S. Nishibayashi, A. Yoshino, K. Yamura, Y. Okawa, M. A. Lara
This paper describes the effects of properties of powder such as specific surface and solid volume percentage on the plastic viscosity of fresh paste with admixtures. Eight types of blended powder and three types of high-range water-reducing admixtures (HRWRA) were used, and the plastic viscosity of paste mixed with these materials was measured, keeping the yield value of paste constant. The fresh paste is considered to be a type of highly concentrated suspension, so a proposed method to predict paste viscosity based on this concept. In this method, the ratio of powder content, the specific surface, and solid volume percentage of powder, the type of admixture, and other factors are taken into account.
{"title":"Effects of Binding Materials and Chemical Admixtures on Rheological Constants of Fresh Paste","authors":"S. Nishibayashi, A. Yoshino, K. Yamura, Y. Okawa, M. A. Lara","doi":"10.14359/6192","DOIUrl":"https://doi.org/10.14359/6192","url":null,"abstract":"This paper describes the effects of properties of powder such as specific surface and solid volume percentage on the plastic viscosity of fresh paste with admixtures. Eight types of blended powder and three types of high-range water-reducing admixtures (HRWRA) were used, and the plastic viscosity of paste mixed with these materials was measured, keeping the yield value of paste constant. The fresh paste is considered to be a type of highly concentrated suspension, so a proposed method to predict paste viscosity based on this concept. In this method, the ratio of powder content, the specific surface, and solid volume percentage of powder, the type of admixture, and other factors are taken into account.","PeriodicalId":21898,"journal":{"name":"SP-173: Fifth CANMET/ACI International Conference on Superplasticizers and Other Chemical Admixtures in Concrete","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1997-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90174521","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
V. Fernon, A. Vichot, N. L. Goanvic, P. Colombet, F.Corazza, U. Costa
The ability of tricalcium aluminate hydration products to absorb polynaphthalene sulfonates (PNS) has been studied by reacting a small excess of saturated lime solution, containing various amounts of PNS, with an aqueous solution of sodium aluminate. Using X-ray diffraction, infrared spectroscopy and transmission electron microscopy, it is shown that well defined organomineral intercalation compounds result from the reaction. They can be described as layered double hydroxides where part of the hydroxyl groups have been replaced by the PNS anions. The consequences of the formation of such compounds upon the rheological characteristics in the early hydration period of portland cement is discussed. Emphasis is laid on the fact that the absorptive behavior of calcium aluminate hydrates in the presence of superplasticizers is not at the origin of the ocasionally observed abnormal early stiffening. This point is illustrated by the investigation of cases of practical interest, based in particular on the analysis of the pore fluid composition in fresh mortars and pastes.
{"title":"Interaction Between Portland Cement Hydrates and Polynapthalene Sulfonates","authors":"V. Fernon, A. Vichot, N. L. Goanvic, P. Colombet, F.Corazza, U. Costa","doi":"10.14359/6186","DOIUrl":"https://doi.org/10.14359/6186","url":null,"abstract":"The ability of tricalcium aluminate hydration products to absorb polynaphthalene sulfonates (PNS) has been studied by reacting a small excess of saturated lime solution, containing various amounts of PNS, with an aqueous solution of sodium aluminate. Using X-ray diffraction, infrared spectroscopy and transmission electron microscopy, it is shown that well defined organomineral intercalation compounds result from the reaction. They can be described as layered double hydroxides where part of the hydroxyl groups have been replaced by the PNS anions. The consequences of the formation of such compounds upon the rheological characteristics in the early hydration period of portland cement is discussed. Emphasis is laid on the fact that the absorptive behavior of calcium aluminate hydrates in the presence of superplasticizers is not at the origin of the ocasionally observed abnormal early stiffening. This point is illustrated by the investigation of cases of practical interest, based in particular on the analysis of the pore fluid composition in fresh mortars and pastes.","PeriodicalId":21898,"journal":{"name":"SP-173: Fifth CANMET/ACI International Conference on Superplasticizers and Other Chemical Admixtures in Concrete","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1997-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"87551912","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A technology to impart high fluidity to concrete with an extremely low W/C range of about 0.2 is required, in order to place ultra high-strength concrete with a compressive strength of over 100 MPa. For this purpose, the authors have developed a methacrylic water-soluable polymer as a superplasticizer (SSP) that imparts adequate workability and excellent cement dispersing capability to concrete mixtures with low W/C. Also, the high cement content of ultra high-strength concrete containing normal portland cement leads of high concrete temperature due to heat of hydration, posing problems of thermal cracking and low long-term strength. This study used a low-heat, belite-rich portland cement recently developed in Japan, together with powder silica fume to produce ultra high-strength concrete containing the SSP with a low water-binder ratio of approximately 0.2. As a result, the belite-rich portland cement was found to reduce the adiabatic temperature rise without causing set retardation when compared with normal portland cement. In addition, the concrete showed high fluidity as well as a high long-term compressive strength of over 150 MPa. These results suggest that the SSP is highly compatible with low-heat cement and is very effective in producing high-performance concrete when used in combination with this type of binder.
{"title":"Properties of Methacrylic Water-Soluable Polymer as a Superplasticicer for Ultra High-Strength Concrete","authors":"M. Kinoshita, T. Suzuki, K. Soeda, T. Nawa","doi":"10.14359/6182","DOIUrl":"https://doi.org/10.14359/6182","url":null,"abstract":"A technology to impart high fluidity to concrete with an extremely low W/C range of about 0.2 is required, in order to place ultra high-strength concrete with a compressive strength of over 100 MPa. For this purpose, the authors have developed a methacrylic water-soluable polymer as a superplasticizer (SSP) that imparts adequate workability and excellent cement dispersing capability to concrete mixtures with low W/C. Also, the high cement content of ultra high-strength concrete containing normal portland cement leads of high concrete temperature due to heat of hydration, posing problems of thermal cracking and low long-term strength. This study used a low-heat, belite-rich portland cement recently developed in Japan, together with powder silica fume to produce ultra high-strength concrete containing the SSP with a low water-binder ratio of approximately 0.2. As a result, the belite-rich portland cement was found to reduce the adiabatic temperature rise without causing set retardation when compared with normal portland cement. In addition, the concrete showed high fluidity as well as a high long-term compressive strength of over 150 MPa. These results suggest that the SSP is highly compatible with low-heat cement and is very effective in producing high-performance concrete when used in combination with this type of binder.","PeriodicalId":21898,"journal":{"name":"SP-173: Fifth CANMET/ACI International Conference on Superplasticizers and Other Chemical Admixtures in Concrete","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1997-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"85964078","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A deflocculation test based on the method proposed by A. Verhasselt and J. Pairon has been applied to different mixes of cement-admixtures in order to distinguish quickly between plasticizer and superplasticizer. The admixtures used were commercial products containing different proportions of lignosulphonates (L), sulphonated naphthalene formaldehyde condensate (SNF) and sulfphonated melamine formaldehyde condensate (SMF). Three different cements were employed: a low tricalcium aluminate (C3A = 2.5%), a high alkali content (Na2O equivalent = 0.95%) and a high fineness portland cement (A.A. Blaine = 5002 cm2/gr). In addition, the minimum active dosage of superplasticizer was calculated. The influence of the type of cement in the minimum active dosage is clearly shown by the results obtained through this method.
{"title":"Application of Deflocculation Test to Characterise the Admixture Plasticizer and Superplasticizer Effect in Different Cements","authors":"A. Macías, S. Goñi","doi":"10.14359/6214","DOIUrl":"https://doi.org/10.14359/6214","url":null,"abstract":"A deflocculation test based on the method proposed by A. Verhasselt and J. Pairon has been applied to different mixes of cement-admixtures in order to distinguish quickly between plasticizer and superplasticizer. The admixtures used were commercial products containing different proportions of lignosulphonates (L), sulphonated naphthalene formaldehyde condensate (SNF) and sulfphonated melamine formaldehyde condensate (SMF). Three different cements were employed: a low tricalcium aluminate (C3A = 2.5%), a high alkali content (Na2O equivalent = 0.95%) and a high fineness portland cement (A.A. Blaine = 5002 cm2/gr). In addition, the minimum active dosage of superplasticizer was calculated. The influence of the type of cement in the minimum active dosage is clearly shown by the results obtained through this method.","PeriodicalId":21898,"journal":{"name":"SP-173: Fifth CANMET/ACI International Conference on Superplasticizers and Other Chemical Admixtures in Concrete","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1997-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88672538","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Galvanized steel and corrosion inhibitors added to concrete are considered methods to protect reinforcement from corrosion. In present paper the simultaneous and separated use of both methods are considered. Concrete specimens have been made for the study. For depassivation the spray salt chamber was used. The results show that if No2 is used the resistance of galvanized steel to chloride attack is improved. Bare steel embedded in concrete with NO2 resists well chloride attack.
{"title":"Protection of Reinforcements from Chloride Attack by the Use of Galvanized Steel and Calcium Nitrite","authors":"C. Alonso, G. Sinibaldi, C. Andrade, R. Cigna","doi":"10.14359/6181","DOIUrl":"https://doi.org/10.14359/6181","url":null,"abstract":"Galvanized steel and corrosion inhibitors added to concrete are considered methods to protect reinforcement from corrosion. In present paper the simultaneous and separated use of both methods are considered. Concrete specimens have been made for the study. For depassivation the spray salt chamber was used. The results show that if No2 is used the resistance of galvanized steel to chloride attack is improved. Bare steel embedded in concrete with NO2 resists well chloride attack.","PeriodicalId":21898,"journal":{"name":"SP-173: Fifth CANMET/ACI International Conference on Superplasticizers and Other Chemical Admixtures in Concrete","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1997-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81538621","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This paper describes the effect of a lithium nitrate-based admixture on the hydroxide ion concentration of the pore solution of hydrating pastes made from portland cement and water. No significant increase in the hydroxide ion concentration results from using this admixture in the mix, which is thus different than any published study with any other lithium compound. It has been reported that underdosing with lithium salts can increase the expansion due to ASR. The authors propose that this is mainly due to increases in hydroxide concentration observed with other lithium salts and therefore this admixture will not show an effect. Mortar bar tests with the new admixture verify the hypothesis that the lithium nitrate-based admixture does not increase expansion at any dose. This is then a much safer admixture to use in the field with respect to risk from damaging ASR expansions. It is also much safer to handle than lithium hydroxide-based admixtures since solutions of lithium nitrate are much closer to neutral pH than lithium hydroxide solutions.
{"title":"A Lithium-Based Admixture for ASR Control that Does Not Increase the Pore Solution pH","authors":"D. Stokes, H. H. Wang, S. Diamond","doi":"10.14359/6216","DOIUrl":"https://doi.org/10.14359/6216","url":null,"abstract":"This paper describes the effect of a lithium nitrate-based admixture on the hydroxide ion concentration of the pore solution of hydrating pastes made from portland cement and water. No significant increase in the hydroxide ion concentration results from using this admixture in the mix, which is thus different than any published study with any other lithium compound. It has been reported that underdosing with lithium salts can increase the expansion due to ASR. The authors propose that this is mainly due to increases in hydroxide concentration observed with other lithium salts and therefore this admixture will not show an effect. Mortar bar tests with the new admixture verify the hypothesis that the lithium nitrate-based admixture does not increase expansion at any dose. This is then a much safer admixture to use in the field with respect to risk from damaging ASR expansions. It is also much safer to handle than lithium hydroxide-based admixtures since solutions of lithium nitrate are much closer to neutral pH than lithium hydroxide solutions.","PeriodicalId":21898,"journal":{"name":"SP-173: Fifth CANMET/ACI International Conference on Superplasticizers and Other Chemical Admixtures in Concrete","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1997-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88110226","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Heat curing is the most common method used for accelerating the strength development in concrete. Accelerated curing finds large applications in the precast industry for quick turnaround of forms and casting beds. The increase in the initial strengths is simply a result of increased rate of hydration caused by higher temperature. However, later strengths are often lower than those of the same concrete cured at 20C. The causes of the strength loss are of physical and chemical nature. The physical cause results in increased porosity and cracking because the concrete constituents have different thermal expansion, (air has the highest). The chemical causes are the differences in the hydration products, microstructure and degree of hydration. Generally, physical causes are the dominating factors for strength loss in hear cured concrete. Results of extensive laboratory and field tests are presented showing that equivalent compressive strengths at 18 hours are obtained with concrete containing the new generation superplasticizers and heat cured concretes at 60C. The 28 day strengths of concretes with admixtures are substantially higher. Thus, with the use of these new generation superplasticizers it's possible to overcome the negative effects of steam curing such as strength loss, permeability, shrinkage, creep and frost resistance.
{"title":"New Admixtures for Eliminating Steam Curing and its Negative Effects on Durability","authors":"R. Khurana, I. Torresan","doi":"10.14359/6179","DOIUrl":"https://doi.org/10.14359/6179","url":null,"abstract":"Heat curing is the most common method used for accelerating the strength development in concrete. Accelerated curing finds large applications in the precast industry for quick turnaround of forms and casting beds. The increase in the initial strengths is simply a result of increased rate of hydration caused by higher temperature. However, later strengths are often lower than those of the same concrete cured at 20C. The causes of the strength loss are of physical and chemical nature. The physical cause results in increased porosity and cracking because the concrete constituents have different thermal expansion, (air has the highest). The chemical causes are the differences in the hydration products, microstructure and degree of hydration. Generally, physical causes are the dominating factors for strength loss in hear cured concrete. Results of extensive laboratory and field tests are presented showing that equivalent compressive strengths at 18 hours are obtained with concrete containing the new generation superplasticizers and heat cured concretes at 60C. The 28 day strengths of concretes with admixtures are substantially higher. Thus, with the use of these new generation superplasticizers it's possible to overcome the negative effects of steam curing such as strength loss, permeability, shrinkage, creep and frost resistance.","PeriodicalId":21898,"journal":{"name":"SP-173: Fifth CANMET/ACI International Conference on Superplasticizers and Other Chemical Admixtures in Concrete","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1997-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82791075","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The importance of molecular weight as a parameter influencing the performance of water-reducing admixture in cement paste was investigated. The sulphomethylolated ALCELL lignin sample was divided into four fractions of different molecular weight. The fractionation was performed by membrane ultrafiltration on a small laboratory unit. Advanced hydrophilic membranes with a 1, 10 and 50 kd nominal MW cut-off and effective area of 63 cm2 were used. Molecular weight distribution, average molecular weights (Mn, Mw, Mz and Mz+1) and polydispersity of the original sample and its fractions were determined by high-performance aqueous size-exclusion chromatography. The five samples, prepared as 20% aqueous solutions, were used as water-reducing admixtures. Their influence on fresh cement pastes was examined using the torque and mini-slump tests. The torque test was performed to investigate the fluidifying and retarding effects of the samples. Changes in torque resistance were monitored by a computerized system, giving an indication of the degree of dispersion and set retardation. The mini-slump test was used to determine the workability and air-entrainment of the fresh cement paste. The dependence of the properties of the fresh cement pastes on the lignosulphonate molecular weight was studied. It was found that the lower the molecular weight of the lignosulphonate sample the higher the set retardation of the cement paste. The highest fluidification and lowest air-entrainment were observed when the intermediate molecular weight samples were used. The optimum molecular weight fraction in each particular experiment was always more effective than the original unfractionated sample. Fractions with extremely low efficiency were also identified. The data obtained can be utilized in optimization of lignosulphonate admixture performance.
{"title":"Influence of Lignosulphonate Molecular Weight Fractions on the Properties of Fresh Cement","authors":"J. Zhor, T. Bremmer","doi":"10.14359/6213","DOIUrl":"https://doi.org/10.14359/6213","url":null,"abstract":"The importance of molecular weight as a parameter influencing the performance of water-reducing admixture in cement paste was investigated. The sulphomethylolated ALCELL lignin sample was divided into four fractions of different molecular weight. The fractionation was performed by membrane ultrafiltration on a small laboratory unit. Advanced hydrophilic membranes with a 1, 10 and 50 kd nominal MW cut-off and effective area of 63 cm2 were used. Molecular weight distribution, average molecular weights (Mn, Mw, Mz and Mz+1) and polydispersity of the original sample and its fractions were determined by high-performance aqueous size-exclusion chromatography. The five samples, prepared as 20% aqueous solutions, were used as water-reducing admixtures. Their influence on fresh cement pastes was examined using the torque and mini-slump tests. The torque test was performed to investigate the fluidifying and retarding effects of the samples. Changes in torque resistance were monitored by a computerized system, giving an indication of the degree of dispersion and set retardation. The mini-slump test was used to determine the workability and air-entrainment of the fresh cement paste. The dependence of the properties of the fresh cement pastes on the lignosulphonate molecular weight was studied. It was found that the lower the molecular weight of the lignosulphonate sample the higher the set retardation of the cement paste. The highest fluidification and lowest air-entrainment were observed when the intermediate molecular weight samples were used. The optimum molecular weight fraction in each particular experiment was always more effective than the original unfractionated sample. Fractions with extremely low efficiency were also identified. The data obtained can be utilized in optimization of lignosulphonate admixture performance.","PeriodicalId":21898,"journal":{"name":"SP-173: Fifth CANMET/ACI International Conference on Superplasticizers and Other Chemical Admixtures in Concrete","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1997-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81491113","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
In the past decade, many efforts have been undertaken to develop highly-workable concrete which consolidates under its own weight without any vibration, specially in Japan. The admixtures used in such concrete are mainly composed of high water reducing superplasticizers, fine limestone dust and viscosity agents to maintain not only high flowability but high segregation resistance of the concrete. The water to cement ratio is generally lower than 0.55. A new admixture is proposed in the present study. It is a mixture of a superplasticizer and an accelerator with plasticizing properties which also acts as a viscosity agent. The composition of the self-leveling concrete is as follows. The total amount of fine materials (cement + fly ash or cement + ground limestone) is 370kg/m^3, while the cement content (normal portland cement CEMI 52.5) is 260kg/m^3. The water to cement ratio is 0.73. The mechanical performances of the hardened concrete is better than those of a usual French building concrete. The compressive strengths at 16 hrs, 7 days, and 28 days are 8, 30, and 3 MPa, respectively. The unrestrained drying shrinkage reaches 600 to 700mm/m, at 90 days as predicted from the 28 day results.
{"title":"Development of an Admixture for Self-Leveling Concrete","authors":"S. Rols, J. Ambroise, J. Pera","doi":"10.14359/6199","DOIUrl":"https://doi.org/10.14359/6199","url":null,"abstract":"In the past decade, many efforts have been undertaken to develop highly-workable concrete which consolidates under its own weight without any vibration, specially in Japan. The admixtures used in such concrete are mainly composed of high water reducing superplasticizers, fine limestone dust and viscosity agents to maintain not only high flowability but high segregation resistance of the concrete. The water to cement ratio is generally lower than 0.55. A new admixture is proposed in the present study. It is a mixture of a superplasticizer and an accelerator with plasticizing properties which also acts as a viscosity agent. The composition of the self-leveling concrete is as follows. The total amount of fine materials (cement + fly ash or cement + ground limestone) is 370kg/m^3, while the cement content (normal portland cement CEMI 52.5) is 260kg/m^3. The water to cement ratio is 0.73. The mechanical performances of the hardened concrete is better than those of a usual French building concrete. The compressive strengths at 16 hrs, 7 days, and 28 days are 8, 30, and 3 MPa, respectively. The unrestrained drying shrinkage reaches 600 to 700mm/m, at 90 days as predicted from the 28 day results.","PeriodicalId":21898,"journal":{"name":"SP-173: Fifth CANMET/ACI International Conference on Superplasticizers and Other Chemical Admixtures in Concrete","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"1997-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88372432","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
京公网安备 11010802042870号
京ICP备2023020795号-1
扫码关注我们
求助内容：
应助结果提醒方式：