ones of compressive strength were between 23 and 89?MPa. Values of E and G were between 117 and 45?GPa and 48–17?GPa, respectively, for samples with 11–45?vol% refractory metal. Poisson's ratio was found to be very sensitive to the bonding between the fine matrix and coarse-grained particles of the composites. Its value increased from 0.25 for good bonding to $\nu =0.43$ in case of poor bonding between the coarse metal particles and the fine ceramic matrix. DTA/TG measurement under air atmosphere showed that the metal-ceramic composites start to oxidise at temperatures above 450?°C.     

Effect of microporous magnesia aggregates on microstructure and properties of periclase-magnesium aluminate spinel castables

The present study investigated three lightweight periclase-magnesium aluminate spinel castables containing microporous magnesia aggregates with a varying apparent porosity (12.8%, 30.8% and 39.3%). The effect of the apparent porosity of the aggregates on the phase composition, microstructure, fracture behavior and strength of the lightweight castables was investigated by XRD, SEM and three-point bending tests. Large cracks between the aggregates with an apparent porosity of 12.8% and the matrix reduced the strength of the castable. For the aggregates with an apparent porosity of 30.8%, an excellent interlocking interface with the matrix increased the strength considerably, but also reduced the fracture toughness. At the highest level of the apparent porosity of the aggregates of 39.3%, the formation of a small number of microcracks between the aggregates and matrix reduced the strength, while the fracture toughness was only slightly affected. The lightweight castables with the best combination of properties were achieved at an apparent porosity of the aggregates of 30.8% since they had a low bulk density of 2.63 g/cm³ as well as a high compressive and flexural strength of 70.2 MPa and 20.9 MPa, respectively.     

Influences of novel Si2BC3N antioxidant on the structure and properties of Al2O3-SiC-C castables: In air and coke bedded atmosphere

The ultra-low cement bonded Al₂O₃-SiC-C castables were prepared with the introduction of a novel Si₂BC₃N antioxidant. The microstructure evolution and the mechanical properties were evaluated in coke bed and air atmosphere. Besides, the thermal properties, including thermal shock, hot modulus of rupture, oxidation and refractories under load, were comparatively investigated. The results show that the Si₂BC₃N powder together with B₄C and Si can satisfy the oxidation resistance requirements over a full temperature range. Si₂BC₃N has mainly two effects depending on the treating temperature: 1) it protects the carbon from oxidation and increases the structure integrity when the specimens are treated below 1100?°C; 2) it stimulates the growth of SiC whiskers under 1400?°C due to the enhanced reaction between SiO and CO. Consequently, the CMOR and CCS of the Si₂BC₃N containing specimens have been improved attributing to the structural integrity and more SiC whiskers formation, regardless of the treating atmosphere. Besides, the thermal properties such as the hot modulus of rupture, thermal shock and refractories under load are also optimized with Si₂BC₃N addition.     

Three-dimensional shape analysis of coarse aggregates: New techniques for and preliminary results on several different coarse aggregates and reference rocks

The shape of aggregates used in concrete is an important parameter that helps determine many concrete properties, especially the rheology of fresh concrete and early-age mechanical properties. This paper discusses the sample preparation and image analysis techniques necessary for obtaining an aggregate particle image in 3-D, using X-ray computed tomography, which is then suitable for spherical harmonic analysis. The shapes of three reference rocks are analyzed for uncertainty determination via direct comparison to the geometry of their reconstructed images. A Virtual Reality Modeling Language technique is demonstrated that can give quick and accurate 3-D views of aggregates. Shape data on several different kinds of coarse aggregates are compared and used to illustrate potential mathematical shape analyses made possible by the spherical harmonic information.     

Non-destructive monitoring of fiber orientation using AC-IS: An industrial-scale application

A comprehensive study has been undertaken to investigate the ability of AC-impedance spectroscopy (AC-IS) to non-destructively monitor the fiber dispersion of conductive fiber-reinforced cement-based materials. Previous work showed that AC-IS effectively monitors various fiber dispersion issues in lab-scale steel fiber-reinforced specimens. In this part of the study, AC-IS was used to study fiber orientation in an industrial-scale pre-cast concrete beam. A conventional method-image analysis (IA)-was used to verify the results of AC-IS measurements. The results of AC-IS and IA were found to match very well in experimental uncertainty. Splitting tensile tests and bending tests were conducted on the parts of the beam to study the effects of fiber orientation on the mechanical performance. The results of the mechanical tests also confirmed the results of AC-IS with splitting tensile strengths increasing as the alignment of fibers increased.     

Enhanced toughness and strength of boron carbide ceramics with reduced graphene oxide fabricated by hot pressing

Boron carbide (B₄C) hybrids with different contents of graphene oxide (GO) were prepared by a heterogeneous co-precipitation method using cetyltrimethyl ammonium bromide (CTAB) as the cationic surfactant. The as-obtained mixtures were further hot-pressed at 1950 °C for 60 min under 30 MPa, by which B₄C–reduced GO (rGO) composites were fabricated. It was found that the addition of only 0.5 wt% rGO could alter the predominance of trans-granular fracture in monolithic B₄C ceramic material to mixed trans-granular and inter-granular modes in B₄C–rGO composites. The flexural strength and fracture toughness of the B₄C–2 wt% rGO were increased by 31% (from 350 to 455 MPa) and 83% (from 3.20 to 5.85 MPa·m^1/2), respectively, compared with those of pure B₄C. The improved mechanical properties are attributed to the mechanisms of pull-out and bridging of rGO and crack deflection, as evidenced by microstructural observations. The energy dissipation in the present B₄C–rGO composites was further verified using two micromechanical models.