The microstructural study was conducted on cement and cement-slag pastes immersed in different concentrations of Mg（NO3）2 solutions utilizing ^29Si, ^27Al NMR spectroscopy and XRD techniques. The results show that t...The microstructural study was conducted on cement and cement-slag pastes immersed in different concentrations of Mg（NO3）2 solutions utilizing ^29Si, ^27Al NMR spectroscopy and XRD techniques. The results show that the hydration of both the cement and cement-slag pastes is delayed when the pastes are cured in Mg（NO3）2 solutions as compared to the pastes cured in water. Moreover, Mg^2＋ ions also exhibit an decalcifying and dealuminizing effect on the C-A-S-H in cement and cement-slag pastes, and thereby decrease Ca/Si and Al[4]/Si ratios of the C-A-S-H. The dealuminization of C-A-S-H is mitigated for cement-slag paste as compared to pure cement paste. The depolymerized calcium and aluminum ions from C-A-S-H gel mainly enter the pore solution to maintain the pH value and form Al^[6] in TAH, respectively. On the other hand, Mg^2＋ ions exert an impact on the intra-transition between Al^[6] species, from AFm and hydrogarnet to hydrotalcite-like phase. NO3^-ions are interstratified in the layered Mg-Al structure and formed nitrated hydrotalcite-like phase（Mg1-xAlx（OH）2（NO3）x·nH2O）. Results from both ^27Al NMR and XRD data show that ettringite seems not to react with Mg^2＋ ions.展开更多
The heavyweight ultra-high performance concrete(HUHPC)was prepared with barite sand partially replaced by titanium-rich heavy slag sand(THS)at replacement proportion of 0%,30%,50%,70%and 100%in this work.The results s...The heavyweight ultra-high performance concrete(HUHPC)was prepared with barite sand partially replaced by titanium-rich heavy slag sand(THS)at replacement proportion of 0%,30%,50%,70%and 100%in this work.The results show that THS incorporation can effectively improve the mechanical properties and reduce the volume shrinkage of HUHPC.The HUHPC with 50%THS replacement reaches an apparent density of 2890 kg/m^(3)(for fresh HUHPC),28 d compressive strength of 129 MPa,28 d flexural strength of 23 MPa,28 d flexural toughness of 28.4,56 d volume shrinkage of 359×10^(-4) and,as expected,excellent durability.Microstructural investigation demonstrates that the internal curing of pre-wetted THS promotes the hydration of the surrounding cement paste thereby strengthening the interfacial transition zone,resulting in the“hard shell”formation around aggregate to“protect”the aggregate.Additionally,the“pin structure”significantly improves the cement paste-aggregate interfacial connection.The combination of“hard shell protection”and“pin structure”remarkably improve the mechanical properties of HUHPC produced with porous THS aggregate.展开更多
The microstructural evolution of C-（A）-S-H gel in Portland cement pastes immersed in pure water and 5.0 wt% Na2SO4 solution for different ages was comparatively investigated, by means of ^（29） Si NMR spectroscopy,...The microstructural evolution of C-（A）-S-H gel in Portland cement pastes immersed in pure water and 5.0 wt% Na2SO4 solution for different ages was comparatively investigated, by means of ^（29） Si NMR spectroscopy, and SEM-EDS analysis. Additionally, molecular dynamics simulation was performed to study the aluminum coordination status and interaction of sulfate ions in C-（A）-S-H gel. The results showed significant changes in the microstructural evolution of C-（A）-S-H gel in Portland cement paste. Sulfate attack has decalcifying and dealuminizing effect on C-（A）-S-H gel which is evident from increase in mean chain length（MCL） and decrease in Ca/Si ＆ Al[4]/Si ratios of C-（A）-S-H gel. Additionally, Molecular dynamics simulation proves that Al[4] substituted in silicate chains of C-（A）-S-H gel is thermodynamically metastable, which may explain its migration from the silicate chains and transformation to Al[6], thus lowering the Al[4]/Si ratio of C-（A）-S-H gel. SO4^（2-）ions can carry the interfacial Ca^（2＋） ions into the pore solution by the diffusion-absorption-desorption process, which unravels the mechanism of sulfate attack on C-（A）-S-H gel.展开更多
Molecular dynamics simulation was utilized to investigate the transport and adsorption of chloride in the nanopore of calcium aluminosilicate hydrate(C-A-S-H)with associated cation types of Ca,Mg,Na and K.The local io...Molecular dynamics simulation was utilized to investigate the transport and adsorption of chloride in the nanopore of calcium aluminosilicate hydrate(C-A-S-H)with associated cation types of Ca,Mg,Na and K.The local ionic structure,atomic dynamics and bond stability were analyzed to elucidate the interaction between cations and chloride ions.The results show that interfacial chloride is absorbed through the ion pairing formation in the vicinity of C-A-S-H substrate.Interfacial cations can simultaneously interact aluminosilicate chains,water molecules and Cl^(-)ions,which restrict the motion of interfacial Cl^(-)ions.Pore solution chloride can be immobilized through the solvation effect of cations.Cations along with their hydration shell can connect to neighboring Cl^(-)ions to decrease their mobility.Owing to the varied ionic chemistry,cations show different interaction strength with neighboring water molecules and anions,which determines the chloride transport behavior in the nanopore of C-A-S-H.The chloride immobilization capacity of C-A-S-H nanopore with different associated cations is listed in following order:Mg^(2+)Ca^(2+)<Na^(+)≈K^(+),which agrees reasonably with previous experiments.展开更多
基金Funded by National Natural Science Foundation of China(Nos.51778513,51402003,51578004)China Ministry of Science and Technology(No.2015CB655101)
文摘The microstructural study was conducted on cement and cement-slag pastes immersed in different concentrations of Mg（NO3）2 solutions utilizing ^29Si, ^27Al NMR spectroscopy and XRD techniques. The results show that the hydration of both the cement and cement-slag pastes is delayed when the pastes are cured in Mg（NO3）2 solutions as compared to the pastes cured in water. Moreover, Mg^2＋ ions also exhibit an decalcifying and dealuminizing effect on the C-A-S-H in cement and cement-slag pastes, and thereby decrease Ca/Si and Al[4]/Si ratios of the C-A-S-H. The dealuminization of C-A-S-H is mitigated for cement-slag paste as compared to pure cement paste. The depolymerized calcium and aluminum ions from C-A-S-H gel mainly enter the pore solution to maintain the pH value and form Al^[6] in TAH, respectively. On the other hand, Mg^2＋ ions exert an impact on the intra-transition between Al^[6] species, from AFm and hydrogarnet to hydrotalcite-like phase. NO3^-ions are interstratified in the layered Mg-Al structure and formed nitrated hydrotalcite-like phase（Mg1-xAlx（OH）2（NO3）x·nH2O）. Results from both ^27Al NMR and XRD data show that ettringite seems not to react with Mg^2＋ ions.
基金Funded by National Natural Science Foundation of China(Nos.52008002,U2006224 and 51878003)Doctoral Scientific Research Startup Foundation of Anhui Jianzhu University(No.2019QDZ66)。
文摘The heavyweight ultra-high performance concrete(HUHPC)was prepared with barite sand partially replaced by titanium-rich heavy slag sand(THS)at replacement proportion of 0%,30%,50%,70%and 100%in this work.The results show that THS incorporation can effectively improve the mechanical properties and reduce the volume shrinkage of HUHPC.The HUHPC with 50%THS replacement reaches an apparent density of 2890 kg/m^(3)(for fresh HUHPC),28 d compressive strength of 129 MPa,28 d flexural strength of 23 MPa,28 d flexural toughness of 28.4,56 d volume shrinkage of 359×10^(-4) and,as expected,excellent durability.Microstructural investigation demonstrates that the internal curing of pre-wetted THS promotes the hydration of the surrounding cement paste thereby strengthening the interfacial transition zone,resulting in the“hard shell”formation around aggregate to“protect”the aggregate.Additionally,the“pin structure”significantly improves the cement paste-aggregate interfacial connection.The combination of“hard shell protection”and“pin structure”remarkably improve the mechanical properties of HUHPC produced with porous THS aggregate.
基金Funded by National Natural Science Foundation of China(Nos.51778513,51578004,51608004)the Major State Basic Research Development Program of China("973"Program)(No.2015CB655101)
文摘The microstructural evolution of C-（A）-S-H gel in Portland cement pastes immersed in pure water and 5.0 wt% Na2SO4 solution for different ages was comparatively investigated, by means of ^（29） Si NMR spectroscopy, and SEM-EDS analysis. Additionally, molecular dynamics simulation was performed to study the aluminum coordination status and interaction of sulfate ions in C-（A）-S-H gel. The results showed significant changes in the microstructural evolution of C-（A）-S-H gel in Portland cement paste. Sulfate attack has decalcifying and dealuminizing effect on C-（A）-S-H gel which is evident from increase in mean chain length（MCL） and decrease in Ca/Si ＆ Al[4]/Si ratios of C-（A）-S-H gel. Additionally, Molecular dynamics simulation proves that Al[4] substituted in silicate chains of C-（A）-S-H gel is thermodynamically metastable, which may explain its migration from the silicate chains and transformation to Al[6], thus lowering the Al[4]/Si ratio of C-（A）-S-H gel. SO4^（2-）ions can carry the interfacial Ca^（2＋） ions into the pore solution by the diffusion-absorption-desorption process, which unravels the mechanism of sulfate attack on C-（A）-S-H gel.
基金Funded by the National Natural Science Foundation of China(Nos.52008002,U21A20149,U2006224 and 51978352)the Open Foundation of the State Key Laboratory of Silicate Materials for Architectures(Wuhan University of Technology)(No.SYSJJ2022-22)Anhui Province Engineering Laboratory of Advanced Building Materials(No.JZCL2202ZR)。
文摘Molecular dynamics simulation was utilized to investigate the transport and adsorption of chloride in the nanopore of calcium aluminosilicate hydrate(C-A-S-H)with associated cation types of Ca,Mg,Na and K.The local ionic structure,atomic dynamics and bond stability were analyzed to elucidate the interaction between cations and chloride ions.The results show that interfacial chloride is absorbed through the ion pairing formation in the vicinity of C-A-S-H substrate.Interfacial cations can simultaneously interact aluminosilicate chains,water molecules and Cl^(-)ions,which restrict the motion of interfacial Cl^(-)ions.Pore solution chloride can be immobilized through the solvation effect of cations.Cations along with their hydration shell can connect to neighboring Cl^(-)ions to decrease their mobility.Owing to the varied ionic chemistry,cations show different interaction strength with neighboring water molecules and anions,which determines the chloride transport behavior in the nanopore of C-A-S-H.The chloride immobilization capacity of C-A-S-H nanopore with different associated cations is listed in following order:Mg^(2+)Ca^(2+)<Na^(+)≈K^(+),which agrees reasonably with previous experiments.
