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This study investigates the effects of nanofillers on the interfacial transition zone (ITZ) between aggregate and cement paste by using nanoindentation and statistical nanoindentation techniques. Moreover, the underlying mechanisms are revealed through micromechanical modeling. The nanoindentation results indicate that incorporating nanofillers increases the degree of hydration in the ITZ, reduces the content of micropores and low-density calcium silicate hydrate (LD C-S-H), and increases the content of high-density C-S-H (HD C-S-H) and ultrahigh-density C-S-H (UHD C-S-H). In particular, a new phase, namely nano-core-induced low-density C-S-H (NCILD C-S-H), with a superior hardness of 2.50 GPa and an indentation modulus similar to those of HD C-S-H or UHD C-S-H was identified in this study. The modeling results revealed that the presence of nanofillers increased the packing density of LD C-S-H and significantly enhanced the interaction (adhesion and friction) among the basic building blocks of C-S-H gels owing to the formation of nano-core-shell elements, thereby facilitating the formation of NCILD C-S-H and further improving the performance of the ITZ. This study provides insight into the effects of nanofillers on the ITZ in concrete at the nanoscale.

Utilizing coral aggregate concrete (CAC) for construction on remote islands can significantly reduce construction cost and period, CO2 emission, and consumption of non-renewable energy. The durability of reinforced CAC structures is critically influenced by their resistance to chloride attack. In this study, a reactive transport modelling was developed to investigate chloride ingress in CAC, in which a COMSOL-PHREEQC interface based on MATLAB language was established. The experiment from the literature was taken as a benchmark example. The results show that the developed numerical model can accurately predict chloride transport in CAC. Differing from ordinary aggregate concrete (OAC), Kuzel’s salt does not appear in cement hydrate compounds of CAC during chloride ingress. The numerical results indicate that the penetration depth of chloride in CAC gradually increases as the exposure time is prolonged. When CAC is exposed to an external chloride solution, the decrease in the pH of the pore solution affects the precipitation of Friedel’s salt, which is detrimental to the chemical binding of chloride.

Energy conservation requires next-generation thermal-insulating materials featured with multiple functions (e.g., ultralight, anticorrosion, mechanically resilient, and highly flame-retardant); how...

We herein report the effects of Sm doping on the thermoelectric performance of (Bi0.2Sb0.8)2(Te0.97Se0.03)3 compounds prepared by a growth-from-the-melt spark-plasma-sintering procedure. In conjunction with micro-morphology and compositional analyses, the electrical conductivity, Seebeck coefficient, and thermal conductivity of (Bi0.2-xSmxSb0.8)2(Te0.97Se0.03)3 samples were measured as a function of temperature (298–473 K) and the Sm doping ratio (x = 0%, 0.2%, 0.4%, and 0.8%). We found that (i) there was no discernible crystal lattice symmetry change upon Sm doping; (ii) Sm doping generally increased the electric conductivity and practically retained the Seebeck coefficient, thereby yielding higher power factors; and (iii) specially, a minimum thermal conductivity ∼ 0.92 Wm−1K−1 was attained in the x = 0.4% sample at 423 K, resulting in a state-of-the-art ZT value ∼ 1.22, which is ∼25% higher than the maximum ZT value of pristine sample. Contrary to the customary view that Sm dopant adopts a trivalent state Sm3+, the variations of lattice constant pointed toward a lower valence state of Sm. The results demonstrated the efficacy and deepened our understanding of rare earth doping in thermoelectric study of Bi2Te3-based materials.

The high autogenous shrinkage of alkali-activated materials made from slag and fly ash is recognised as a major drawback with regard to the use as construction materials. In this study, metakaolin was introduced into the alkali-activated slag-fly ash (AASF) paste to mitigate the autogenous shrinkage. The shrinkage mitigation mechanism of metakaolin was explained by studying the influences of metakaolin on the microstructure, shrinkage related properties, and mechanical properties of AASF paste. It was found that adding metakaolin could significantly reduce the chemical and autogenous shrinkage of AASF paste. This shrinkage mitigation is accompanied by a decrease in the alkalinity of AASF paste pore solution, a reduced drop in internal relative humidity, and an increase in porosity of AASF paste. Moreover, the incorporation of metakaolin does not change the type of the reaction products, but greatly delays the formation of the reaction products of AASF paste. The addition of metakaolin, above 5% of the binder, results in lower 28-day compressive and flexural strength of AASF paste.

Shrinkage-induced cracking can impair the durability of concrete structures. In the past few decades, this topic has drawn more and more attention. Shrinkage of mortar and concrete is actually the result of the interaction between the shrinking cement paste and the non-shrinking aggregates. In recent years, different models that consider the restraining effect of aggregates, i.e. Series model and Hobbs’ model, have been proposed to predict the autogenous shrinkage of mortar and concrete. However, in these models both aggregate particles and cement paste matrix are considered as elastic materials. In fact, cement paste is not ideally elastic. Creep also plays an important role in autogenous shrinkage of mortar and concrete. In this paper an extended Pickett model, which takes the effect of creep into consideration, is proposed. The autogenous shrinkage of CEM I and CEM III/B cement mortar and concrete is simulated by using this model and compared with the experimental results to evaluate the accuracy of the predictions. The results show that the extended Pickett model can well predict the autogenous shrinkage of mortar and concrete.

This study aims to predict the autogenous shrinkage of alkali-activated concrete (AAC) based on slag and fly ash. A variety of analytical and numerical models are available for the prediction of autogenous shrinkage of ordinary Portland cement (OPC) concrete, but these models are found to show dramatic discrepancies when applied for AAC due to the different behaviours of these two systems. In this study, a new numerical approach is developed to predict the autogenous shrinkage of alkali-activated slag (AAS) and alkali-activated slag-fly ash (AASF) concrete from the experimental results on corresponding paste. In this approach, the creep of AAS and AASF and the restraining effect of the aggregate are particularly considered. By this approach, a fairly good prediction is obtained. Moreover, the microcracking in paste caused by restraining aggregates is evaluated. The results indicate that AAC is subjected to high tendency of development of microcracking.

Impressed current cathodic protection (ICCP) is an effective and direct method for controlling the corrosion of reinforced concrete (RC) structures. However, few investigations related to ICCP in cracked RC structures have been reported. In this study, the effect of cracks in concrete cover on ICCP of chloride-contaminated RC structures was investigated through a numerical model including steel polarisation, electrode reactions, and ionic migration. In the developed numerical model, cracked concrete cover is assumed to consist of sound concrete and cracks, and cracks have their own ionic diffusion coefficients. The results indicate that the ICCP can maintain its ability to remove Cl− if concrete cover does not completely crack. Once the complete cracking in concrete cover occurs, the Cl− removal ability of ICCP would decrease or even disappear. Cracking does not cause any adverse effect on the pH improvement of ICCP. In this case, a stronger cathodic polarisation is recommended.

This study aims to compare the developments of mechanical properties and autogenous shrinkage related properties of alkali-activated materials-based concrete (AC) and ordinary Portland cement-based concrete (OC) against curing age and degree of reaction. Temperature Stress Testing Machines are utilized to monitor the evolution of the internal tensile stress and the cracking occurrence in the restrained concrete. It is found that AC shows lower tensile strength-to-compressive strength ratios than OC. The mechanical properties of both OC and AC can be modelled by a power law against the degree of reaction. AC shows higher autogenous shrinkage, but later cracking than OC when under restrained condition. However, the degrees of reaction at which AC and OC cracked are very similar. From the autogenous shrinkage, the elastic modulus and the self-induced stress, the elastic and creep deformations of the concrete can be calculated. AC is found to show much higher creep coefficient than OC., SCOPUS: ar.j, info:eu-repo/semantics/published

The influence of heat treatment on the strain-controlled fatigue behavior of cast NZ30 K alloy was investigated. Compared with the as-cast and solutionized (T4) alloys, the peak-aged (T6) and over-aged (T7) counterparts have a higher cyclic stress and a lower plastic strain value due to the precipitation strengthening. The as-cast and T4-treated alloys have a higher fatigue strength/yield strength ratio than the aged alloys, which is mainly attributed to their higher cyclic hardening. Under stress-controlled loading, the aged alloys show lower hysteresis energies than the as-cast and T4-treated counterparts, leading to longer fatigue lifetimes. For the T4-treated alloy, the cyclic hardening and fatigue failure are controlled by the dislocations-slip and twinning, while for both the as-cast and T6-treated counterparts, they are controlled by the dislocation-slip. For the T7-treated alloy, cyclic deformation and failure behavior are mainly dependent on dislocations-slip and grain boundary sliding.

Oil flooded compression has been found to be able to improve the energy efficiency of vapor injection cycle significantly by reducing the specific compression work. In order to optimize the coupling and study the thermodynamic characteristics, six coupled cycles were proposed and studied in heating conditions. A detailed analysis on each cycle using R32 working fluid was conducted. COP and heating capacity at various conditions were obtained and compared to the respective values of the vapor injection cycle. The respective COP improvement of oil flooded compression in the coupled cycles were investigated and compared to that of oil flooded compression in the single stage vapor compression cycle. The effects of oil temperature and regenerator efficiency on the cycles were also investigated and revealed. The results can provide theoretical guide to the design of a coupled cycle system.

Environmental energy harvesting based on flexible thermoelectric generator (f-TEG) provides an ideal micro-power source to drive the node sensors for the Internet of Things (IoT) and wearable electronics. However, the existing flexible thermoelectric devices have some disadvantages, such as low strength, large thermal resistance, complex manufacturing processes, and low reliability. Here, we report a high-performance f-TEG with a dimension of 2×16×0.6 mm3, encapsulating 18 pairs of thermoelectric legs with the size of 0.38×0.38×0.38 mm3. The flexible polyimide (PI) films with patterned electrodes act as the substrates, of which the upper one is cut into blocks to ensure the generator a minimum bending radius of 4.5 mm. The f-TEG can produce an open circuit voltage of 236 mV and output power of 4.19 mW under a temperature difference of 50 K, which remain almost unchanged even after 7400 times of bending tests. The power density reaches up to 13.1 mW/cm2 at ΔT =50 K, and the normalized output power density is 5.26 μW/cm2·K2. These results are among the best performances reported for f-TEGs, which opens a new avenue for flexible micro-power system design and promotes the development of the next generation self-powered sensors and charge-free electronic devices.

Alternative current impedance spectroscopy (ACIS) is a promising non-destructive testing method to monitor long-term change and assess the durability of concrete. This study investigates the influences of Ethylene Diamine Tetraacetic Acid (EDTA) on the hydration of hardening cement by ACIS. It is found that EDTA retards the early-age hydration of cement but can facilitate the later age reaction. Pastes with EDTA show comparable or higher compressive strength than Control at 28 d, especially when the dosage is higher than 0.4%. Microstructural characterization results reveal the working mechanism of EDTA originating from its complexing effect on free ions. The resistivity evolution of the pastes detected by ACIS can well reflect the effects of EDTA on the cement hydration in different ages. Proportional relations are identified between the resistivity and other hydration parameters, such as reaction degree, chemical shrinkage, compressive strength. The results of this study indicate a wider prospect of ACIS in monitoring the microstructure evolution and macro-properties of cementitious materials.

Oil flooded compression is able to improve the performance of vapor compression cycle by approaching a quasi-isothermal compression process. In conventional Oil Flooded Compression Cycle (OFCC), oil is usually injected in the compression chambers just after the compression pockets are sealed off from the suction chambers. In this research, OFCC with intermediate pressure port (OFCC-IPP) is proposed. The performance characteristics of OFCC-IPP with and without regenerator using R410A and R32 working fluids are investigated and compared to conventional OFCC with and without regenerator. Theoretical results show that the improvement of ratios of COPc and cooling capacity in the OFCC-IPP systems can be up to 4.5% an 5.3% respectively compared to the OFCC systems under the operating conditions of 40 °C condensing temperature and evaporating temperature in the range of −25 °C and 5 °C. Besides, the performance improvements of the OFCC and OFCC-IPP systems are different for the R410A and R32 working fluids. The results suggest that OFCC-IPP has huge potential for applications as an alternative to the conventional OFCC.

Composites of nominal composition (PbTe)1−x(CoTe)x (x = 0–0.18) were fabricated by high pressure (6 GPa) sintering (773 K) method. The thermoelectric performances were investigated in the temperature range of 293–773 K. The experimental results show that CoTe utilized as the secondary phase can remarkably enhance the TE properties of PbTe, of which the highest ZT value reaches 0.88 at 473 K when x = 0.14. The enhancement of TE performance owes much to its high electric conductivity of CoTe. Meanwhile, the high pressure sintering (HPS) samples consist of nanoparticle, which significantly enhances the boundary scattering on carriers, decreases thermal conductivity, and increases Seebeck coefficient. All the results indicate that HPS method and the addition of CoTe-composite are effective methods to enhance the thermoelectric performance of PbTe as a potential TE material.

One-part alkali-activated slag (AAS) binders are more promising in large-scale constructions because one-part mixing procedure is safer and easier to handle compared to normal two-part method. Thus, this study aims at investigating different properties of one-part AAS binders and control samples prepared using the two-part method. Experimental results of the former suggested the hardening time was greatly extended, but the workability showed little difference. Besides, compared to the control, one-part AAS binders had similar early compressive strength (within 7 days) and flexural strength in all tested curing stages but lower later compressive strength (from 28 days). The characterisation of the porous structures suggests that there were fewer C-A-S-H gels for one-part binders evidenced by less volume of mesopores but with larger amount of bigger pores. In addition, mineralogical and microstructural analyses imply that there was no hydrotalcite formed in the one-part AAS binders. Moreover, one-part AAS binders are more susceptible to efflorescence, presumably further affecting their surface and long-term durability.

This paper aims to investigate the effect of nano/micro-structures on anti-impact property of nano-boron nitride (nano-BN) filled cementitious composites, and to predict the dynamic mechanical behavior of the composites by introducing a damage constitutive model. It is found that nano-BN can fundamentally modify the micro- and even nano-structures of cementitious composites, and therefore greatly improve the anti-impact property of the composites at the macro level. Specifically, nano-BN refines the pore structure (especially at nano-scale) of composite matrix and increases its compactness. Furthermore, by enhancing polymerization state of silicate tetrahedron in C-S-H gel, nano-BN improves the micromechanical properties of hydration products at different micro-zones. Consequently, dynamic mechanical property of cementitious composites at macro-level is enhanced, and the energy absorption capacity is improved. In addition, dynamic damage model indicates that nano-BN inhibits the damage of cementitious composites, but its inhibition form changes with the strain rate.

The present paper compares the low cycle fatigue characteristics between the solution-treated (T4) and peak-aged (T6) Mg-3Nd-0.2Zn-0.5Zr magnesium alloys produced by semi-continuous casting. The cyclic stress amplitudes of the T6-treated counterpart are higher than those of the T4-treated alloy, which is due to precipitate strengthening. At the same stress amplitude, the T4-treated alloy with higher hysteresis energies undergoes more fatigue damage, resulting in shorter fatigue lives. For the T4-treated alloy, the cyclic stress amplitude is found to increase with increasing the cycle number, which is attributed to its soft matrix and the increased volume fraction and number density of twins under continuous loading. Cyclic hardening followed by the decrease of the stress is observed in the T6-treated alloy. It is shown that the cyclic deformation behavior and failure mechanism of the T4-treated alloy are dependent on the dislocations-slip plus twinning, while those of the T6-treated counterpart are dependent on the dislocations-slip. Based on discussion, the Coffin-Manson law and Basquin equation or the energy-based concepts can be well applied to the prediction of the LCF lives of the NZ30K alloys.

Effects of natural aging and test temperature on the tensile behaviors have been studied for a high-performance cast aluminum alloy Al–10Si–1.2Cu–0.7Mn. Based on self-strengthening mechanism and spheroidization microstructures, the alloy tested at room temperature (RT) exhibits higher 0.2% proof stress (YS) of 206 MPa, ultimate tensile strength (UTS) of 331 MPa and elongation of 10%. Increasing aging time improves the YS and UTS and reduces the ductility of the alloy. Further increasing aging time beyond 72 h does not significantly increase the tensile strengths. Increasing test temperature significantly decreases the tensile strengths and increases the ductility of the alloy. The UTS of the alloy can be estimated by using the hardness. The Portevin–Le Chatelier effect occurs at RT due to the interactions between solid solution atoms and dislocations. Similar behaviors occurring at 250 °C are attributed to dynamic strain aging mechanism. Increasing aging time leads to decrease in the strain-hardening exponent ( n ) value and increase in the strain-hardening coefficient ( k ) value. Increasing test temperature apparently decreases the n and k values. Eutectic phase particles cracking and debonding determine the fracture mechanism of the alloy. Final failure of the alloy mainly depends on the global instability (high temperature, necking) and local instability (RT, shearing). Different tensile behaviors of the alloy are mainly attributed to different matrix strengths, phase particle strengths and damage rate.

The fatigue characteristics of the AZ91D-T6 alloy samples taken from engine blocks have been investigated at 20 °C and elevated temperature (150 °C). The fatigue strength and cyclic stress amplitude of the alloy significantly decrease with the increase of the test temperature, although cyclic hardening occurs continuously until failure for both temperatures. With the increase of the temperature, the decreased fatigue life of the alloy tested at the same stress amplitude is mainly attributed to the decreased matrix strength and the increased hysteresis energies. Fatigue failure of the engine blocks made of AZ91D-T6 alloy is mainly controlled by casting defects. For the defect-free specimens, the crack initiation behavior is determined by the single-slip (20 °C) and by environment-assisted cyclic slip (150 °C) during fatigue, respectively. The low-cycle fatigue lives of the alloy can be predicted using the Coffin-Manson relation and Basquin laws, the three-parameter equation and the energy-based concepts, while the high-cycle fatigue lives of the alloy fitted well with the developed long crack life model and MSF life models. Keywords: Magnesium alloy, Engine blocks, Fatigue properties, Cyclic deformation behavior, Temperature, Fatigue life prediction

In this study, internal curing by superabsorbent polymers (SAP) is utilized to mitigate self-desiccation and autogenous shrinkage of alkali-activated slag (AAS) pastes. Absorption and desorption kinetics of SAP incorporated in AAS pastes were studied with X-ray tomography. Internal curing delayed the peak of the rate of heat liberation but increased the total reaction degree of AAS pastes. Internal curing by SAP mitigated effectively the drop of internal relative humidity and the self-desiccation-induced autogenous shrinkage of AAS pastes. The cracking tendency of AAS pastes undergoing shrinkage in restrained conditions was also significantly reduced with SAP. Nonetheless, adding SAP, regardless of their content, cannot eliminate the autogenous shrinkage of AAS pastes, suggesting the existence of other autogenous shrinkage mechanism(s) besides self-desiccation., Cement and Concrete Research, 135, ISSN:0008-8846

This study investigates the influences of internal curing on reducing the autogenous shrinkage of alkaliactivated slag/fly ash (AASF) paste. The influences of internal curing with superabsorbent polymers (SAPs) on the reactions and microstructure of AASF paste are investigated. It is found that the SAPs absorb liquid mainly before the initial setting time of the paste. Afterwards, the liquid is gradually released, keeping the internal relative humidity of the paste close to 100%. The internal curing with SAPs can significantly mitigate the autogenous shrinkage of AASF paste, especially after the acceleration period of the reaction. The mitigating effect of internal curing is due to the mitigated self-desiccation in the paste, rather than the formation of a denser microstructure or expansive crystals. The cracking potential of AASF under restrained condition is also greatly mitigated by internal curing. Despite the slight reductions in the elastic modulus and the compressive strength, great improvement is obverted in the flexural strength of the paste. This work confirms the effectiveness of internal curing of AASF with SAPs and further provides a promising way to reduce the autogenous shrinkage of AASF without compromising its mechanical properties., Zhenming Li and Xuhui Liang would like to acknowledge the funding supported by the China Scholarship Council (CSC) under grant No. 201506120072 and No. 201806050051. This work is supported also by the grant from the Netherlands Organisation for Scientific Research (NWO). José Granja and Miguel Azenha would like to acknowledge Por tuguese Foundation for Science and Technology (FCT) to fund the Research Project IntegraCrete (PTDC/ECM-EST/1056/2014 - POCI 01-0145-FEDER-016841). The grant UID/ECI/04029/2019 - ISISE, funded by national funds through the FCT/MCTES (PIDDAC) is also acknowledged.

A computational model for studying the mechanical performance of steel-concrete columns under combined torsion is established via ABAQUS. The model is validated by experimental results. Through numerical simulations, the influence of the axial load ratio, torsion-bending ratio, concrete strength, steel ratio, longitudinal reinforcement ratio, stirrup ratio, and shear-span ratio on the torsional behaviour of steel-concrete columns is comprehensively investigated. The initial torsion stiffness and ultimate torsion strength of the column increase with increasing concrete strength and decreasing shear-span ratio. The parameters in descending order of influence on the ultimate torsion strength are steel ratio, torsion-bending ratio, stirrup ratio, longitudinal reinforcement ratio, and axial load ratio. Furthermore, the seven parameters in descending order of influence on the ductility coefficient are the steel ratio, shear-span ratio, concrete strength, axial load ratio, stirrup ratio, torsion-bending ratio and longitudinal reinforcement ratio.

In 3D concrete printing (3DCP), it is necessary to meet contradicting rheological requirements: high fluidity during pumping and extrusion, and high stability and viscosity at rest to build the layered structure. In this paper, the impact of the hydroxypropyl methylcellulose (HPMC)-based viscosity-modifying admixture (VMA) on the 3D printability and mechanical performance of a limestone and calcined clay based cementitious material is investigated. A combination of VMA and superplasticizer was used for that purpose. In this case, controlling the competitive effects between VMA and superplasticizer becomes critical. The main strategy for 3D printing in this study was to add an optimal dosage of VMA in the solid suspension that was already mixed with water and superplasticizer. A lab-scale 3DCP setup was developed and demonstrated as well. A series of tests was performed to characterize the effects of VMA on flowability, extrudability, open time, buildability, green strength, hydration, compressive strength, and air void content and distribution. Experiments performed in this study showed that the mixture containing 0.24% (of the binder mass) of VMA exhibited satisfactory 3D printability and optimal mechanical performance. Finally, the results, limitations, and perspectives of the current research were discussed.

This study aims to provide a better understanding of the autogenous shrinkage of slag and fly ash-based alkali-activated materials (AAMs) cured at ambient temperature. The main reaction products in AAMs pastes are C-A-S-H type gel and the reaction rate decreases when slag is partially replaced by fly ash. Due to the chemical shrinkage and the fine pore structure of AAMs pastes, drastic drop of internal relative humidity is observed and large pore pressure is generated. The pore pressure induces not only elastic deformation but also a large creep of the paste. Besides the pore pressure, other driving forces, like the reduction of steric-hydration force due to the consumption of ions, also cause a certain amount of shrinkage, especially in the acceleration period. Based on the mechanisms revealed, a computational model is proposed to estimate the autogenous shrinkage of AAMs. The calculated autogenous shrinkage matches well with the measured results.

In recent years more and more attention has been given to autogenous shrinkage due to the increasing use of high-performance concrete, which always contains supplementary materials. With the addition of supplementary materials&mdash, e.g., fly ash and blast furnace slag&mdash, internal relative humidity, chemical shrinkage and mechanical properties of cement paste will be affected. These properties significantly influence the autogenous shrinkage of cement paste. In this study, three supplementary materials&mdash, i.e., silica fume, fly ash and blast furnace slag&mdash, are investigated. Measurements of final setting time, internal relative humidity, chemical shrinkage, compressive strength and autogenous deformation of the cement pastes with and without supplementary materials are presented. Two water-binder ratios, 0.3 and 0.4, are considered. The effects of different supplementary materials on autogenous shrinkage of cement paste are discussed.

In recent decades, several simulation models have been proposed to predict autogenous shrinkage of cementitious systems. In most of these models, however, only the elastic deformation caused by self-desiccation of the hydrating cement paste is considered. In fact, cement paste is not an ideal elastic material. Also the time-dependent deformation, i.e. creep, has been proposed an important component of autogenous shrinkage, especially at the early age. In this study, a simulation model for autogenous deformation is proposed, which includes an elastic part and a time-dependent part. The time-dependent part of this model is based on the activation energy concept. The capillary tension is considered as the driving force of the autogenous shrinkages. In order to evaluate the accuracy of the prediction with the proposed model, CEM I and CEM III/B pastes are studied in this paper. The simulated autogenous shrinkages are compared with experimental results.

This study aims to investigate the influences of different grades of calcined clay on 3D printability, compressive strength (7 days), and hydration of limestone and calcined clay-based cementitious materials. Calcined clays that contained various amounts of metakaolin were achieved by blending low-grade calcined clay (LGCC) and high-grade calcined clay (HGCC) in three different proportions. The results revealed that increasing the HGCC% ranging from 0 wt% to 50 wt% in calcined clay could: (1) increase the flow consistency; (2) impressively improve the buildability, and reduce the printability window of the fresh mixtures; (3) enhance and accelerate the cement hydration. The reduction of mean interparticle distance induced by increasing HGCC% may be the primary reason for the enhancement of buildability and very early-age hydration. However, increasing HGCC% led to an increase of air void content in the interface region of the printed sample, which weakened the compressive strength of the printed sample at 7 days. Besides, it confirmed that the cold-joint/weak interface was easily formed by using the fresh mixture with a high structuration rate.

This study aims to investigate the cracking potential of alkali-activated slag (AAS) and alkali-activated slag-fly ash (AASF) concrete subjected to restrained autogenous shrinkage. Temperature Stress Testing Machine (TSTM) is utilized, for the first time, to monitor the stress evolution and to measure the cracking time of alkali-activated concrete (AAC) under restraint condition. The stresses in AAS and AASF concrete are calculated based on the experimental results while taking into consideration the influence brought by creep and relaxation. It is found that AAS and AASF concrete showed lower autogenous shrinkage-induced stress and later cracking compared to ordinary Portland cement (OPC) based concrete with similar compressive strength, despite the higher autogenous shrinkage of AAS and AASF concrete. The low autogenous shrinkage-induced stress in the AAC is mainly attributed to the pronounced stress relaxation. A good prediction of the stress evolution in AAC is obtained by taking into account the elastic part of the autogenous shrinkage and the stress relaxation. In contrast, calculations ignoring the creep and relaxation would lead to a significant overestimation of the stress in AAC.

The goal of this study is to investigate the effects of different grades of calcined clay on the extrudability and early-age strength development under ambient conditions. Four mix designs were proposed. Three of them contained high, medium, and low grades of calcined clay, respectively, and one was the reference without calcined clay. In terms of extrudability, an extrusion test method based on the ram extruder was introduced to observe the quality of extruded material filaments, and to determine the extrusion pressure of tested materials at different ages. For evaluating the very early-age strength development, the penetration resistance test, the green strength test, and the ultrasonic pulse velocity test were applied. Furthermore, the mechanical properties of the developed mix designs were determined by the compressive strength test at 1, 7 and 28 days. Finally, the main finding of this study was that increasing the metakaolin content in calcined clay could significantly increase the extrusion pressures and green strength, shorten the initial setting time and enhance the compressive strength at 1, 7, and 28 days.

In this study, a compact thermoelectric generator (TEG) configuration composed of T-shaped electrodes and connected P–N thermoelectric legs was developed to improve its performance. Based on the 1D self-consistent numerical model, the electrical impedance matching relations and optimal thermoelectric leg sizes for T-shaped TEG were semi-analytically derived, and demonstrated with a high accurate prediction. The comparison between the developed T-shaped and the conventional π-shaped TEGs was conducted, which indicated that the specific output power was greatly increased by 90.1% under the same given thermal conditions. Furthermore, impacts of the coupled thermal-electrical conditions, thermal conditions and geometric parameters on the performance of T-shaped TEG have been studied in detail. Single-parameter and two-parameter geometric optimizations were also discussed. Then, the optimal geometric size could be obtained under the given heat transfer conditions and external electrical loading. Finally, the optimization of geometric parameters for T-shaped TEG was conducted in different heat transfer conditions. Under the constant heat transfer conditions, the maximum power has improved by 21.9%. The novel structure of the T-shaped TEG proposed could be applied in many application scenarios.

In order to enhance the performance of the automobile waste heat recovery system, a novel two-stage automobile thermoelectric generator with a cascade configuration along the temperature gradient of exhaust gas was designed. The corresponding integrated one-dimensional numerical model considering temperature-dependent thermoelectric materials was developed to predict the performance accurately. The impedance matching condition for the maximum output power was derived for the novel two-stage automobile thermoelectric generator. The developed model was demonstrated through comparisons with the reported experimental data and simulation results. The results indicated significant discrepancies in output power, conversion efficiency, and the temperature difference between the cold/hot sides of thermocouples for single-stage and two-stage configurations. The designed thermoelectric generator with the two-stage configuration could capture the temperature change along the flow direction of exhaust gas and obtain better performance by adjusting the length of the two-stage module. Compared to the traditional single-stage thermoelectric generator, the designed two-stage configuration could achieve the output power increments by 13.5% under the same working conditions. The effects of working conditions (exhaust gas temperature and mass flow rate) and TEG geometry size on the output power are studied. This work could provide some guidance on the design and performance enhancement of automobile thermoelectric generators.

This study experimentally evaluates the performance of a two-stage indoor air filtration system with the focus on main-stage filter media selection. The impact of the main-stage filter media properties on the effectiveness of adding a pre-stage filter media is compared among three main-stage filter media types commonly used in heating, ventilation, and air conditioning (HVAC) filtration system: cellulose, melt-blown, and expanded PTFE membrane. The pre-stage filter media effectiveness was found to be significantly affected by the incoming aerosol size distribution. Although counter-intuitive, having an additional pre-stage filter was found to cause a service lifetime drop for cellulose-type main-stage filters depending on incoming aerosol size distribution, while universally enhanced life spans were found for melt-blown and PTFE membrane-coated main-stage filters. The mechanism that causes a shortened service life of cellulose main-stage filter with the presence of a pre-stage filter was explored in detail and was found to be associated with the special particle loading behavior of conventional cellulose media. The rapid pressure drop increase of cellulose media in depth and transitional filtration regimes was strongly affected by the incoming aerosol size distribution, with a steeper slope (faster pressure drop increase) associated with finer particles penetrating more deeply into the inhomogeneous pores of cellulose media. The pre-stage filter alters the particle size distribution of the main-stage faces, with an increased fraction of fine particles that negatively impacts the main-stage filter loading behavior. This observation and mechanism analysis are the first time being reported, which suggests a careful selection of the combination of pre-stage and main-stage filter types with the consideration of incoming contaminant properties. It is essential for researchers and engineers in designing energy-efficient two-stage filtration systems.

The push–pull fatigue characteristics of the peak-aged Mg–0.2Zn–0.5Zr alloys with different addition levels of neodymium (Nd) have been investigated. The fatigue strength (σf) of the Mg–xNd–0.2Zn–0.5Zr (NZx0K) alloy increases proportionally with the increase of the Nd content (CNd) as follows: σf (T6) ≈ (13.8–14.0) CNd + 46 (for x between 0 and 3.0 wt%). The cyclic stress amplitude also increases but the plastic strain value decreases with the increase of the Nd content. The studied alloys exhibit the strain hardening followed by cyclic softening during fatigue test. During the low-cycle fatigue (LCF) test, the cracks originate from the cyclic deformation and cumulative damage. In high-cycle fatigue (HCF), the failure is due to the cyclic deformation and damage irreversibly caused by environment-assisted cyclic slip. The LCF lives of the alloys fitted well with the Coffin–Manson relation and Basquin laws, the three-parameter equation, and the energy-based concepts. The developed multi-scale fatigue (MSF) life models can be used to predict the LCF and HCF lives of the alloys. Among these models, the MSF life can well capture the influence of Nd addition on fatigue.

This paper studied the tensile crack initiation behaviors in cast NZ30K alloys under as-cast, T4- and T6-treated conditions. Results indicate that tensile failure of cast magnesium alloys mainly nucleates from the tensile-sensitive microstructural constituent such as eutectic particles, grain boundaries and twin grain boundaries. For the as-cast alloy, the micro-cracks form mainly by cracking of the eutectic phases (~83%). In addition, twin grain boundaries are also found to be preferential sites for crack nucleation (~17%). The tensile failure in the T4- and T6-treated alloys is mainly caused by twin nucleation (T4~60% and T6~44%) and grain boundaries cracking (T4~28% and T6~52%). Interactions among dislocation-slip, twinning and grain boundaries as well as eutectic phases determine the crack initiation behavior of cast magnesium alloys.

This paper investigates the fatigue properties and deformation behavior of a newly developed Mg-2.96Nd-0.21Zn-0.39Zr magnesium alloy wheel in both as-cast and T6 conditions. Compared with the as-cast alloy, the T6-treated alloy shows a significant increase in fatigue strength and cyclic stress amplitude. This is believed to be attributed to the change of defect type from porosity to oxides and the increased matrix strength in the T6 (peak-aged) condition. For the as-cast alloy wheel, fatigue failure mainly originated from the cast defects including porosity, oxide film, and inclusion at or near the sample surface. In the T6-treated alloy, however, oxides and inclusions or slip bands initiate the fatigue cracks. Solution treatment appears to reduce or eliminate the shrinkage porosity because of grain growth and dissolution of as-cast eutectic phases in the grain boundaries. The cyclic stress amplitude of the as-cast alloy increases with increasing the number of cycles, while the T6-treated alloy shows cyclic softening after the stress reaches a maximum value. The Coffin–Manson law and Basquin equation can be used to evaluate the life of low cycle fatigue. The developed long crack model and multi-scale fatigue (MSF) models can be used to predict high-cycle fatigue life of the Mg-2.96Nd-0.21Zn-0.39Zr alloys with or without casting defects.

Effects of chemical compositions on the tensile properties, deformation behavior and fracture mechanism have been studied in a die-casting aluminum alloys Al-10Si-yCu-xMn-zFe. The test specimens were taken from the engine support brackets and tested at 20 °C, 150 °C and 300 °C. Addition of Mn and Cu elements in the Al-10Si alloys can significantly increase the YS and UTS of the alloys. The as-cast Al-10Si-1.5Cu-0.8Mn-0.15Fe alloy exhibits the highest tensile properties (RT: YS of 190 MPa and UTS of 308 MPa, 150 °C: YS of 176 MPa and UTS of 249 MPa, 300 °C: YS of 94 MPa and UTS of 111 MPa). Increasing test temperature reduces the YS and UTS and improves the ductility of the alloys. Chemical compositions (such as Mn, Cu and Fe) do not significantly affect the work-hardening behavior of the alloys. Increasing test temperature significantly decreases the n and k values. Phase particles (both Si and (Fe/Mn)-rich) cracking and debonding determine the fracture mechanism of the alloys. Final failure of the alloys mainly depends on the global instability (HT, necking) and local instability (RT, shearing).

The effect of grain size on tensile and fatigue properties has been investigated in cast Mg alloys of Mg-2.98Nd-0.19Zn (1530 μm) and Mg-2.99Nd-0.2Zn-0.51Zr (41 μm). The difference between RB and push–pull fatigue testing was also evaluated in both alloys. The NZ30K05-T6 alloy shows much better tensile strengths (increased by 246 pct in YS and 159 pct in UTS) and fatigue strength (improved by ~80 pct) in comparison with NZ30-T6 alloy. RB fatigue testing results in higher fatigue strength compared with push–pull fatigue testing, mainly due to the stress/strain gradient in the RB specimen cross section. The material with coarse grains could be hardened more in the cyclic loading condition than in the monotonic loading condition, corresponding to the lower σ f and the higher σ f/σ b or σ f/σ 0.2 ratio compared to the materials with fine grains. The fatigue crack initiation sites and failure mechanism are mainly determined by the applied stress/strain amplitude. In LCF, fatigue failure mainly originates from the PSBs within the surface or subsurface grains of the samples. In HCF, cyclic deformation and damage irreversibly caused by environment-assisted cyclic slip is the crucial factor to influence the fatigue crack. The Coffin–Manson law and Basquin equation, and the developed MSF models and fatigue strength models can be used to predict fatigue lives and fatigue strengths of cast magnesium alloys.

A synergy of fine-tuning the carrier concentration and implementing scatter of heat-carrying phonons is vital to the thermoelectric study of narrow bandgap semiconductors. In this work, we adopted a synergistic charge-balanced compensation-doping approach derived from the interplay between heterovalent Sb dopants and Sn vacancies in narrow band gap semiconductor SnTe, a lead-free alternative to the classic thermoelectric material PbTe. Specifically, we designed a composition series of Sn1-xSb2x/3Te (0 ≤ x ≤ 0.20 in steps of 0.02), in which every three Sn2+ are substituted by two Sb3+ and one Sn vacancy. To the first order, such chemical composition does not contribute net charge carriers in the context of electron counting and also ensures that the composition of the primary phase is not altered by the formation of secondary phase Sb2Te3, which greatly simplifies the data analysis. Interestingly, we found that the measured carrier concentration gradually reduced with increasing Sb content and in turn increased the Seebeck coefficient. Meanwhile, the coexistence of Sb dopants and Sn vacancies effectively scattered heat-carrying phonons at elevated temperatures. Consequently, a peak ZT value of ~1.1 at 873K was achieved in the x = 0.12 sample. These results offer a new avenue for thermoelectric study of narrow bandgap semiconductors.

The resonance effect has been used as a theoretical strategy in enhancing the thermoelectric performance. However, its origin and mechanisms are still ambiguous, which complicates the rapid screening of resonance dopants and thereby hinders the improvement of the thermoelectric performance. In this work, we attempt to disclose the mechanism of resonance effect and propose a straightforward method to screen the resonance dopants suitable for SnTe. Hence, two triggering conditions of achieve the resonance effect are proposed. One is the energy correlation between impurity and host states. The other is the position correlation between impurity state level and Fermi level. Based on the two conditions, a variety of elements (including Y, Ru, In, Sb, La, Gd, Lu, Os, Tl and Bi) are selected as the potential resonance dopants suitable for SnTe. Among these elements, we herein re-study the effect of Bi doping on the thermoelectric performance of SnTe. Consequently, a maximum PF of ∼32.46μWcm−1K−2 and the optimal ZT of ∼1.23 were achieved in Sn0·94Bi0·04Te at 873 K due to a high Seebeck coefficient and low thermal conductivity after Bi doping.

Alkali-activated slag and fly ash (AASF) materials are emerging as promising alternatives to conventional Portland cement. Despite the superior mechanical properties of AASF materials, they are known to show large autogenous shrinkage, which hinders the wide application of these eco-friendly materials in infrastructure. To mitigate the autogenous shrinkage of AASF, two innovative autogenous-shrinkage-mitigating admixtures, superabsorbent polymers (SAPs) and metakaolin (MK), are applied in this study. The results show that the incorporation of SAPs and MK significantly mitigates autogenous shrinkage and cracking potential of AASF paste and concrete. Moreover, the AASF concrete with SAPs and MK shows enhanced workability and tensile strength-to-compressive strength ratios. These results indicate that SAPs and MK are promising admixtures to make AASF concrete a high-performance alternative to Portland cement concrete in structural engineering.

Chemical deformation (chemical shrinkage/expansion), the absolute volume change during reactions, is a key parameter influencing the volume stability, especially the autogenous deformation of a binder material. This work, for the first time, reports an in-depth investigation on the chemical deformation of metakaolin based geopolymer (MKG). Unlike ordinary Portland cement-based binders with monotonic chemical shrinkage, MKG experiences three stages of chemical deformations: chemical shrinkage in the first stage, chemical expansion afterward and chemical shrinkage again in the final stage. Various experimental techniques (XRD, FTIR and NMR) plus theoretical calculations are applied to explore the mechanisms behind the chemical deformation of MKG. Clear correlations are found between the chemical deformations and the reaction processes during geopolymerization. A conceptual chemical deformation model for geopolymer is summarised. The insights into the chemical deformation provided by this study will play a fundamental role in further understanding, controlling and even utilizing the deformation behaviours of geopolymers.

This study investigates the effectiveness of metakaolin (MK)in mitigating the autogenous shrinkage of alkali-activated slag (AAS). It is found that the autogenous shrinkage of AAS paste can be reduced by 40% and 50% when replacing 10% and 20% slag with MK, respectively. By providing additional Si and Al, and decreasing the pH of the pore solution, the incorporation of MK retards the formation of aluminium-modified calcium silicate hydrate (CASH)gels, the main reaction products in the studied pastes. The chemical shrinkage and pore refinement are consequently mitigated, resulting in a substantial reduction in the pore pressure. Meanwhile, the elastic modulus of AAS paste is only slightly influenced after MK addition. As a result, the autogenous shrinkage of AAS is significantly mitigated by incorporating MK. In addition, the introduction of MK would extend the setting time, slightly decrease the compressive strength, but greatly increase the flexural strength of AAS.

The effect of curing conditions (sealed and unsealed) on the pore solution composition and carbonation resistance of different binary alkali-activated fly ash (FA) and ground granulated blast furnace slag (GBFS) pastes is investigated in this study. The studied mixtures were with FA/GBFS ratios of 100:0, 70:30; 50:50, 30:70, 0:100. Ordinary Portland cement (OPC) and Cement III/B (70 wt% of GBFS and 30 wt% OPC) pastes with the same precursor content were also studied to provide a baseline for comparison. Accelerated carbonation conditions (1% (v/v) CO2, 60% RH for 500 days) were considered for evaluating the carbonation resistance of the pastes. The results show a substantial lower [Na+] in the pore solution of the unsealed cured samples compared to the sealed cured samples. It is also found that unsealed curing of the samples leads to a faster carbonation rate. Additionally, it is observed that the carbonation rate decreases with increasing GBFS content independent of the curing conditions. The potential risks with respect to carbonation of the pore solution are also identified and discussed.

Herein the effect of Co element doping on the thermoelectric properties of n-type Bi2(Te0.97Se0.03)3 alloys fabricated by a melting synthesis technique followed by spark plasma sintering was reported. The micro-morphology and composition of the specimens was characterized by means of X-ray diffraction and scanning electron microscopy. The electrical and thermal properties of (Bi1-xCox)2(Te0.97Se0.03)3 with x = 0%, 2%, 4% and 8% were measured from 298K to 473K. We found that (i) the micro-scale secondary phase CoTe2 was embedded into the matrix due to the low solubility of Co element; (ii) Co doping generally improved the effective mass due to the scattering by local magnetic moments carried by Co ion and practically retained the carrier concentration, thereby yielding the higher Seebeck coefficient; (iii)both the electronic and lattice thermal conductivity decreased simultaneously by introducing the hierarchical defects (point defects and the micro-scale defects). Specifically, the x = 8% sample obtained a minimum thermal conductivity ~1.26 Wm−1K−1 at 398K. This results in a maximum ZT value of ~0.77, approximately ~20% higher than that of pristine Bi2(Te0.97Se0.03)3.

Strain-controlled fatigue characteristics of peak-aged and over-aged Mg96.47Nd2.9Zn0.21 magnesium alloys containing 0.42Zr, including stress response, strain resistance, hysteresis loops, strain–life and corresponding low-cycle fatigue life prediction model, were studied. In the peak-aged state (T61: 540 °C × 8 h + 200 °C × 14 h), the alloy shows higher cyclic stress response, but lower ductility than the alloy in the over-aged state (T62: 540 °C × 8 h + 200 °C × 400 h). The yield strength and ultimate tensile strength of the alloy under T61- and T62-treated conditions are close. Compared with T61-treated alloy, the steady stress amplitude occurred in T62-treated alloy is due to higher ductility and more homogenous deformation. In T61 state, the fatigue cracks in the alloy first initiate along the cracked persistent slip bands and then propagate in the trans-granular mode, while in the T62 state, the fatigue cracks initiate along grain boundaries and then propagate in the inter-granular mode.

This paper investigates the fatigue properties and deformation behavior of the T6-treated Mg–2.99Nd–0.18Zn–0.38Zr (NZ30K04) and an AZ91D-T6 magnesium alloys with the same grain size. Compared with the AZ91-T6 alloy, the NZ30K04-T6 alloy shows significant increases in mechanical properties, achieving 17 MPa in YS, 51 MPa in UTS, 2.4 % in elongation, and 15 MPa in FS, respectively. The NZ30K04-T6 alloy also shows higher cyclic stress amplitudes in comparison with the AZ91-T6 alloy. This is attributed to increased matrix strength from the higher precipitate strengthening in the NZ30K04-T6 alloy. The cyclic stress amplitude of the AZ91-T6 alloy increases with increasing the number of cycles. For the NZ30K04-T6 alloy, the cyclic stress amplitude first increases and then decreases during cyclic deformation. Hysteresis loops were symmetrical at all total strain amplitudes due to the lessened extents of texture in these cast magnesium alloys and the absence of twinning–detwinning during cyclic deformation. For both AZ91-T6 and NZ30K04-T6 alloys, the cracked slip bands initiate the fatigue cracks and also assist the crack propagation. The Coffin–Manson law and Basquin equation can be used to evaluate the fatigue parameters and predict the LCF lives of both AZ91-T6 and NZ30K04-T6 alloys. The multi-scale fatigue (MSF) life models and the same model parameters were also used to predict the HCF lives of these alloys.

Laser scanning confocal microscopy (LSCM) and Electron back-scattered diffraction (EBSD) were applied to the study of surface morphology variation of as-cast Mg–3.0Nd–0.2Zn–Zr (NZ30K) (wt.%) alloy under tension-compression fatigue test at room temperature. Two kinds of typical damage morphologies were observed in fatigued NZ30K alloy: One was parallel lines on basal planes led by the cumulation of basal slips, called persistent slip markings (PSMs), and the other was lens shaped, thicker and in less density, led by the formation of twinning. The surface fatigue damage morphology evolution was analyzed in a statistical way. The influences of stress amplitude and grain orientation on fatigue deformation mechanisms were discussed and the non-uniform deformation among grains and the PSMs, within twinning were described quantitatively.

This paper investigates the effects of strain rate and test temperature on the tensile properties and deformation behavior of a recently developed high-ductility cast aluminum alloy Al–5Mg–0.6Mn. The as-cast alloy tested at room temperature and the lowest strain rate of ~1.67×10−4 s−1 shows the highest yield strength of ~212 MPa, ultimate tensile strength of ~357 MPa and elongation (~17.6%). Increasing strain rate reduces the ultimate tensile strength and ductility of the as-cast alloy. With the increasing of test temperature, the as-cast alloy shows significantly decreases in tensile strengths and improvements in elongation. The tensile failure of the alloy is mainly originated from the cracking and debonding eutectic particles. The Portevin–Le Chatelier effect occurs in the alloy tested at RT. Strain rate in current study ranges does not significantly affect the work-hardening behavior of the alloy. Increasing test temperature apparently reduces the strain-hardening exponent and coefficient. For the alloy tested at RT, all tensile failures occur prior to global instability, indicating the existence of localized damage. In contrast, for the alloy tested at HT, the global instability occurs at strains below the logarithmic fracture strains, suggesting that there is still a postnecking damage.