The effects of Sm, Si and Ca on the microstructure and mechanical property of AZ91 magnesium alloy were investigated by means of optical microscopy (OM), differential scanning calorimetry (DSC), scanning electronic mi...The effects of Sm, Si and Ca on the microstructure and mechanical property of AZ91 magnesium alloy were investigated by means of optical microscopy (OM), differential scanning calorimetry (DSC), scanning electronic microscopy (SEM), X-ray diffraction (XRD) and tensile testing. The results indicated that the addition of 1.5 wt.% Sm with or without 0.8 Si/Ca led to a decrease in the volume fraction of the β-Mg17Al12 phase and the formation of the intermetallic compounds of Al-Sm, Mg2Si, MgAlCa and Al2Ca. The microstructure of AZ91 alloy was significantly refined and distribution became discrete with additions of Sm and Ca;the average grain size of the α-Mg matrix was reduced from 239.7 ± 16.9 μm to 66.34 ± 5.10 μm. The AZ91-Sm-Ca alloy exhibited a good combination of yield strength at 135 MPa, ultimate tensile strength at 199 MPa and elongation at 4.32%, which was ascribed to grain refinement strengthening. Furthermore, the T6 treated AZ91-Sm-Ca alloy possessed yield strength of 154 MPa and elongation of 7.1%, which was due to grain refinement strengthening and reduction in discontinuous precipitates.展开更多
The effect of fly ash on controlling alkail-silica rection （ASR） in simudated alkali solution was studied. The expausion of mortar bars and the content of Ca（ OH）2 in cement paste cured at 80 °G for 91 d were...The effect of fly ash on controlling alkail-silica rection （ASR） in simudated alkali solution was studied. The expausion of mortar bars and the content of Ca（ OH）2 in cement paste cured at 80 °G for 91 d were measured. Traasmission electron microscopy （TEM） and high-resolution transmission electron microscot9 （HRTEM） were employed to study the microstructure of C-S-H. TEM/ energy dispersive spectroscopy （EDS） leas then used to determine the composition of C-S-H. The pore structure of the paste was analyzed by mercury intntsion porosimetry （MIP）. The results show that the contents of fly ash of 30% and 45% can well inhibit ASR. And the content of Ca（OH） 2 decreases with the increase of fly ash. That fly ash reacted with Ca（OH）2 to produce C-S-H with a low Ca/Si molar ratio could bind more Na^＋ and K^＋ ious, and produce a reduction in the amount of soluble alkali available for ASR. At the same time, the C- S- H produced by pozzolanic reaction converted large pores to snudler ones （ gel pores smaller than 10 nm ） to deusify the pore structure. Perhaps that could inhibit alkali trausport to aggregate for ASR.展开更多
The nanostructure and chloride binding capacity evolution of C-A-S-H gel exposed to aggressive solutions were investigated,utilizing 29Si NMR,27Al NMR,SEM-EDS,and XRD techniques.The experimental results show that whil...The nanostructure and chloride binding capacity evolution of C-A-S-H gel exposed to aggressive solutions were investigated,utilizing 29Si NMR,27Al NMR,SEM-EDS,and XRD techniques.The experimental results show that while Cl-ions show a smaller effect on the microstructure of C-A-S-H sample,and SO4^2-ions can react with C-A-S-H,resulting in decreasing Ca/Si and Al[4]/Si for the C-A-S-H gel (i e,decalcification and dealuminization).The presence of Mg^2+ ions can aggravate the decalcifying and dealuminizing effects of SO4^2-ions on the C-A-S-H.With decreasing Ca/Si ratio and aluminum substitution for the original C-A-S-H gel,the depolymerization degree of silicate tetrahedra increases and the calcium aluminosilicate skeleton strengthens.C-A-S-H gel with lower Ca/Si ratio and higher Al[4]/Si ratio shows gentler nanostructure evolution under chemical attack,i e,improving thermodynamic stability under chemical attack.Furthermore,the chloride binding capacity of C-A-S-H gel is decreased after the sulfate attack.Aluminum substitution can also help C-A-S-H gel resist the degraded chloride binding capacity induced by sulfate attack.展开更多
An accelerated laboratory method（saturated ammonium nitrate solution immersion method） was used to analyze the degradation of cement decalcification process. By studying the changes of intensity, volume, elastic mod...An accelerated laboratory method（saturated ammonium nitrate solution immersion method） was used to analyze the degradation of cement decalcification process. By studying the changes of intensity, volume, elastic modulus, quality, p H value, the Ca/Si, and mineral phase, it could be found that the first cement decalcification degradation process was the decalcification of calcium hydroxide, and then CSH gel, AFm, etc. The secondary ettringite deposition happened and the decalcification degradation depth was proportional to the square root of time. Moreover, the corresponding strength of cement would be gradually reduced, cement rock volume shrinkage occurred, p H values decreased, the surface elastic modulus decreased down to a certain level, and slightly changed and the Ca/Si was 3.1 from the beginning and lasted down to 1.3.展开更多
The time-dependent viscoelastic response of cement-based materials to applied deformation is far from fully understood at the atomic level.Calcium silicate hydrate(C-S-H),the main hydration product of Portland cement,...The time-dependent viscoelastic response of cement-based materials to applied deformation is far from fully understood at the atomic level.Calcium silicate hydrate(C-S-H),the main hydration product of Portland cement,is responsible for the viscoelastic mechanism of cement-based materials.In this study,a molecular model of C-S-H was developed to explain the stress relaxation characteristics of C-S-H at different initial deformation states,Ca/Si ratios,temperatures,and water contents,which cannot be accessed experimentally.The stress relaxation of C-S-H occurs regardless of whether it is subjected to initial shear,tensile,or compressive deformation,and shows a heterogeneous characteristic.Water plays a crucial role in the stress relaxation process.A large Ca/Si ratio and high temperature reduce the cohesion between the calcium-silicate layer and the interlayer region,and the viscosity of the interlayer region,thereby accelerating the stress relaxation of C-S-H.The effect of the hydrogen bond network and the morphology of C-S-H on the evolution of the stress relaxation characteristics of C-S-H at different water contents was elucidated by nonaffine mean squared displacement.Our results shed light on the stress relaxation characteristics of C-S-H from a microscopic perspective,bridging the gap between the microscopic phenomena and the underlying atomic-level mechanisms.展开更多
目前研究表明混合材对碱-硅酸反应(ASR)有很好的抑制作用,但对混合材抑制ASR机理的认识需进一步完善。本文采用TEM／EDS、FT-IR和29Si MAS NMR等现代测试方法对不同粉煤灰掺量的水泥基材料主要水化产物C-S-H凝胶化学组成和结构进行研究...目前研究表明混合材对碱-硅酸反应(ASR)有很好的抑制作用,但对混合材抑制ASR机理的认识需进一步完善。本文采用TEM／EDS、FT-IR和29Si MAS NMR等现代测试方法对不同粉煤灰掺量的水泥基材料主要水化产物C-S-H凝胶化学组成和结构进行研究,指出了低Ca／Si比的C-S-H凝胶在抑制ASR中的重要作用(能够结合孔溶液中的碱,减少可发生ASR的有效碱量),并分析了低Ca／Si比的C-S-H凝胶的持碱机制,对混合材抑制ASR的机理有了新的认识, 完善了混合材抑制ASR的机理。研究结果不仅对于建立比较完整的碱一集料反应预防理论是十分必要的,而且对于抑制和防止碱集料反应具有一定的指导意义。展开更多
基金financially supported by the National Key Research and Development Program of China(2016YFB0701204)
文摘The effects of Sm, Si and Ca on the microstructure and mechanical property of AZ91 magnesium alloy were investigated by means of optical microscopy (OM), differential scanning calorimetry (DSC), scanning electronic microscopy (SEM), X-ray diffraction (XRD) and tensile testing. The results indicated that the addition of 1.5 wt.% Sm with or without 0.8 Si/Ca led to a decrease in the volume fraction of the β-Mg17Al12 phase and the formation of the intermetallic compounds of Al-Sm, Mg2Si, MgAlCa and Al2Ca. The microstructure of AZ91 alloy was significantly refined and distribution became discrete with additions of Sm and Ca;the average grain size of the α-Mg matrix was reduced from 239.7 ± 16.9 μm to 66.34 ± 5.10 μm. The AZ91-Sm-Ca alloy exhibited a good combination of yield strength at 135 MPa, ultimate tensile strength at 199 MPa and elongation at 4.32%, which was ascribed to grain refinement strengthening. Furthermore, the T6 treated AZ91-Sm-Ca alloy possessed yield strength of 154 MPa and elongation of 7.1%, which was due to grain refinement strengthening and reduction in discontinuous precipitates.
基金Founded bythe National Basic Research Programof China"973"(No.2001CB610706)
文摘The effect of fly ash on controlling alkail-silica rection （ASR） in simudated alkali solution was studied. The expausion of mortar bars and the content of Ca（ OH）2 in cement paste cured at 80 °G for 91 d were measured. Traasmission electron microscopy （TEM） and high-resolution transmission electron microscot9 （HRTEM） were employed to study the microstructure of C-S-H. TEM/ energy dispersive spectroscopy （EDS） leas then used to determine the composition of C-S-H. The pore structure of the paste was analyzed by mercury intntsion porosimetry （MIP）. The results show that the contents of fly ash of 30% and 45% can well inhibit ASR. And the content of Ca（OH） 2 decreases with the increase of fly ash. That fly ash reacted with Ca（OH）2 to produce C-S-H with a low Ca/Si molar ratio could bind more Na^＋ and K^＋ ious, and produce a reduction in the amount of soluble alkali available for ASR. At the same time, the C- S- H produced by pozzolanic reaction converted large pores to snudler ones （ gel pores smaller than 10 nm ） to deusify the pore structure. Perhaps that could inhibit alkali trausport to aggregate for ASR.
基金Funded by the Key Science and Technology Project of Heilongjiang Transport Department(No.2017HLJ0029)the National Natural Science Foundation of China(Nos.51778513 and 51878003)the Key Projects of Anhui Province Outstanding Young Talents Support Plan(gxyqZD2019055)。
文摘The nanostructure and chloride binding capacity evolution of C-A-S-H gel exposed to aggressive solutions were investigated,utilizing 29Si NMR,27Al NMR,SEM-EDS,and XRD techniques.The experimental results show that while Cl-ions show a smaller effect on the microstructure of C-A-S-H sample,and SO4^2-ions can react with C-A-S-H,resulting in decreasing Ca/Si and Al[4]/Si for the C-A-S-H gel (i e,decalcification and dealuminization).The presence of Mg^2+ ions can aggravate the decalcifying and dealuminizing effects of SO4^2-ions on the C-A-S-H.With decreasing Ca/Si ratio and aluminum substitution for the original C-A-S-H gel,the depolymerization degree of silicate tetrahedra increases and the calcium aluminosilicate skeleton strengthens.C-A-S-H gel with lower Ca/Si ratio and higher Al[4]/Si ratio shows gentler nanostructure evolution under chemical attack,i e,improving thermodynamic stability under chemical attack.Furthermore,the chloride binding capacity of C-A-S-H gel is decreased after the sulfate attack.Aluminum substitution can also help C-A-S-H gel resist the degraded chloride binding capacity induced by sulfate attack.
基金Funded by the Natural Science Foundation of Ministry of Housing and Urban-Rural Development of the People's Republic of China(No.2009-K4-27)
文摘An accelerated laboratory method（saturated ammonium nitrate solution immersion method） was used to analyze the degradation of cement decalcification process. By studying the changes of intensity, volume, elastic modulus, quality, p H value, the Ca/Si, and mineral phase, it could be found that the first cement decalcification degradation process was the decalcification of calcium hydroxide, and then CSH gel, AFm, etc. The secondary ettringite deposition happened and the decalcification degradation depth was proportional to the square root of time. Moreover, the corresponding strength of cement would be gradually reduced, cement rock volume shrinkage occurred, p H values decreased, the surface elastic modulus decreased down to a certain level, and slightly changed and the Ca/Si was 3.1 from the beginning and lasted down to 1.3.
基金This work is supported by the National Natural Science Foundation of China(Nos.51602229 and U2040222)the Opening Project of Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education(Tongji University)the Water Conservancy Science and Technology Project of Hunan Province(No.XSKJ2021000-15),China.
文摘The time-dependent viscoelastic response of cement-based materials to applied deformation is far from fully understood at the atomic level.Calcium silicate hydrate(C-S-H),the main hydration product of Portland cement,is responsible for the viscoelastic mechanism of cement-based materials.In this study,a molecular model of C-S-H was developed to explain the stress relaxation characteristics of C-S-H at different initial deformation states,Ca/Si ratios,temperatures,and water contents,which cannot be accessed experimentally.The stress relaxation of C-S-H occurs regardless of whether it is subjected to initial shear,tensile,or compressive deformation,and shows a heterogeneous characteristic.Water plays a crucial role in the stress relaxation process.A large Ca/Si ratio and high temperature reduce the cohesion between the calcium-silicate layer and the interlayer region,and the viscosity of the interlayer region,thereby accelerating the stress relaxation of C-S-H.The effect of the hydrogen bond network and the morphology of C-S-H on the evolution of the stress relaxation characteristics of C-S-H at different water contents was elucidated by nonaffine mean squared displacement.Our results shed light on the stress relaxation characteristics of C-S-H from a microscopic perspective,bridging the gap between the microscopic phenomena and the underlying atomic-level mechanisms.
文摘目前研究表明混合材对碱-硅酸反应(ASR)有很好的抑制作用,但对混合材抑制ASR机理的认识需进一步完善。本文采用TEM／EDS、FT-IR和29Si MAS NMR等现代测试方法对不同粉煤灰掺量的水泥基材料主要水化产物C-S-H凝胶化学组成和结构进行研究,指出了低Ca／Si比的C-S-H凝胶在抑制ASR中的重要作用(能够结合孔溶液中的碱,减少可发生ASR的有效碱量),并分析了低Ca／Si比的C-S-H凝胶的持碱机制,对混合材抑制ASR的机理有了新的认识, 完善了混合材抑制ASR的机理。研究结果不仅对于建立比较完整的碱一集料反应预防理论是十分必要的,而且对于抑制和防止碱集料反应具有一定的指导意义。
