We focused on the efflorescence induced microstructural evolution of ettringite-rich systems prepared with calcium aluminate cement(CAC)and anhydrite.The effects of anhydrite on the visible efflorescence,and the corre...We focused on the efflorescence induced microstructural evolution of ettringite-rich systems prepared with calcium aluminate cement(CAC)and anhydrite.The effects of anhydrite on the visible efflorescence,and the corresponding capillary absorption of CAC-anhydrite mortars were revealed.The composition and microstructure of efflorescence-causing substances were investigated by optical microscope,in-situ Raman spectroscopy,scanning electron microscope,energy dispersive spectrometer,thermogravimetric analysis,and differential scanning calorimetry,at multi-scales.Results indicate that,besides the calcium carbonate,ettringite is another main component of efflorescence-causing substances.Compared with the neat CAC mortars,the addition of anhydrite has a significant effect on the degree of efflorescence by acting on the composition of hydration products and pore structure.In addition,methods are proposed for the prevention of efflorescence of CAC-anhydrite binary system.展开更多
The isothermal absorption properties and kinetic model of Cr(VI) and Cr(III) onto ettringite were investigated using the batch adsorption method. IR analysis was used to study the difference and mechanism of the adsor...The isothermal absorption properties and kinetic model of Cr(VI) and Cr(III) onto ettringite were investigated using the batch adsorption method. IR analysis was used to study the difference and mechanism of the adsorption of chromium ions with different valence states. The results show that the adsorption of Cr(III) onto ettringite at 20 ℃ agrees with Langmuir’s isothermal model. The ion binding stability was significantly greater than that of Cr(VI). While the adsorption of Cr(VI) onto ettringite agrees with Freundlich’s isothermal model, the D-R model fits the adsorption isotherms of two types of valence Cr(R2>0.994). It can be concluded that the adsorption of Cr(III) onto ettringite is mainly by chemical adsorption and that the adsorption of Cr(VI) onto ettringite is mainly by physical adsorption. Dynamic model fitting and model parameter analyses show that the adsorption of Cr(III) onto ettringite agrees with the pseudo second order kinetics model given by Lagergren. The formation of chemical bonds is the main factor causing the fast adsorption. Cr(VI) adsorption is mainly dominated by liquid film diffusion, and the adsorption rate is much slower than that of Cr(III) adsorption.展开更多
The formation and transformation of ettringite were studied by measuring the ion concentration in liquid phase and analyzing the composition in solid phase. The effects of C3A, gypsum lime and C-S-H gel on ettringite...The formation and transformation of ettringite were studied by measuring the ion concentration in liquid phase and analyzing the composition in solid phase. The effects of C3A, gypsum lime and C-S-H gel on ettringite formation and transformation were also investigated. The experimental results shuw, that, when gypsum was presented in solid phase, the composition of liquid phase was in favor of ettringite formation. Ettringite formation consisted of three reactions including the formation of [ Al （OH）6 ]^3- octahedral, formation of Ca-Al polyhedra prism in which Ca and Al polyhedra arranged alternately, as well as entrance of SO4^2- into the channel of polyhedra. [ Al （OH）6 ]^3- formation, which was the slowest reaction, controlled ettringite formation. The concentration of AlO2^- was a main factor that influenced ettringite formation. After gypsum in solid phase was consumed, [ SO4^2- ] decreased quickly and [ AlO2^- ] increased, and it was possible that ettringite transformed into hydrated monsulfate calcium aluminate （ be called M in short） in thermodymanics. The rate of transformation was controlled by diffusion of AlO2^- .展开更多
By employing different forms and amounts of materials, many kinds of ettringite type expansive agents had been prepared. The relationship between the compositions and properties of expansive agents was analyzed. The d...By employing different forms and amounts of materials, many kinds of ettringite type expansive agents had been prepared. The relationship between the compositions and properties of expansive agents was analyzed. The design methods of expansive agent have been put forward according to the property, requirement of expansive concrete.展开更多
Two different mechanisms have been proposed for the formation of ettringite, i.e. through-solution reaction and topochemical reaction. In this paper, the formation mechanism of secondary ettringite in concrete due to ...Two different mechanisms have been proposed for the formation of ettringite, i.e. through-solution reaction and topochemical reaction. In this paper, the formation mechanism of secondary ettringite in concrete due to sulfate attack was reviewed and analyzed. It was deduced that the formation mechanism of secondary ettringite is mainly by topochemical mechanism. The sample made from AFm (4CaO·Al2O3·CaSO4·12H2O) and Ca (OH)2 was immersed in 5% sodium sulfate at 20℃for 20 days. X-ray diffraction (XRD) was used to analyze the sample. The results verified that the formation mechanism of secondary ettringite should be attributed to topochemical reaction.展开更多
In the present study, the average modulus of delayed ettringite is evaluated by an experimental method combined with theoretical analysis. Firstly, the delayed ettringite crystal is synthesized by chemical reaction of...In the present study, the average modulus of delayed ettringite is evaluated by an experimental method combined with theoretical analysis. Firstly, the delayed ettringite crystal is synthesized by chemical reaction of Aluminum sulfate and calcium hydroxide. Secondly, specimens are obtained by compressing the delayed ettringite crystal under different pre-loads. Thirdly, the variation of the modulus of the specimen with different pre-loads is tested using Instron material test machine and the SHPB technique, respectively. It is found that the experimental data may be suitably fitted by Boltzmann Function. Finally, the porosity of the specimen is detected using the saturation method, and the effect of the porosity on the modulus is analyzed by the Eshelby＇s equivalent inclusion method and the Mori-Tanaka＇s scheme. The static and dynamic modulli of the equivalent homogeneous ettringite obtained in present study are approximately 10.64 GPa and 24.61 GPa, respectively.展开更多
The hydration reaction of a mixture of tricalcium aluminate (C3A) and gypsum with the molar ratio of 1:3 was carried out at room temperature and a water/solid ratio of 4.0. The hydration was carried out in presence of...The hydration reaction of a mixture of tricalcium aluminate (C3A) and gypsum with the molar ratio of 1:3 was carried out at room temperature and a water/solid ratio of 4.0. The hydration was carried out in presence of 0, 1 and 3% stearic acid and the mixes were designated as A, B and C, respectively. Ettringite was the only hydration product formed in the presence and absence of stearic acid. Phase composition, microstructure, infra-red analysis as well as degree of hydration were carried out for the different hydration mixtures. The rate of ettringite formation in the presence of 3% stearic acid was accelerated during the first half hour of hydration, and then retardation was occurred. In the presence of 1% stearic acid the ettringite formation was accelerated first till 3 days, then retardation was observed at later hydration ages.展开更多
Ettringite is a main hydrate of cement, and the Sr-bearing ettringite is a main hydrate of Sr-bearing calcium suplhoaluminate. In this paper the two hydrates are studied by a quantum chemistry method, the self-consist...Ettringite is a main hydrate of cement, and the Sr-bearing ettringite is a main hydrate of Sr-bearing calcium suplhoaluminate. In this paper the two hydrates are studied by a quantum chemistry method, the self-consistent-field discrete variation X(alpha) method (SCC-DV-X(alpha)) The results show: their bond order of Al and covalent bond order of Al-O bond are alike; that the bond order of Sr and the covalent bond order of Sr-O in Sr-bearing ettringite are higher than these of ettringite is the main reason, that the strength of Sr-bearing ettringite is higher than that of ettringite.展开更多
On parabolic flights,the growth of ettringite,[Ca3Al(OH)6·12H2O]2·(SO4)3·2H2O,a major reaction product of cement with water which forms instantaneously,was crystallized under microgravity conditions and...On parabolic flights,the growth of ettringite,[Ca3Al(OH)6·12H2O]2·(SO4)3·2H2O,a major reaction product of cement with water which forms instantaneously,was crystallized under microgravity conditions and studied.In the experiments,Ca(OH)2/Al2(SO4)3 solutions were combined and reacted for 10?s,followed by immediate filtration of the suspension and subsequent quenching with acetone.For the ettringite crystals,the size,aspect ratios,quantity and morphology were determined and the results were compared with those from identical experiments performed under terrestric gravity.Under microgravity,generally smaller crystals (l-2.9 μm) precipitated in larger amount than under normal gravity (l-3.5 μm).The aspect ratios of the crystals grown under terrestric or microgravity condition were comparable at about 5.6.It is assumed that the reason for the smaller ettringite crystals is the absence of convection leading to more initial nuclei,but slower crystal growth which is diffusion limited.Apparently,no preference relative to the ion transport to the different faces of the crystals exists.The results contribute to the understanding of the mineralization of inorganic salts under microgravity conditions for which hitherto only a handful of examples were reported.展开更多
In part 1 of this two-part series, the formation mechanism of secondary ettringite has been discussed. In this part, the expansive mechanisms of secondary ettringite due to the ettringite form of sulfate attack were r...In part 1 of this two-part series, the formation mechanism of secondary ettringite has been discussed. In this part, the expansive mechanisms of secondary ettringite due to the ettringite form of sulfate attack were reviewed and analyzed. Three theories have been proposed for explaining the expansive mechanism of secondary ettringite, that is, the crystallization pressure theory (or the crystal growth theory), the theory increasing in solid volume and the swelling theory. According to the analysis, swelling theory should be responsible for the expansion of secondary ettringite. The experimental results also verified that the expansion, subsequent the cracking and spalling of concrete can be caused by the swelling of fine ettringite particles formed at the interface zone of coarse aggregate particles.展开更多
Effects of polycarboxylate type admixture（PCA）on calcium monocarboaluminate hydrate（AFmc）formation in hydrated cement paste containing limestone filler（LF）are investigated by the Fourier transform infrared spect...Effects of polycarboxylate type admixture（PCA）on calcium monocarboaluminate hydrate（AFmc）formation in hydrated cement paste containing limestone filler（LF）are investigated by the Fourier transform infrared spectroscopy（FTIR）,the scanning electron microscopy（SEM）,the derivative thermogravimetric（DTG）analysis and the adsorption amount measurement.Experimental results indicate that AFmc forms during the initial hydration period of cement as early as 15 min.It is found that PCA accelerates the early age AFmc formation and enhances cement hydration by promoting C4AF hydration at the early age,and,as a consequence,the iron associated AFmc phase forms more readily.The phenomenon is not observed when PCA is replaced by a naphthalene formaldehyde sulphonate condensate water reducer.Compatibility between PCA and cement is modified due to the presence of AFmc along with ettringite（AFt）,which results in a less adsorption amount of PCA on the surface of cement minerals.As a kind of high-range water reducer,PCA may be the preferred choice for concrete containing LF.展开更多
The feasibility of sulphoaluminate cement （SAC） utilization in support mortar was studied. Setting time and strength of as-received sulphoaluminate cement （SAC） paste were examined, hydration kinetics behavior was...The feasibility of sulphoaluminate cement （SAC） utilization in support mortar was studied. Setting time and strength of as-received sulphoaluminate cement （SAC） paste were examined, hydration kinetics behavior was determined through Isothermal Calorimeter, and hydration mechanism was investigated by X-Ray diffraction analysis （XRD） and field emission scanning electron microscopy analysis （FSEM）. Results showed that as-received SAC contained 61% of anhydrous calcium sulfate （3CA＇CaSO4） and dicalcium silicate （C2S）. The strength after 1 day or 3 days grew to 68.6% or 85.7% of that after 28 days respectively, while most of hydration heat was released within 1 day. The emergency of three exothermic peaks at acceleration stage was found and hydration kinetics model was established choosing the terminal time of the first exothermic peak at accelerating stage as the beginning of accelerating stage. XRD analysis suggested that large amount of ettringite （AFt） was produced at early age and FSEM observation revealed that ettringite （AFt） formed in sulphoaluminate cement （SAC） paste was characterized of different morphology which was proved to be caused by different ion concentrations.展开更多
In order to improve the engineering properties oforganic soil, a new stabilization agent is developed by theaddition of phosphor gypsum and calcium aluminate cement.The artificial organic soil is applied in the study ...In order to improve the engineering properties oforganic soil, a new stabilization agent is developed by theaddition of phosphor gypsum and calcium aluminate cement.The artificial organic soil is applied in the study and a series oflaboratory tests were carried out to explore new stabilizationagents and determine the optimal dosage. Unconfinedcompressive strength （UCS） and the pH value of soil poresolution were measured. The influence of organic content,agent composition and curing time on the UCS of sampleswere also researched. The test results show that the UCS ofstabilized organic soils by a new agent achieves approximately800 and 1 200 kPa at 28 and 90 d curing time, respectively.The pH test results show that a high alkaline environment is anecessary and not a sufficient condition for high strength. Thestrength of stabilized soil is related to the hydration product ofstabilization agent. The mechanism of strength formation wasalso explored by X-ray diffraction （XRD）, mercury intrusionporosimetry （MIP） and scanning electron microscope （SEM）tests. A large amount of ettringite is produced to fill the largepores of organic soils, which contribute to the high UCS valueof stabilized organic soils. The new agent can solidify theorganic soil successfully as well as provide a new approach totreat the organic soil.展开更多
Supersulphated phosphogysum-slag cement （SSC） is a newly developed non-burned cementitious material mainly composed of phosphogysum （PG） and ground granulated blast furnace slag （GGBFS）, with small amount of ste...Supersulphated phosphogysum-slag cement （SSC） is a newly developed non-burned cementitious material mainly composed of phosphogysum （PG） and ground granulated blast furnace slag （GGBFS）, with small amount of steel slag （SS） and clinker （CL）. SSC is a kind of environmentally-friendly cementitious material due to its energy-saving, low-carbon emission, and waste-utilization. We prepared concretes with supersulphated phosphogysum-slag cement, and studied the mechanical properties, micro- properties and resistance to chloride penetration of concrete in comparison with those of portland slag cement （PSC） and ordinary portland cement （OPC） concrete. The test results show that the compressive strength of SSC concrete can reach 38.6 MPa after 28 d, close to PSC concrete and OPC concrete. Microanalyses indicate that large quantities of ettringite and C-S-H, and little amount of Ca（OH）2 are generated during the hydration of SSC. The dense cement paste structure of SSC is formed by ettringite and C-S-H, surrounded unreacted phosphogysum. The property of resistance to chloride penetration of SSC concrete is better than PSC and OPC concrete due to the fact that SSC can form much more ettringite to solidify more Cl^-.展开更多
The influences of water/cement ratio and admixtures on carbonation resistance of sulphoaluminate cement-based high performance concrete （HPC） were investigated. The experimental results show that with the decreasing...The influences of water/cement ratio and admixtures on carbonation resistance of sulphoaluminate cement-based high performance concrete （HPC） were investigated. The experimental results show that with the decreasing water/cement ratio, the carbonation depth of sulphoaluminate cement-based HPC is decreased remarkably, and the carbonation resistance capability is also improved with the adding admixtures. The morphologies and structure characteristics of sulphoaluminate cement hydration products before and after carbonation were analyzed using SEM and XRD. The analysis results reveal that the main hydration product of sulphoaluminate cement, that is ettringite （AFt）, decomposes after carbonation.展开更多
This paper presents an experimental study and micro-mechanism discussion on gypsum role in the mechanical improvements of cement-based stabilized clay(CBSC).A soft marine clay at two initial water contents(i.e.50%and ...This paper presents an experimental study and micro-mechanism discussion on gypsum role in the mechanical improvements of cement-based stabilized clay(CBSC).A soft marine clay at two initial water contents(i.e.50%and 70%)was treated by reconstituted cementitious binders with varying gypsum to clinker(G/C)ratios and added metakaolin to facilitate the formation of ettringite,followed by the measurements of final water contents,dry densities and strengths in accordance with ASTM standards as well as microstructure by mercury intrusion porosimetry(MIP)and scanning electron microscopy(SEM).Results reveal that the gypsum fraction has a significant influence on the index and mechanical properties of the CBSC,and there exists a threshold of the G/C ratio,which is 10%and 15%for clays with 50%and 70%initial water contents,respectively.Beyond which adding excessive gypsum cannot improve the strength further,eliminating the beneficial role.At these thresholds of the G/C ratio,the unconfined compressive strength(UCS)values for clays with 50%and 70%initial water contents are 1.74 MPa and 1.53 MPa at 60 d of curing,respectively.Microstructure characterization shows that,besides the common cementation-induced strengthening,newly formed ettringite also acts as significant pore infills,and the associated remarkable volumetric expansion is responsible,and may be the primary factor,for the beneficial strength gain due to the added gypsum.Moreover,pore-filling ettringite also leads to the conversion of relatively large inter-aggregate to smaller intra-aggregate pores,thereby causing a more homogeneous matrix or solid skeleton with higher strength.Overall,added gypsum plays a vital beneficial role in the strength development of the CBSC,especially for very soft clays.展开更多
Due to the environmental policies and economic reasons,the water used in some flotation operations of complex sulfide ores is recirculated,causing the ion concentration of some species to increase over time,affecting ...Due to the environmental policies and economic reasons,the water used in some flotation operations of complex sulfide ores is recirculated,causing the ion concentration of some species to increase over time,affecting the flotation of the minerals of interest.In this work,an experimental and thermodynamic analysis of the synthetic solutions was presented with a high content of calcium and sulfate ions.The study focused on evaluating the use of two aluminum compounds for the precipitation of Ca^(2+)and SO_(4)^(2−)in the form of ettringite.The amorphous aluminum hydroxide was found to be more efficient than the crystalline one,giving rise to 83%calcium and 91%sulfate removal.The XRD analysis of the solids showed the main reaction product of ettringite,accompanied by small amounts of calcite,due to the absorption of atmospheric carbon dioxide.The final solution after the chemical treatment showed residual calcium and sulfate concentrations below 200 mg/L.Finally,the kinetics of calcium removal appeared to correspond to a second order reaction with respect to calcium concentration,with an apparent activation energy of 53.48 kJ/mol,which may suggest that the ettringite precipitation process is governed by the chemical reaction.展开更多
The influences of different nano-SiO2（NS） contents on the mechanical properties and rheological behavior of sulfoaluminate cement（SAC） based composite materials were studied.Results show that with increasing conte...The influences of different nano-SiO2（NS） contents on the mechanical properties and rheological behavior of sulfoaluminate cement（SAC） based composite materials were studied.Results show that with increasing content of NS,the apparent viscosity,and shearing strength of fresh paste gradually increase but the fluidity decreases.With a dosage of 3.0%NS,the tensile and flexural strengths of mortars at 56 days were increased by 87.0%and 84.6%,respectively,compared with that in the absence of NS,indicating that the toughness of hardened mortars is significantly improved.Besides,the exothermic peaks of hydration are obviously increased and will earlier occur,and the second and the third peaks appear 2.61 hours and 2.56 hours earlier,respectively than that in the absence of NS,and the hydration of SAC before 8 hours is accelerated.The forming mechanism of strengths was revealed by scanning electron microscopy（SEM）,hydration heat,X-ray diffraction（XRD） and derivative thermogravimetry（DTG）.The micro-aggregate filling effect and nucleation effect at early age and weak pozzolanic effect at late age of NS make the microstructure more compact,which obviously enhances the strength of SAC mortars.展开更多
The assessment of magnesium sulphate attack on concretes containing rice husk ash （RHA, 20wt% of the cementitious materials） with various average particle sizes was investigated. The total cementitious materials wer...The assessment of magnesium sulphate attack on concretes containing rice husk ash （RHA, 20wt% of the cementitious materials） with various average particle sizes was investigated. The total cementitious materials were 390 kg and the water-to-binder ratio （W/B） was 0.53 for all mixtures. Specimens were initially cured in water for 7 d and then immersed in the 3wt% magnesium sulphate solution for up to 111 d of exposure. The specimens were subjected to drying-wetting cycles to accelerate sulphate attack. In addition to the visual monitoring of the specimens, the concrete specimens were subsequently tested for compressive strength, dynamic modulus of elasticity, and length and mass changes. The results show that the specimens exposed to sulphate attack exhibit higher strength and dynamic modulus than those kept in water. The length change is negligible and can be attributed to the normal swelling of concrete. On the other hand, concretes suffers mass loss and surface spalling and softening; the fine RHA-concrete results in a better resistance. For the accelerated sulphate attack method used in this study, mass change and visual monitoring are recommended for assessing the deterioration degree and the effectiveness of supplementary cementitious materials to resist sulphate attack.展开更多
The properties and microscopic structure of tailings solidification bodies, the hardening mechanism of the fluorgypsum-based binder material （FBBM） and tailings solidification mechanism were investigated. FBBM consi...The properties and microscopic structure of tailings solidification bodies, the hardening mechanism of the fluorgypsum-based binder material （FBBM） and tailings solidification mechanism were investigated. FBBM consisted of 40% fluorgypsum,25%-50% blast furnace slag,10%-35% ordinary Portland cement clinker（OPCC） and 1% activator, which was good material in binding iron ore tailings.XRD analysis showed that the properties of tailings solidification bodies was related to its ettringite content.SEM and XRD analyses of tailings solidification bodies showed that the chemical reaction was produced between FBBM and tailings in the whole hydration process. The hydration of the resulting gel material covered the surface of the tailings particles and formed a gel structure. A large number of ettringite crystals and the remaining board-like gypsum crystals took each end and constituted the structural skeleton. The filamentous network-like calcium-silicate-hydrate（C-S-H） gel bonded firmly together with ettringite crystals, dehydrated gypsum crystals and granular tailings, and formed an extremely dense whole.展开更多
基金Funded by the National Key Research and Development Program of China(No.2022YFC3803400)National Natural Science Foundation of China(Nos.52378255,52278270)+1 种基金Shanghai Municipal Science and Technology Major Project(No.2021SHZDZX0100)the Fundamental Research Funds for the Central Universities and the Experimental Center of Materials Science and Engineering in Tongji University。
文摘We focused on the efflorescence induced microstructural evolution of ettringite-rich systems prepared with calcium aluminate cement(CAC)and anhydrite.The effects of anhydrite on the visible efflorescence,and the corresponding capillary absorption of CAC-anhydrite mortars were revealed.The composition and microstructure of efflorescence-causing substances were investigated by optical microscope,in-situ Raman spectroscopy,scanning electron microscope,energy dispersive spectrometer,thermogravimetric analysis,and differential scanning calorimetry,at multi-scales.Results indicate that,besides the calcium carbonate,ettringite is another main component of efflorescence-causing substances.Compared with the neat CAC mortars,the addition of anhydrite has a significant effect on the degree of efflorescence by acting on the composition of hydration products and pore structure.In addition,methods are proposed for the prevention of efflorescence of CAC-anhydrite binary system.
基金Supported by the National Natural Science Foundation of China(No.2010CB735803)
文摘The isothermal absorption properties and kinetic model of Cr(VI) and Cr(III) onto ettringite were investigated using the batch adsorption method. IR analysis was used to study the difference and mechanism of the adsorption of chromium ions with different valence states. The results show that the adsorption of Cr(III) onto ettringite at 20 ℃ agrees with Langmuir’s isothermal model. The ion binding stability was significantly greater than that of Cr(VI). While the adsorption of Cr(VI) onto ettringite agrees with Freundlich’s isothermal model, the D-R model fits the adsorption isotherms of two types of valence Cr(R2>0.994). It can be concluded that the adsorption of Cr(III) onto ettringite is mainly by chemical adsorption and that the adsorption of Cr(VI) onto ettringite is mainly by physical adsorption. Dynamic model fitting and model parameter analyses show that the adsorption of Cr(III) onto ettringite agrees with the pseudo second order kinetics model given by Lagergren. The formation of chemical bonds is the main factor causing the fast adsorption. Cr(VI) adsorption is mainly dominated by liquid film diffusion, and the adsorption rate is much slower than that of Cr(III) adsorption.
文摘The formation and transformation of ettringite were studied by measuring the ion concentration in liquid phase and analyzing the composition in solid phase. The effects of C3A, gypsum lime and C-S-H gel on ettringite formation and transformation were also investigated. The experimental results shuw, that, when gypsum was presented in solid phase, the composition of liquid phase was in favor of ettringite formation. Ettringite formation consisted of three reactions including the formation of [ Al （OH）6 ]^3- octahedral, formation of Ca-Al polyhedra prism in which Ca and Al polyhedra arranged alternately, as well as entrance of SO4^2- into the channel of polyhedra. [ Al （OH）6 ]^3- formation, which was the slowest reaction, controlled ettringite formation. The concentration of AlO2^- was a main factor that influenced ettringite formation. After gypsum in solid phase was consumed, [ SO4^2- ] decreased quickly and [ AlO2^- ] increased, and it was possible that ettringite transformed into hydrated monsulfate calcium aluminate （ be called M in short） in thermodymanics. The rate of transformation was controlled by diffusion of AlO2^- .
基金Funded by the State "the Nineth Five-Plan"of Communica-tion Ministry(95 - 05 - 02 - 25)
文摘By employing different forms and amounts of materials, many kinds of ettringite type expansive agents had been prepared. The relationship between the compositions and properties of expansive agents was analyzed. The design methods of expansive agent have been put forward according to the property, requirement of expansive concrete.
基金Funded by the National Natural Science Foundation of China (No. 50378092)
文摘Two different mechanisms have been proposed for the formation of ettringite, i.e. through-solution reaction and topochemical reaction. In this paper, the formation mechanism of secondary ettringite in concrete due to sulfate attack was reviewed and analyzed. It was deduced that the formation mechanism of secondary ettringite is mainly by topochemical mechanism. The sample made from AFm (4CaO·Al2O3·CaSO4·12H2O) and Ca (OH)2 was immersed in 5% sodium sulfate at 20℃for 20 days. X-ray diffraction (XRD) was used to analyze the sample. The results verified that the formation mechanism of secondary ettringite should be attributed to topochemical reaction.
基金supported by the National Basic Research Program of China(973 Program,2009CB623203)the National Nature Science Foundation of China(Nos.10572064 and 10802039)+1 种基金Natural Science Foundation of Zhejiang Province (No.Y107780)K.C.Wong Magna Fund in Ningbo University.
文摘In the present study, the average modulus of delayed ettringite is evaluated by an experimental method combined with theoretical analysis. Firstly, the delayed ettringite crystal is synthesized by chemical reaction of Aluminum sulfate and calcium hydroxide. Secondly, specimens are obtained by compressing the delayed ettringite crystal under different pre-loads. Thirdly, the variation of the modulus of the specimen with different pre-loads is tested using Instron material test machine and the SHPB technique, respectively. It is found that the experimental data may be suitably fitted by Boltzmann Function. Finally, the porosity of the specimen is detected using the saturation method, and the effect of the porosity on the modulus is analyzed by the Eshelby＇s equivalent inclusion method and the Mori-Tanaka＇s scheme. The static and dynamic modulli of the equivalent homogeneous ettringite obtained in present study are approximately 10.64 GPa and 24.61 GPa, respectively.
文摘The hydration reaction of a mixture of tricalcium aluminate (C3A) and gypsum with the molar ratio of 1:3 was carried out at room temperature and a water/solid ratio of 4.0. The hydration was carried out in presence of 0, 1 and 3% stearic acid and the mixes were designated as A, B and C, respectively. Ettringite was the only hydration product formed in the presence and absence of stearic acid. Phase composition, microstructure, infra-red analysis as well as degree of hydration were carried out for the different hydration mixtures. The rate of ettringite formation in the presence of 3% stearic acid was accelerated during the first half hour of hydration, and then retardation was occurred. In the presence of 1% stearic acid the ettringite formation was accelerated first till 3 days, then retardation was observed at later hydration ages.
文摘Ettringite is a main hydrate of cement, and the Sr-bearing ettringite is a main hydrate of Sr-bearing calcium suplhoaluminate. In this paper the two hydrates are studied by a quantum chemistry method, the self-consistent-field discrete variation X(alpha) method (SCC-DV-X(alpha)) The results show: their bond order of Al and covalent bond order of Al-O bond are alike; that the bond order of Sr and the covalent bond order of Sr-O in Sr-bearing ettringite are higher than these of ettringite is the main reason, that the strength of Sr-bearing ettringite is higher than that of ettringite.
文摘On parabolic flights,the growth of ettringite,[Ca3Al(OH)6·12H2O]2·(SO4)3·2H2O,a major reaction product of cement with water which forms instantaneously,was crystallized under microgravity conditions and studied.In the experiments,Ca(OH)2/Al2(SO4)3 solutions were combined and reacted for 10?s,followed by immediate filtration of the suspension and subsequent quenching with acetone.For the ettringite crystals,the size,aspect ratios,quantity and morphology were determined and the results were compared with those from identical experiments performed under terrestric gravity.Under microgravity,generally smaller crystals (l-2.9 μm) precipitated in larger amount than under normal gravity (l-3.5 μm).The aspect ratios of the crystals grown under terrestric or microgravity condition were comparable at about 5.6.It is assumed that the reason for the smaller ettringite crystals is the absence of convection leading to more initial nuclei,but slower crystal growth which is diffusion limited.Apparently,no preference relative to the ion transport to the different faces of the crystals exists.The results contribute to the understanding of the mineralization of inorganic salts under microgravity conditions for which hitherto only a handful of examples were reported.
基金Funded by the National Natural Science Foundation of China(No. 0378092)
文摘In part 1 of this two-part series, the formation mechanism of secondary ettringite has been discussed. In this part, the expansive mechanisms of secondary ettringite due to the ettringite form of sulfate attack were reviewed and analyzed. Three theories have been proposed for explaining the expansive mechanism of secondary ettringite, that is, the crystallization pressure theory (or the crystal growth theory), the theory increasing in solid volume and the swelling theory. According to the analysis, swelling theory should be responsible for the expansion of secondary ettringite. The experimental results also verified that the expansion, subsequent the cracking and spalling of concrete can be caused by the swelling of fine ettringite particles formed at the interface zone of coarse aggregate particles.
基金The Natural Science Foundation of Jiangsu Province(No.BK2009712)the National Construction Research Project(No.2009-K4-9)
文摘Effects of polycarboxylate type admixture（PCA）on calcium monocarboaluminate hydrate（AFmc）formation in hydrated cement paste containing limestone filler（LF）are investigated by the Fourier transform infrared spectroscopy（FTIR）,the scanning electron microscopy（SEM）,the derivative thermogravimetric（DTG）analysis and the adsorption amount measurement.Experimental results indicate that AFmc forms during the initial hydration period of cement as early as 15 min.It is found that PCA accelerates the early age AFmc formation and enhances cement hydration by promoting C4AF hydration at the early age,and,as a consequence,the iron associated AFmc phase forms more readily.The phenomenon is not observed when PCA is replaced by a naphthalene formaldehyde sulphonate condensate water reducer.Compatibility between PCA and cement is modified due to the presence of AFmc along with ettringite（AFt）,which results in a less adsorption amount of PCA on the surface of cement minerals.As a kind of high-range water reducer,PCA may be the preferred choice for concrete containing LF.
基金Funded by the National Natural Science Foundation of China(Nos.51379163 and 51109170)the National Key Research Program(973 Program)(No.2013CB035901)
文摘The feasibility of sulphoaluminate cement （SAC） utilization in support mortar was studied. Setting time and strength of as-received sulphoaluminate cement （SAC） paste were examined, hydration kinetics behavior was determined through Isothermal Calorimeter, and hydration mechanism was investigated by X-Ray diffraction analysis （XRD） and field emission scanning electron microscopy analysis （FSEM）. Results showed that as-received SAC contained 61% of anhydrous calcium sulfate （3CA＇CaSO4） and dicalcium silicate （C2S）. The strength after 1 day or 3 days grew to 68.6% or 85.7% of that after 28 days respectively, while most of hydration heat was released within 1 day. The emergency of three exothermic peaks at acceleration stage was found and hydration kinetics model was established choosing the terminal time of the first exothermic peak at accelerating stage as the beginning of accelerating stage. XRD analysis suggested that large amount of ettringite （AFt） was produced at early age and FSEM observation revealed that ettringite （AFt） formed in sulphoaluminate cement （SAC） paste was characterized of different morphology which was proved to be caused by different ion concentrations.
基金The National Natural Science Foundation of Chin(No.51578148)the Project of China Communications Construction(No.2015-ZJKJ-26)the Fundamental Research Funds for the Centra Universities,the Scientific Innovation Research of College Graduates in Jiangsu Province(No.SJLX15_0062)
文摘In order to improve the engineering properties oforganic soil, a new stabilization agent is developed by theaddition of phosphor gypsum and calcium aluminate cement.The artificial organic soil is applied in the study and a series oflaboratory tests were carried out to explore new stabilizationagents and determine the optimal dosage. Unconfinedcompressive strength （UCS） and the pH value of soil poresolution were measured. The influence of organic content,agent composition and curing time on the UCS of sampleswere also researched. The test results show that the UCS ofstabilized organic soils by a new agent achieves approximately800 and 1 200 kPa at 28 and 90 d curing time, respectively.The pH test results show that a high alkaline environment is anecessary and not a sufficient condition for high strength. Thestrength of stabilized soil is related to the hydration product ofstabilization agent. The mechanism of strength formation wasalso explored by X-ray diffraction （XRD）, mercury intrusionporosimetry （MIP） and scanning electron microscope （SEM）tests. A large amount of ettringite is produced to fill the largepores of organic soils, which contribute to the high UCS valueof stabilized organic soils. The new agent can solidify theorganic soil successfully as well as provide a new approach totreat the organic soil.
基金Funded by the National High Technology Research and Development Program of China(863 Program)(No.2012AA06A112)
文摘Supersulphated phosphogysum-slag cement （SSC） is a newly developed non-burned cementitious material mainly composed of phosphogysum （PG） and ground granulated blast furnace slag （GGBFS）, with small amount of steel slag （SS） and clinker （CL）. SSC is a kind of environmentally-friendly cementitious material due to its energy-saving, low-carbon emission, and waste-utilization. We prepared concretes with supersulphated phosphogysum-slag cement, and studied the mechanical properties, micro- properties and resistance to chloride penetration of concrete in comparison with those of portland slag cement （PSC） and ordinary portland cement （OPC） concrete. The test results show that the compressive strength of SSC concrete can reach 38.6 MPa after 28 d, close to PSC concrete and OPC concrete. Microanalyses indicate that large quantities of ettringite and C-S-H, and little amount of Ca（OH）2 are generated during the hydration of SSC. The dense cement paste structure of SSC is formed by ettringite and C-S-H, surrounded unreacted phosphogysum. The property of resistance to chloride penetration of SSC concrete is better than PSC and OPC concrete due to the fact that SSC can form much more ettringite to solidify more Cl^-.
基金Funded by the National Natural Science Foundation of China(No.50872043)
文摘The influences of water/cement ratio and admixtures on carbonation resistance of sulphoaluminate cement-based high performance concrete （HPC） were investigated. The experimental results show that with the decreasing water/cement ratio, the carbonation depth of sulphoaluminate cement-based HPC is decreased remarkably, and the carbonation resistance capability is also improved with the adding admixtures. The morphologies and structure characteristics of sulphoaluminate cement hydration products before and after carbonation were analyzed using SEM and XRD. The analysis results reveal that the main hydration product of sulphoaluminate cement, that is ettringite （AFt）, decomposes after carbonation.
基金supported by the National Key R&D Program of China (Grant No. 2019YFC1806004)National Natural Science Foundation of China (Grant Nos. 51878159 and 41572280)
文摘This paper presents an experimental study and micro-mechanism discussion on gypsum role in the mechanical improvements of cement-based stabilized clay(CBSC).A soft marine clay at two initial water contents(i.e.50%and 70%)was treated by reconstituted cementitious binders with varying gypsum to clinker(G/C)ratios and added metakaolin to facilitate the formation of ettringite,followed by the measurements of final water contents,dry densities and strengths in accordance with ASTM standards as well as microstructure by mercury intrusion porosimetry(MIP)and scanning electron microscopy(SEM).Results reveal that the gypsum fraction has a significant influence on the index and mechanical properties of the CBSC,and there exists a threshold of the G/C ratio,which is 10%and 15%for clays with 50%and 70%initial water contents,respectively.Beyond which adding excessive gypsum cannot improve the strength further,eliminating the beneficial role.At these thresholds of the G/C ratio,the unconfined compressive strength(UCS)values for clays with 50%and 70%initial water contents are 1.74 MPa and 1.53 MPa at 60 d of curing,respectively.Microstructure characterization shows that,besides the common cementation-induced strengthening,newly formed ettringite also acts as significant pore infills,and the associated remarkable volumetric expansion is responsible,and may be the primary factor,for the beneficial strength gain due to the added gypsum.Moreover,pore-filling ettringite also leads to the conversion of relatively large inter-aggregate to smaller intra-aggregate pores,thereby causing a more homogeneous matrix or solid skeleton with higher strength.Overall,added gypsum plays a vital beneficial role in the strength development of the CBSC,especially for very soft clays.
文摘Due to the environmental policies and economic reasons,the water used in some flotation operations of complex sulfide ores is recirculated,causing the ion concentration of some species to increase over time,affecting the flotation of the minerals of interest.In this work,an experimental and thermodynamic analysis of the synthetic solutions was presented with a high content of calcium and sulfate ions.The study focused on evaluating the use of two aluminum compounds for the precipitation of Ca^(2+)and SO_(4)^(2−)in the form of ettringite.The amorphous aluminum hydroxide was found to be more efficient than the crystalline one,giving rise to 83%calcium and 91%sulfate removal.The XRD analysis of the solids showed the main reaction product of ettringite,accompanied by small amounts of calcite,due to the absorption of atmospheric carbon dioxide.The final solution after the chemical treatment showed residual calcium and sulfate concentrations below 200 mg/L.Finally,the kinetics of calcium removal appeared to correspond to a second order reaction with respect to calcium concentration,with an apparent activation energy of 53.48 kJ/mol,which may suggest that the ettringite precipitation process is governed by the chemical reaction.
基金Funded by the Fundamental Research Funds for the Central Universities(No.2013-YB-25)the National Natural Science Foundation of China(No.51378408)
文摘The influences of different nano-SiO2（NS） contents on the mechanical properties and rheological behavior of sulfoaluminate cement（SAC） based composite materials were studied.Results show that with increasing content of NS,the apparent viscosity,and shearing strength of fresh paste gradually increase but the fluidity decreases.With a dosage of 3.0%NS,the tensile and flexural strengths of mortars at 56 days were increased by 87.0%and 84.6%,respectively,compared with that in the absence of NS,indicating that the toughness of hardened mortars is significantly improved.Besides,the exothermic peaks of hydration are obviously increased and will earlier occur,and the second and the third peaks appear 2.61 hours and 2.56 hours earlier,respectively than that in the absence of NS,and the hydration of SAC before 8 hours is accelerated.The forming mechanism of strengths was revealed by scanning electron microscopy（SEM）,hydration heat,X-ray diffraction（XRD） and derivative thermogravimetry（DTG）.The micro-aggregate filling effect and nucleation effect at early age and weak pozzolanic effect at late age of NS make the microstructure more compact,which obviously enhances the strength of SAC mortars.
文摘The assessment of magnesium sulphate attack on concretes containing rice husk ash （RHA, 20wt% of the cementitious materials） with various average particle sizes was investigated. The total cementitious materials were 390 kg and the water-to-binder ratio （W/B） was 0.53 for all mixtures. Specimens were initially cured in water for 7 d and then immersed in the 3wt% magnesium sulphate solution for up to 111 d of exposure. The specimens were subjected to drying-wetting cycles to accelerate sulphate attack. In addition to the visual monitoring of the specimens, the concrete specimens were subsequently tested for compressive strength, dynamic modulus of elasticity, and length and mass changes. The results show that the specimens exposed to sulphate attack exhibit higher strength and dynamic modulus than those kept in water. The length change is negligible and can be attributed to the normal swelling of concrete. On the other hand, concretes suffers mass loss and surface spalling and softening; the fine RHA-concrete results in a better resistance. For the accelerated sulphate attack method used in this study, mass change and visual monitoring are recommended for assessing the deterioration degree and the effectiveness of supplementary cementitious materials to resist sulphate attack.
基金Funded by the National High Technology Research and Development Program (863 Project) (No.2009AA064004)the Fundamental Research Funds for the Central Universities (No.5082011 and No.1104003)
文摘The properties and microscopic structure of tailings solidification bodies, the hardening mechanism of the fluorgypsum-based binder material （FBBM） and tailings solidification mechanism were investigated. FBBM consisted of 40% fluorgypsum,25%-50% blast furnace slag,10%-35% ordinary Portland cement clinker（OPCC） and 1% activator, which was good material in binding iron ore tailings.XRD analysis showed that the properties of tailings solidification bodies was related to its ettringite content.SEM and XRD analyses of tailings solidification bodies showed that the chemical reaction was produced between FBBM and tailings in the whole hydration process. The hydration of the resulting gel material covered the surface of the tailings particles and formed a gel structure. A large number of ettringite crystals and the remaining board-like gypsum crystals took each end and constituted the structural skeleton. The filamentous network-like calcium-silicate-hydrate（C-S-H） gel bonded firmly together with ettringite crystals, dehydrated gypsum crystals and granular tailings, and formed an extremely dense whole.
