Mechanical Property and Microstructure of Cement Mortar with Carbonated Recycled Powder

Carbonated recycled powder as cementitious auxiliary material can reduce carbon emissions and realize high-quality recycling of recycled concrete.In this paper,microscopic property of recycled powder with three carbonation methods was tested through XRD and SEM,the mechanical property and microstructure of recycled powder mortar with three replacement rates were studied by ISO method and SEM,and the strengthening mechanism was analyzed.The results showed that the mechanical property of recycled powder mortar decreased with the increasing of replacement rate.It is suggested that the replacement rate of recycled powder should not exceed 20%.The strength index and activity index of carbonated recycled powder mortar were improved,in which the flexural strength was increased by 27.85%and compressive strength was increased by 20%at the maximum.Recycled powder can be quickly and completely carbonated,and the improvement effect of CH pre-soaking carbonation was the best.The activity index of carbonated recycled powder can meet the requirements of Grade II technical standard for recycled powder.Microscopic results revealed the activation mechanism of carbonated recycled powder such as surplus calcium source effect,alkaline polycondensation effect and carbonation enhancement effect.

Microstructure and mechanical properties of nickel-based superalloy fabricated by laser powder-bed fusion using recycled powders

Evaluating the recyclability of powders in additive manufacturing has been a long-term challenge.In this study,the microstructure and mechanical properties of a nickel-based superalloy fabricated by laser powder-bed fusion(LPBF)using recycled powders were investigated.Re-melted powder surfaces,satellite particles,and deformed powders were found in the recycled powders,combined with a high-oxygencontent surface layer.The increasing oxygen content led to the formation of high-density oxide inclusions;moreover,printing-induced cracks widely occurred and mainly formed along the grain boundaries in the as-built LPBF nickel-based superalloys fabricated using recycled powders.A little change in the Si or Mn content did not increase the hot cracking susceptibility(HCS)of the printed parts.The changing aspect ratio and the surface damage of the recycled powders might contribute to increasing the crack density.Moreover,the configuration of cracks in the as-built parts led to anisotropic mechanical properties,mainly resulting in extremely low ductility vertical to the building direction,and the cracks mainly propagated along the cellular boundary owing to the existence of a brittle precipitation phase.

Tensile Behavior of Strain Hardening Cementitious Composites (SHCC) Containing Reactive Recycled Powder from Various C&D Waste

This work investigates the feasibility of utilizing reactive recycled powder(RP)from construction and demolition(C&D)waste as supplementary cementitious material(SCM)to achieve a ductile strain hardening cementitious composites(SHCC).The recycled mortar powder(RMP)from mortar waste,recycled concrete powder(RCP)from concrete waste and recycled brick powder(RBP)from clay brick waste were first prepared,and the micro-properties and tensile behavior of SHCC containing various types and replacement ratios of RPs were determined.The incorporated RP promotes pozzolanic and filler effects,while the hydration products in cementitious materials decrease with RP incorporation;therefore,the incorporated RP decreases the compressive strength of SHCC.Attributed to the reduction in the matrix strength,the incorporated RP increases the crack-bridging extent and ductility of SHCC;the irregular micro-structure and high reactivity of RP also help the strain-hardening performance of the prepared SHCC.In addition,the strainhardening performance of SHCC containing RMP and RBP is surperior to that of SHCC with RCP and is slightly lower than that of SHCC with fly ash(FA);for instance,the ultimate strain of SHCC containing 54%FA,RMP,RCP and RBP is 3.67%,3.61%,2.52%and 3.53%,respectively.In addition,the strain-hardening behavior of an SHCC doubled mix with FA and RMP or RBP has a similar ultimate strain and a higher ultimate stress than SHCC containing only FA.

Development and Application of a High-Volume Recycled Powder Solidifying Material for Waterworks Sludge

Recycled powder(RP)is produced as a by-product during the process of recycling construction and demolition(C&D)wastes,presenting a low additional value.Using RP-based solidifying material can not only improve its utilization efficiency,but also reduce the cost of commercial solidifying materials.To date,this is the best solidifying material utilized to dispose the original waterworks sludge(OWS)with high moisture contents(60%),and the product could be used to fabricate non-fired bricks.This has become a new environment-friendly technology of“using waste to treat waste”.In this paper,the influence of different particle sizes and dosages of RP on the prepared solidifying material was studied.Besides,unconfined compression strength(UCS),volume stability,chemical composition,and heat of hydration,pore structure of the solidifying material were characterized.Then,non-fired bricks were prepared by using the solidifying material,recycled aggregate,and original waterworks sludge.The UCS and softing coefficient(SC)of the non-fired bricks were evaluated.As a result,the 28-day UCS of the solidifying material with optimal(M30)was 35.40 MPa,which could reach 84.37%of Portland cement(PC).The addition of RP increased the volume stability of the solidifying material.The addition of a large amount of RP reduced the heat flux and cumulative heat release of the solidifying material,while its porosity increased.The UCS of non-fired brick(NF20)in 28 days was 15.19 MPa and the SC after 28 days was 78.35%.In conclusion,the preparation of solidifying material using RP could be a promising approach and has a great potential in disposal of original waterworks sludge.

Study on the Durability of Recycled Powder Concrete against Sulfate Attack under Partial Immersion Condition

In order to make full use of waste recycled fine powder(RFP)in concrete and achieve the goal of carbon neutrality in the concrete industry,the durability of sulfate resistance is an important aspect of evaluating the performance of recycled powder concrete(RPC).Therefore,the durability of RPC under partial sulfate immersion was studied to provide theoretical guidance for understanding the erosion mechanism of RPC.The compressive strength,mass loss,and microstructure change patterns of RPC under partial immersion of 5%Na2SO4 and MgSO4 solutions were analyzed by cubic compressive strength,mass loss rate,SEM-EDS,and XRD.The results showed that the surface crystalline matter of concrete in Na2SO4 solution was mainly white powders,and that of concrete in MgSO4 solution was mainly transparent paste,both of which had a little spalling on the outer surface of the concrete.The compressive strength and mass loss rate of concrete with 20%RFP was relatively good,indicating that concrete with 20%RFP had better durability against sulfate.The compressive strength of the lower part of the concrete partially immersed in Na2SO4 solution was higher than that of the upper part and the strength of the lower part of RPC-2 was 3.11%higher than the upper part at 180 d;The pattern was reversed in the MgSO4 solution,where the strength of the lower part of RPC-2 was 19.74%lower than the upper part at 180 d.Microscopic analysis showed that the hydration products of RPC were mainly gypsum and ettringite,while the RPC produced more hydration products with the promotion of magnesium ion in the MgSO4 solution.The higher the replacement rate of RFP,the more frequent the gypsum-type failures in the concrete.

Stress-strain relationship for reactive powder concrete with recycled powder under uniaxial compression

The recycled powder(RP)from construction wastes can be used to partially replace cement in the preparation of reactive powder concrete.In this paper,reactive powder concrete mixtures with RP partially replacing cement,and natural sand instead of quartz,are developed.Standard curing is used,instead of steam curing that is normally requested by standard for reactive powder concrete.The influences of RP replacement ratio(0%,10%,20%,30%),silica fume proportion(10%,15%,20%),and steel fiber proportion(0%,1%,2%)are investigated.The effects of RP,silica fume,and steel fiber proportion on compressive strength,elastic modulus,and relative absorption energy are analyzed,and theoretical models for compressive strength,elastic modulus,and relative absorption energy are established.A constitutive model for the uniaxial compressive stress-strain relationship of reactive powder concrete with RP is developed.With the increase of RP replacement ratio from 0% to 30%,the compressive strength decreases by 42% and elastic modulus decreases by 24%.

Influence of Recycled Concrete Fine Powder on Durability of Cement Mortar

In this paper,the durability of cement mortar prepared with a recycled-concrete fine powder(RFP)was examined;including the analysis of a variety of aspects,such as the carbonization,sulfate attack and chloride ion erosion resistance.The results indicate that the influence of RFP on these three aspects is different.The carbonization depth after 30 days and the chloride diffusion coefficient of mortar containing 10%RFP decreased by 13.3%and 28.19%.With a further increase in the RFP content,interconnected pores formed between the RFP particles,leading to an acceleration of the penetration rate of CO_(2)and Cl^(−).When the RFP content was less than 50%,the corrosion resistance coefficient of the compressive strength of the mortar was 0.84-1.05 after 90 days of sulfate attack.But the expansion and cracking of the mortar was effectively alleviated due to decrease of the gypsum production.Scanning electron microscope(SEM)analysis has confirmed that 10%RFP contributes to the formation of a dense microstructure in the cement mortar.

Effect of recycled powder on the yield stress of cement paste with varied superplasticizers

The influence of superplasticizer on the yield stress of cement pastes with recycled powder(RP)was examined in the study.Four superplasticizers were used to obtain the similar fluidity by adjusting the dosage.The results show that the 10%RP decreases the yield stress of paste compared to the reference paste at the same fluidity,but 20%and 30%RP increases the yield stress,ranging from 11 to 599%.The superplasticizer with adsorptive group of phosphate-type minimizes the yield stress of paste than that of polycarboxylate-type,but it made a significant increment in yield stress as the incorporating of RP increased.Besides,the polycarboxylate superplasticizer with the higher molecular weight of side chain and charge density led to lower yield stress.Based on the Yodel model,the yield stress of paste with RP was analyzed by the polymer adsorption and particle packing density of particles to reveal the influence of RP with different superplasticizers on the colloidal interaction and contact network among the particles.The packing density of particles with recycled powder was a little higher than the reference paste,but the higher fraction of fine particles made a stronger PSD effect,which improved the particle contact interaction.On the other hand,due to the higher polymer adsorption of recycled powder than cement,especially for superplasticizer with phosphate group,the average surface coverage was increased,which extended the separation distance,so that colloidal interaction among particles was weaken.

Strength and Microstructural Analysis of Geopolymer Prepared with Recycled Geopolymer Powder

The strength and microstructural analysis of recycled geopolymer are presented in this paper.Five kinds of geopolymers containing 0%,20%,50%,80%and 100%of recycled geopolymer powder were prepared using metakaolin as the source material.The alkali activator solution was a mixture of sodium silicate(Na_(2)SiO_(3))and 12 M sodium hydroxide(NaOH).The change laws of compressive and flexural strength of recycled geopolymer specimens were investigated.And the microscopic characteristics were carried out by SEM,XRD and FTIR to observe the internal morphology and analyze changes in components of recycled geopolymers at different substitution rates.The results show that,with the increase of substitution rate of recycled geopolymer powder,the mechanical properties of recycled geopolymers degenerate and the looser structure are formed.When the substitution rate is less than 50%,the recycled geopolymer specimen meets the use requirements of heavy traffic load class.And the specimen with 80%of substitution rate satisfies the requirements of plastering mortar.

Utilization of Recycled Concrete Powder in Cement Composite:Strength,Microstructure and Hydration Characteristics

Recycled concrete powder(RCP)is used more and more in cement-based materials,but its influence on the hydration process is still unclear.Therefore,this paper studied the influence of recycled concrete powder(RCP)on the hydration process of cement and provides a theoretical basis for the hydration mechanism of cement composite materials.The hydration heat method was used to systematically analyze the thermal evolution process of cement paste with or without RCP.Hydration products were identified using X-ray diffraction(XRD)and thermal analysis(TG–DSC).The pore structure change of cement pastes was analyzed by mercury intrusion porosimetry(MIP)method.The mechanical properties of mortar were also evaluated.Four recycled concrete powder(RCP)dosages,such as 10%,20%,30%and 40%are considered.The results indicate that with the increase of RCP content,the hydration heat release rate and total heat release amount of paste decreased,but the second heat release peak of hydration reaction advanced;the proportion of harmful pores and more harmful pores increases,the total porosity and the most probable pore size also increase;the fluidity and mechanical strength of mortar decrease,but the crystal type of hydration products does not change.When the content of RCP is less than 20%,it has little effect on the mechanical strength of mortar.When fly ash and silica fume are mixed,the fluidity difference of mortar decreases,and when the content of fly ash is the highest,the fluidity of mortar is the highest,which is 15mm higher than that of the control group.When RCP content is 15%,fly ash and silica fume content is 15%(FA:SF=3:2),the hydration heat of the clean pulp is the highest among all the compounding ratios,and the hydration reaction is the most complete;the proportion of harmless pores increased by 9.672%,the proportion of harmful pores and more harmful pores decreased,and the compactness of material structure increased;the compressive strength and flexural strength of mortar reached 50.6 MPa and 9 MPa respectively,both exceeding those of control mortar.

Effect of Recycled Mixed Powder on the Mechanical Properties and Microstructure of Concrete

In this paper,recycled bricks and recycled concrete were applied to prepare eco-friendly recycled mixed powder(RMP)cementitious material,as a supplementary to replace conventional cement for improve the recycling of construction and demolition waste.Based on the effect of cementitious materials on the hydration of silicate cement,the effects of RMP on the workability,mechanical properties and microstructure of recycled mixed powder concrete(RMPC)with the different replacement ratios and the 8:4 and 6:4 mixing ratio of recycled brick powder(RBP)and recycled concrete powder(RCP)were investigated.The results showed that the fluidity of the mix decreased with increasing of the replacement ratio and the mixing ratio of RBP and RCP,but the influence of the fluidity was smaller within 15%replacement ratio.As the replacement ratio increases,the internal pore structure of RMPC tends to be loose and porous,which exhibits a significant pore volume distribution characteristic.The number of large capillaries was considerably increased at replacement ratio of 45%.The 7 d compressive strength of RMPC was slightly lower than that of ordinary concrete.The compressive and splitting tensile strengths of RMPC at 28 d increased by 4.2%and 10.1%,respectively,with increasing curing age at 15%replacement ratio and 6:4 mixing ratio.The RMPC mechanical strengths with RBP and RCP at the mixing ratio of 6:4 was higher than those of 8:2.Finally,a basis for the recycling of RBP and RCP in the construction industry can be provided by the results of this study.

Experimental study and assessment of thermal energy storage mortar with paraffin/recycled brick powder composite phase change materials

Thermal energy storage recycled powder mortar(TESRM)was developed in this study by incorporating paraffin/recycled brick powder(paraffin/BP)composite phase change materials(PCM).Fourier transform infrared and thermogravimetric analysis results showed that paraffin/BP composite PCM had good chemical and thermal stability.The onset melting temperature and latent heat of the composite PCM were 46.49°C and 30.1 J·g−1.The fresh mortar properties and hardened properties were also investigated in this study.Paraffin/BP composite PCM with replacement ratio of 0%,10%,20%,and 30%by weight of cement were studied.The results showed that the static and dynamic yield stresses of TESRM were 699.4%and 172.9%higher than those of normal mortar,respectively.The addition of paraffin/BP composite PCM had a positive impact on the mechanical properties of mortar at later ages,and could also reduce the dry shrinkage of mortar.The dry shrinkage of TESRM had a maximum reduction about 26.15%at 120 d.The thermal properties of TESRM were better than those of normal mortar.The thermal conductivity of TESRM was 36.3%less than that of normal mortar and the heating test results showed that TESRM had good thermal energy storage performance.

Performance improvement of a crumb rubber modified bitumen using recycled glass powder

In this study, the efficiency of bitumen and asphalt mixtures modified with crumb rubber(CR) and recycled glass powder(RGP) is evaluated. From an environmental point of view, the application of RGP in asphalt mixtures is considered beneficial since it prevents accumulation of waste glass in the natural environment. Rheological and mechanical properties of modified bitumen samples were investigated by laboratory methods such as bitumen conventional tests and dynamic shear rheometer(DSR), and also by asphalt mixture performance tests including Marshall stability, indirect tensile strength(ITS),compressive strength, and indirect tensile stiffness modulus. The results showed that the application of RGP in place of CR has no negative impact on the efficiency of bitumen and asphalt mixtures and even improves their engineering properties except for the toughness index(TI). Moreover, a modification with 5% CR and 5% RGP in asphalt mixtures results in the best overall performance.

Calcium hydride reduced high-quality Nd-Fe-B powder from Nd-Fe-B sintered magnet sludge

The structural and magnetic properties were studied for recycling Nd-Fe-B powders from Nd-Fe-B sintered magnets sludge via reduction diffusion(RD)with calcium hydride(CaH_(2))particles.For comparison,traditional reducing agent calcium granules were applied to prepare recycled Nd-Fe-B powders.Finer particle size and better size distribution as well as lower impurity content are achieved by using CaH_(2)instead of Ca.In detail,the average particle size of the recycled Nd-Fe-B powder is reduced from 4.66 to 3.43μm,and the bimodal distribution disappears.Moreover,the residual calcium content and oxygen content are reduced to about 0.080 wt%and 0.32 wt%.As a consequence,the roomtemperature magnetization of the CaH_(2)-recycled Nd-Fe-B powder is increased to 146.30 emu/g,6.8%and 33%,respectively,higher than that of Ca-reduced powder and the initial sludge.Further analysis indicates that CaH_(2)is able to reduce the sludge at lower tempe rature to fabricate well-dispersed,unifo rm recycled powder with high magnetization arising from a combination factors of its low melting point,low thermodynamic behavior,and the release of hydrogen during the reaction.

Magnetic property recovery in Nd-Fe-B bonded magnet wastes with chemical reaction and physical dissolution

The main difficulty for the recovery of Nd-Fe-B bonded magnet wastes is how to completely remove the epoxy resins.In this study,chemical reaction and physical dissolution were combined to remove the epoxy resins by adding ammonia-water and mixed organic solvents.Ammonia-water can react with the epoxy functional group of epoxy resin to generate polyols.Mixed organic solvents of alcohol,dimethyl formamide(DMF),and tetrahydrofuran(THF) can dissolve the generated polyols and residual epoxy resins.Under the optimum processing conditions,the epoxy resins in the waste magnetic powders are substantially removed.The oxygen and carbon contents in the recycled magnetic powder are reduced from 13500 × 10^(-6) to 1600 × 10^(-6) and from 19500 × 10^(-6) to 2100 × 10^(-6) with the reduction ratio of88.1% and 89.2%,respectively.The recycled magnetic powder presents improved magnetic properties with MS of 1.306 × 10^(-1) A·m^(2)/g,Mr of 0.926 × 10^(-1) A·m^(2)/g,Hcj of 1.170 T,and(BH)max of 125.732 kJ/m^(3),which reach 99.8%,99.4%,95.9%,and 96.9% of the original magnetic powders,respectively.