This article investigated the factors and mechanisms that affected the workability and mechanical properties of cement paste incorporating nano-TiO_(2).The findings indicated that,for nano-TiO_(2)aqueous solution conc...This article investigated the factors and mechanisms that affected the workability and mechanical properties of cement paste incorporating nano-TiO_(2).The findings indicated that,for nano-TiO_(2)aqueous solution concentrations of 3%,6%,9%,and 12%,the optimal dispersion effect was achieved with an ultrasonic dispersion time of 20 minutes.Specifically,at a 6%nano-TiO_(2)content,both the workability and mechanical performance of the cement paste were enhanced.Furthermore,while nano-TiO_(2)did not alter the types of hydration products present in the cement paste,it did increase the amount of C-S-H gels.This enhancement was attributed to a higher number of nucleation sites for hydration products,which promoted hydration and reduced the porosity of the cement paste.展开更多
The reflected optical caustics method is applied to study dynamic fracture problems in hardened cement paste. First both the unreinforced cement paste and the glass fibres reinforced cement paste specimens were fabric...The reflected optical caustics method is applied to study dynamic fracture problems in hardened cement paste. First both the unreinforced cement paste and the glass fibres reinforced cement paste specimens were fabricated and the reflective coating on the surface of the specimen was prepared. Secondly the crack path and the shadow spot patterns during the crack propagation process for the two specimens were recorded by using a multi-spark high speed camera.Thirdly some dynamic parameters of two cement paste specimens including crack onset time the dynamic stress intensity factor and crack growth velocity were determined and analyzed comparatively.This indicates that the glass fibres can improve the fracture resistance and delay fracture time.These results will play an important role in evaluating the dynamic fracture properties of cement paste.展开更多
Based on effective media theory, a predictive model, relating chloride diffusivity to the capillary pores, gel pores, tortuosity factor, and pore size distribution of hardened cement, is proposed. To verify the propos...Based on effective media theory, a predictive model, relating chloride diffusivity to the capillary pores, gel pores, tortuosity factor, and pore size distribution of hardened cement, is proposed. To verify the proposed model, the diffusion coefficient of chloride ions, the degree of hydration, and peak radius of capillary pores of cement paste specimens were measured. The predicted results for chloride diffusivity were compared with published data. The results showed that the predicted chloride diffusivity of hardened cement paste was in good agreement with the experimental results. The effect of the evolution of pore structures in cement paste on chloride diffusivity could be deduced simultaneously using the proposed model.展开更多
The effects of activated coal gangue on compressive strength, porosity and pore size distribution of hardened cement pastes were investigated. Activated coal gangue with two different kaolin contents, one higher and o...The effects of activated coal gangue on compressive strength, porosity and pore size distribution of hardened cement pastes were investigated. Activated coal gangue with two different kaolin contents, one higher and one lower, were used to partially replace Portland cement at 0%, 10%, and 30% by weight. The water to binder ratio（w/b） of 0.5 was used for all the blended cement paste mixes. Experimental results indicate that the blended cement of activated coal gangue mortar with higher kaolin mineral content has a higher compressive strength than that with lower kaolin mineral content. The porosity and pore size of blended cement mortar were significantly affected by the replacement of activated coal gangue.展开更多
Hydration shrinkage generated by cement hydration is the cause of autogenous shrinkage of high strength concrete. It may result in the volume change and even cracking of mortar and concrete. According to the data anal...Hydration shrinkage generated by cement hydration is the cause of autogenous shrinkage of high strength concrete. It may result in the volume change and even cracking of mortar and concrete. According to the data analysis in a series of experimental studies, the influence of ultra-fine fly ash on the hydration shrinkage of composite cementitious materials was investigated. It is found that ultra-fine fly ash can reduce the hydration shrinkage of cement paste effectively, and the more the ultra-fine fly ash, the less the hydration shrinkage. Compared with cement paste without the ultra-fine fly ash, the shrinkage ratio of cement paste reduces from 23.4% to 39.7% when the ultra-fine fly ash replaces cement from 20% to 50%. Moreover, the microscopic mechanism of the ultra-fine fly ash restraining the hydration shrinkage was also studied by scanning electron microscopy, X-ray diffraction and hydrated equations. The results show that the hydration shrinkage can be restrained to a certain degree because the ultra-fine fly ash does not participate in the hydration at the early stage and the secondary hydration products are different at the later stage.展开更多
The early stage hydration mechanism of cellulose ether modified thin layer cement pastes was studied, using brick as the matrix. Samples of 6 h, 24 h, and 3 d and 7 d hydration time were analyzed to study the hydratio...The early stage hydration mechanism of cellulose ether modified thin layer cement pastes was studied, using brick as the matrix. Samples of 6 h, 24 h, and 3 d and 7 d hydration time were analyzed to study the hydration law on the surface of high water-absorbing matrix. Hydration products were qualitatively and semi-quantitatively analyzed using XRD, TG-DSC-DTG, FTIR and SEM. The experimental results show that there is no enough water for 2 mm thick cement pastes to hydrate, because of rapid water absorption of matrix. Trace amounts of Ca （OH）2 was detected after three days hydration. With the prolongation of hydration time, the category and concentration of hydration products do not change. Compared with 2 mm thick cement pastes, 6 mm thick cement pastes and 10 mm thick cement pastes have lower dehydration rate and water loss. The humidity field of the cement paste show different changes within the same time. 6 mm thick cement paste and 10 mm thick cement pastes have longer time and more water to hydrate. Ca（OH）2 and ettringite were detected after 6 hours hydration and the concentrations of hydration products increased from 24 hours to 7 days.展开更多
To understand the effect of steam curing temperature on the hydrate and microstructure of hardened cement paste,several measuring methods including X-ray diffraction(XRD),atomic absorption spectroscopy(ASS),ion chroma...To understand the effect of steam curing temperature on the hydrate and microstructure of hardened cement paste,several measuring methods including X-ray diffraction(XRD),atomic absorption spectroscopy(ASS),ion chromatography,conductivity meter,alternating-current impedance spectroscopy and nuclear magnetic resonance(NMR)are employed to investigate the hydration characteristics,pore solution composition and conductivity,resistivity and pore structure during the steam curing process.Experimental results show that steam curing promotes the hydration process,greatly raises the resistivity,and decreases the porosity of specimen at early age.Compared with being treated at 45℃,higher temperature leads to a fast decomposition of ettringite at initial stage of the constant temperature treatment period,which improves the relative content and ionic activity of the conductive ions in pore solution.Furthermore,the number of pores larger than 200 nm increases significantly,which reduces the resistivity of the hardened cement paste.Cement paste treated at 45℃ has a more stable and denser microstructure with less damages.展开更多
The effect of curing regime on degree ofAl3＋ substituting for Si^4＋ （Al/Si ratio） in C-S-H gels of hardened Portland cement pastes was investigated by 29Si magic angel spinning （MAS） nuclear magnetic resonance ...The effect of curing regime on degree ofAl3＋ substituting for Si^4＋ （Al/Si ratio） in C-S-H gels of hardened Portland cement pastes was investigated by 29Si magic angel spinning （MAS） nuclear magnetic resonance （NMR） with deconvolution technique. The curing regimes included the constant temperature （20, 40, 60 and 80 ℃） and variable temperature （simulated internal temperature of mass concrete with 60 ℃ peak）. The results indicate that constant temperature of 20 ℃ is beneficial to substitution ofAl3＋ for Si4＋, and AI/Si ratio changes to be steady after 180 d. The increase of Al/Si ratio at 40 ℃is less than that at 20℃ for 28 d. The other three regimes of high temperature increase Al/Si ratio only before 3 d, on the contrary to that from 3 to 28 d. However, the 20 ℃ curing stage from 28 to 180 d at variable temperature regime, is beneficial to the increase of AI/Si ratio which is still lower than that at constant temperature regime of 20 ℃ for the same age. A nonlinear relation exists between the Al/Si ratio and temperature variation or mean chain length （MCL） of C-S-H gels, furthermore, the amount ofAl3＋ which can occupy the bridging tetrahedra sites in C-S-H structure is insufficient in hardened Portland cement pastes.展开更多
Using the tomography image, a method to characterize the 3D spatial distributions of increased porosity was proposed, and the increased porosity distributions of cement pastes with different leaching degrees were give...Using the tomography image, a method to characterize the 3D spatial distributions of increased porosity was proposed, and the increased porosity distributions of cement pastes with different leaching degrees were given using the current method. The leaching processes of CH/C-S-H and the contribution of CH/C-S-H leaching to porosity evolution were discussed. The proposed method can be applied to all cement- based materials with any leaching degrees. From the quantitative increased porosity results, we find that the CH leaching finished quickly on the sharp CH leaching front.展开更多
The effect of curing regime on the distribution ofAl3＋ coordination in hardened cement pastes within 28 d were investigated by 29Si and 27Al magic angle spinning （MAS） nuclear magnetic resonance（NMR） with deconvo...The effect of curing regime on the distribution ofAl3＋ coordination in hardened cement pastes within 28 d were investigated by 29Si and 27Al magic angle spinning （MAS） nuclear magnetic resonance（NMR） with deconvolution technique. The results indicate that the tetrahedral coordination Al3＋ incorporated in C-S-H structure mainly originate from the AP＋ incorporated in the alite and belite phases in the Portland cement. The curing regime of constant temperature of 20 ℃ is beneficial to the octahedral coordination Al3＋ transforming to tetrahedral coordination AP＋ incorporated in C-S-H structure. However, at curing regime of variable temperature, the temperature rising process is more advantageous to the transformation from ettringite to monosulphate, substitution of Al3＋ for Si4＋ in the C-S-H structure and the formation of the third aluminate hydrate （TAH） than that at constant temperature of 20 ℃. The high temperature of 60 ℃ in the holding temperature process promotes the decomposition of ettringite, and enhances the consumption of the Al3＋ incorporated in C-S-H phases and the Al3＋ in TAH for the monosulphate forming. The temperature decreasing promotes the transformation from monosulphate to ettringite, and increases the consumption of the Al3＋ incorporated in C-S-H phases, and then increases the quantity of the TAH.展开更多
Micro computed tomography (Micro-CT) was applied to obtain three-dimensional images of the microstructure of cement paste (water-to-cement mass ratio of 0.5) at different ages. By using the Amira software, component p...Micro computed tomography (Micro-CT) was applied to obtain three-dimensional images of the microstructure of cement paste (water-to-cement mass ratio of 0.5) at different ages. By using the Amira software, component phases of the cement paste such as pores, hydration products, and unhydrated clinker particles were segmented from each other based on their 3D image grey levels; their relative contents were also calculated with the software, and the data are 61.2%, 0% and 38.8% at the beginning of hydration and 11.8%, 78.5% and 9.7% at 28 d age, respectively. The hydration degree of cement paste at different ages was compared with the experimental data acquired by loss on ignition (LOI) tests. The results show that the calculated and measured data reasonably agree with each other, which indicates that micro-CT is a useful and reliable approach to characterize the micro structure evolution of hydrating cement paste.展开更多
Four cellulose ethers（CEs） were compared for their effects on the pore structure of cement paste using mercury intrusion porosimetry. The experimental results show that the total pore volume and porosity of cement p...Four cellulose ethers（CEs） were compared for their effects on the pore structure of cement paste using mercury intrusion porosimetry. The experimental results show that the total pore volume and porosity of cement pastes containing the four cellulose ethers are significantly higher than that of the pure cement pastes and the total pore volume and porosity of cement pastes containing HEC（hydroxyethyl cellulose ether） or low viscosity cellulose ethers are low in four CEs. By changing the surface tension and viscosity of liquid phase and the strengthening of liquid film between air voids in cement pastes, CEs affect the formation, diameter evolution and upward movement of air voids and the pore structure of hardening cement paste. For the four CEs, the pore volume of cement pastes containing HEC or low viscosity cellulose ethers is higher with the diameter of 30-70 nm while lower with the diameter larger than 70 nm. CEs affect the pore structure of cement paste mainly through their effects on the evolvement of the small air voids into bigger ones when the pore diameter is below 70 nm and their effects on the entrainment and stabilization of air voids when the pore diameter is above 70 nm.展开更多
This paper is aimed to study the factors influencing rheological properties of cement paste with fly ash,including the quantity of cement replaced by fly ash,types of the fly ash,degree of cement hydration and the mic...This paper is aimed to study the factors influencing rheological properties of cement paste with fly ash,including the quantity of cement replaced by fly ash,types of the fly ash,degree of cement hydration and the microstructure of cement paste. By changing the water-cement ratio,mixing different amounts of fly ash,adding different types fly ash,the rheological parameters of cement paste are measured with a rheometer,and the needle penetration are tested by Vical needle. A ultrasound velocity test was aslo employed to monitor hydration of paste at early ages. Combinations of different factors on the rheological properties of cement paste are studied. The objective is to reveal the effect mechanism of material nature and proportion of each component on the rheological properties of cement paste with fly ash.展开更多
The microstructure characteristics and meso-defect volume changes of hardened cement paste before and after carbonation were investigated by three-dimensional （3D） X-ray computed tomograpby （XCT）, where three type...The microstructure characteristics and meso-defect volume changes of hardened cement paste before and after carbonation were investigated by three-dimensional （3D） X-ray computed tomograpby （XCT）, where three types water-to-cement ratio of 0.53, 0.35 and 0.23 were considered. The high-resolution 3D images of microstructure and filtered defects were reconstructed by an XCT VG Studio MAX 2.0 software, The meso- defect volume fractions and size distribution were analyzed based on 3D images through add-on modules of 3D defect analysis. The 3D meso-defects volume fractions before carbonation were 0.79%, 0.38% and 0.05% corresponding to w/c ratio=0.53, 0.35 and 0.23, respectively. The 3D meso-defects volume fractions after carbonation were 2.44%, 0.91% and 0.14% corresponding to w/c ratio=0.53, 0.35 and 0.23, respectively. The experimental results suggest that 3D meso-defects volume fractions after carbonation for above three w/c ratio increased significantly. At the same time, meso-cracks distribution of the carbonation shrinkage and gray values changes of the different w/c ratio and carbonation reactions were also investigated.展开更多
Hydration heat effect of cement pastes and mechanism of hydroxypropyl methyl cellulose ether （HPMC） and expanded perlite in cement pastes were studied by means of hydration exothermic rate, hydration heat amount, FT...Hydration heat effect of cement pastes and mechanism of hydroxypropyl methyl cellulose ether （HPMC） and expanded perlite in cement pastes were studied by means of hydration exothermic rate, hydration heat amount, FTIR and TG-DTG. The results show that HPMC can significantly delay the hydration induction period and acceleration period of cement pastes. As mixing amount increased, hydration induction period of cement pastes enlarged and accelerated period gradually went back. At the same time, the amount of hydration heat gradually decreased. Expanded perlite had worse delay effects and less change of hydration heat amount of cement pastes than HPMC. HPMC changed the structure of C-S-H during cement hydration. The more amount of HPMC, the more obvious effect. However, EXP had little influence on the structure of C-S-H. At the same age, the content of Ca （OH）2 in cement pastes gradually decreased as the mixing amount increase of HPMC and expanded perlite, and had better delay effect than that single-doped with HPMC or expanded perlite when HPMC and expanded nerlite were both dooed in cement pastes.展开更多
Cement pastes containing 0%, 15%, 25% and 35% fly ash were prepared. After being cured for 90 days, all fly ash blended cement pastes were crushed and ground into powders with a particle size less than 80 μm and then...Cement pastes containing 0%, 15%, 25% and 35% fly ash were prepared. After being cured for 90 days, all fly ash blended cement pastes were crushed and ground into powders with a particle size less than 80 μm and then the powders were immersed in alkali solutions. Adsorption characteristics of K^+ and Na^+ ions in the pastes were investigated. Meawhile, the desorption characteristics of the adsorbed alklai ions and the inherent K^+ and Na^+ ions in the pastes were also investigated. Results showed that the contents of K^+ and Na^+ ions adsorbed by the pastes increased with increasing the substitution levels of fly ash and/or the concentrations of alkali solutions. Each paste was characterized by having the same adsorption capacity for K^+ or Na^+ that was essentially independent of alkali concentration. Adsorption mechanism of K^+ and Na^+ ions by the pastes is believed to be an effect of charge compensation of the C-S-H gel. Adsorption-desorption of the adsorbed K^+ and Na^+ ions in the pastes is reversible. The inherent K^+ and Na^+ ions in the pastes entered rapidly into the de-ionized water during the first 120 minutes, and then they were released at a relatively slow rate. A steady-state alkali partition was reached at about 720 minutes. Some K^+ and Na^+ ions which were originally "bound" by the hydration products were considered to be released into de-ionized water. Leaching tests showed that there was no significant effect of fly ash on the retaining of available alkalis in the pastes. A part of the released alkali ions exists in the pore solutions and the other part may be physically adsorbed by the hydration products.展开更多
The dominant factors during early hydration process of cement paste containing 10% metakaolin replacement （MK10） and 10% kaolin replacement （K10） are investigated in comparison to neat cement paste （NCP）, and X-...The dominant factors during early hydration process of cement paste containing 10% metakaolin replacement （MK10） and 10% kaolin replacement （K10） are investigated in comparison to neat cement paste （NCP）, and X-ray Diffraction （XRD） analysis is employed to identify the crystalline phases of all specimens. Thermogravimetric （TG） and Differential Scanning Calorimetry （DSC） are used to identify the phase constituents. The amount of acid-insoluble residue （AIR） of all specimens is used to evaluate the unreacted materials. The results indicate that, after the first day, MK act as nuclei for the formation of C-S-H during hydration of C3S and C2S, densifying the microstructure of cement paste. Its contribution is mainly due to the fine nature of the MK. From 3 days to 7 days, more and more MK reacts with CH to form C-S-H, making the microstructure denser. The strength contribution is mainly due to the chemical activity of MK.展开更多
The chemical and physical interactions in the interfacial transition zone （ITZ） between three different types of coarse aggregates （limestone, granite and basalt） and cement paste were investigated. The results sh...The chemical and physical interactions in the interfacial transition zone （ITZ） between three different types of coarse aggregates （limestone, granite and basalt） and cement paste were investigated. The results show that all the aggregates are chemically active. Significant amounts of Ca2＋, K＋, and Na＋ are absorbed by all the aggregates from the cement solution, granite and basalt also absorb significant amounts of OH- and release significant amounts of Si4＋ into cement solution. The XRD, EDXA and pore structure results of the ITZ also show that more clinkers participate in the cement hydration in the ITZ of granite and basalt, and more hydrates are generated, hence resulting in a denser ITZ structure with a lower content of maeropores. Although the limestone has the least activity, the connection between it and cement paste is tight, due to its rough surface and higher water absorption. Whereas the granite with smooth surface and lower water absorption has a loose connection with cement paste, many pores and cracks are visible, which is very detrimental to the concrete durability.展开更多
To further strengthen the protective effect of aerogel cement paste (ACP) coating on self-compacting concrete (SCC) in tunnel fire under the optimal mix proportion,the effect of curing temperature (from 5 to 80 ℃,bas...To further strengthen the protective effect of aerogel cement paste (ACP) coating on self-compacting concrete (SCC) in tunnel fire under the optimal mix proportion,the effect of curing temperature (from 5 to 80 ℃,based on site construction curing temperature and surrounding rock temperature) on fire insulation of ACP was investigated.The mechanical properties,thermal conductivity and porosity of ACP were tested.The microstructure of ACP was characterized by means of SEM,XRD and TG/DTG.The results reveal that 50 ℃ is the optimal curing temperature for ACP with good mechanical properties and fire insulation.Relatively high curing temperature can facilitate hydration and pozzolanic reactions,contributing to the generation of more stable substances (such as C-S-H gels,tobermorite and thenardite,etc).ACP under excessive low curing temperature brings inhomogeneous microstructure with coarse pores,leading to producing wider and longer microcracks when it is exposed to tunnel fire.The microcracks make the heat convection and thermal radiation more significant and thus accelerate the damage of ACP under fire.In case of the less than 7% difference of thermal conductivity,dense microstructure and stable substances are more conducive to strengthening fire insulation of ACP.In practical engineering applications,the thickness of protective layer of ACP can be further optimized when ACP is cured under about 50 ℃.展开更多
The rheologicalbehaviors of fresh cement paste with polycarboxylate superplasticizer were systematically investigated.Influentialfactors including superplasticizer to cement ratio（Sp/C）,water to cement ratio（w/c）,...The rheologicalbehaviors of fresh cement paste with polycarboxylate superplasticizer were systematically investigated.Influentialfactors including superplasticizer to cement ratio（Sp/C）,water to cement ratio（w/c）,temperature,and time were discussed.Fresh cement pastes with Sp/Cs in the range of 0 to 2.0% and varied W/Cs from 0.25 to 0.5 were prepared and tested at 0,20 and 40 °C,respectively.Flowability and rheologicaltests on cement pastes were conducted to characterize the development of the rheologicalbehavior of fresh cement pastes over time.The exprimentalresults indicate that the initialflowability and flowability retention over shelf time increase with the growth in superplasticizer dosage due to the plasticizing effect and retardation effect of superplasticizer.Higher temperature usually leads to a sharper drop in initialflowability and flowability retention.However,for the cement paste with high Sp/C or w/c,the flowability is slightly affected by temperature.Yield stress and plastic viscosity show similar variation trends to the flowability under the abovementioned influentialfactors at low Sp/C.In the case of high Sp/C,yield stress and plastic viscosity start to decline over shelf time and the decreasing rate descends at elevated temperature.Moreover,two equations to roughly predict yield stress and plastic viscosity of the fresh cement pastes incorporating Sp/C,w/c,temperature and time are developed on the basis of the existing models,in which experimentalconstants can be extracted from a database created by the rheologicaltest results.展开更多
基金Funded by National Natural Science Foundation of China(No.52108188)State Key Laboratory of Silicate Materials for Architectures(Wuhan University of Technology)(No.SYSJJ2024-15)+3 种基金State Key Laboratory of Mountain Bridge and Tunnel Engineering,Chongqing Jiaotong University(No.SKLBT-2301)Opening Project of State Key Laboratory of Green Building Materials(No.2022GBM10)Open Research Fund of Key Laboratory of Engineering Materials of Ministry of Water Resources,China Institute of Water Resources and Hydropower Research(No.EMF202407)General Project of Science and Technology Plan of Beijing Municipal Commission of Education(No.KM202110005018)。
文摘This article investigated the factors and mechanisms that affected the workability and mechanical properties of cement paste incorporating nano-TiO_(2).The findings indicated that,for nano-TiO_(2)aqueous solution concentrations of 3%,6%,9%,and 12%,the optimal dispersion effect was achieved with an ultrasonic dispersion time of 20 minutes.Specifically,at a 6%nano-TiO_(2)content,both the workability and mechanical performance of the cement paste were enhanced.Furthermore,while nano-TiO_(2)did not alter the types of hydration products present in the cement paste,it did increase the amount of C-S-H gels.This enhancement was attributed to a higher number of nucleation sites for hydration products,which promoted hydration and reduced the porosity of the cement paste.
基金The Ph.D.Programs Foundation of Ministry of Education of China(No.20120023120020)the National Natural Science Foundation of China(No.51404273)
文摘The reflected optical caustics method is applied to study dynamic fracture problems in hardened cement paste. First both the unreinforced cement paste and the glass fibres reinforced cement paste specimens were fabricated and the reflective coating on the surface of the specimen was prepared. Secondly the crack path and the shadow spot patterns during the crack propagation process for the two specimens were recorded by using a multi-spark high speed camera.Thirdly some dynamic parameters of two cement paste specimens including crack onset time the dynamic stress intensity factor and crack growth velocity were determined and analyzed comparatively.This indicates that the glass fibres can improve the fracture resistance and delay fracture time.These results will play an important role in evaluating the dynamic fracture properties of cement paste.
基金Project supported by the National Basic Research Program (973) of China (No. 2009CB623200)the National High-Tech R&D Program (863) of China (No. 2008AA030794)the Postgraduates Research Innovation in University of Jiangsu Province, China (No. CX10B-064Z)
文摘Based on effective media theory, a predictive model, relating chloride diffusivity to the capillary pores, gel pores, tortuosity factor, and pore size distribution of hardened cement, is proposed. To verify the proposed model, the diffusion coefficient of chloride ions, the degree of hydration, and peak radius of capillary pores of cement paste specimens were measured. The predicted results for chloride diffusivity were compared with published data. The results showed that the predicted chloride diffusivity of hardened cement paste was in good agreement with the experimental results. The effect of the evolution of pore structures in cement paste on chloride diffusivity could be deduced simultaneously using the proposed model.
基金the National Basic Research Program of China(No.2001CB610703)the Basic Research of Preparation and Application of High Performance Cement
文摘The effects of activated coal gangue on compressive strength, porosity and pore size distribution of hardened cement pastes were investigated. Activated coal gangue with two different kaolin contents, one higher and one lower, were used to partially replace Portland cement at 0%, 10%, and 30% by weight. The water to binder ratio（w/b） of 0.5 was used for all the blended cement paste mixes. Experimental results indicate that the blended cement of activated coal gangue mortar with higher kaolin mineral content has a higher compressive strength than that with lower kaolin mineral content. The porosity and pore size of blended cement mortar were significantly affected by the replacement of activated coal gangue.
文摘Hydration shrinkage generated by cement hydration is the cause of autogenous shrinkage of high strength concrete. It may result in the volume change and even cracking of mortar and concrete. According to the data analysis in a series of experimental studies, the influence of ultra-fine fly ash on the hydration shrinkage of composite cementitious materials was investigated. It is found that ultra-fine fly ash can reduce the hydration shrinkage of cement paste effectively, and the more the ultra-fine fly ash, the less the hydration shrinkage. Compared with cement paste without the ultra-fine fly ash, the shrinkage ratio of cement paste reduces from 23.4% to 39.7% when the ultra-fine fly ash replaces cement from 20% to 50%. Moreover, the microscopic mechanism of the ultra-fine fly ash restraining the hydration shrinkage was also studied by scanning electron microscopy, X-ray diffraction and hydrated equations. The results show that the hydration shrinkage can be restrained to a certain degree because the ultra-fine fly ash does not participate in the hydration at the early stage and the secondary hydration products are different at the later stage.
基金Funded by the Youth Fund of National Natural Science Foundation of China (50902107)
文摘The early stage hydration mechanism of cellulose ether modified thin layer cement pastes was studied, using brick as the matrix. Samples of 6 h, 24 h, and 3 d and 7 d hydration time were analyzed to study the hydration law on the surface of high water-absorbing matrix. Hydration products were qualitatively and semi-quantitatively analyzed using XRD, TG-DSC-DTG, FTIR and SEM. The experimental results show that there is no enough water for 2 mm thick cement pastes to hydrate, because of rapid water absorption of matrix. Trace amounts of Ca （OH）2 was detected after three days hydration. With the prolongation of hydration time, the category and concentration of hydration products do not change. Compared with 2 mm thick cement pastes, 6 mm thick cement pastes and 10 mm thick cement pastes have lower dehydration rate and water loss. The humidity field of the cement paste show different changes within the same time. 6 mm thick cement paste and 10 mm thick cement pastes have longer time and more water to hydrate. Ca（OH）2 and ettringite were detected after 6 hours hydration and the concentrations of hydration products increased from 24 hours to 7 days.
基金Projects(U1534207,11790283,51878583)supported by the National Natural Science Foundation of China。
文摘To understand the effect of steam curing temperature on the hydrate and microstructure of hardened cement paste,several measuring methods including X-ray diffraction(XRD),atomic absorption spectroscopy(ASS),ion chromatography,conductivity meter,alternating-current impedance spectroscopy and nuclear magnetic resonance(NMR)are employed to investigate the hydration characteristics,pore solution composition and conductivity,resistivity and pore structure during the steam curing process.Experimental results show that steam curing promotes the hydration process,greatly raises the resistivity,and decreases the porosity of specimen at early age.Compared with being treated at 45℃,higher temperature leads to a fast decomposition of ettringite at initial stage of the constant temperature treatment period,which improves the relative content and ionic activity of the conductive ions in pore solution.Furthermore,the number of pores larger than 200 nm increases significantly,which reduces the resistivity of the hardened cement paste.Cement paste treated at 45℃ has a more stable and denser microstructure with less damages.
基金Funded by the Major State Basic Research Development Program of China(973 Program)(No.2009CB623201)National Natural Science Foundation of China(No.51302070)
文摘The effect of curing regime on degree ofAl3＋ substituting for Si^4＋ （Al/Si ratio） in C-S-H gels of hardened Portland cement pastes was investigated by 29Si magic angel spinning （MAS） nuclear magnetic resonance （NMR） with deconvolution technique. The curing regimes included the constant temperature （20, 40, 60 and 80 ℃） and variable temperature （simulated internal temperature of mass concrete with 60 ℃ peak）. The results indicate that constant temperature of 20 ℃ is beneficial to substitution ofAl3＋ for Si4＋, and AI/Si ratio changes to be steady after 180 d. The increase of Al/Si ratio at 40 ℃is less than that at 20℃ for 28 d. The other three regimes of high temperature increase Al/Si ratio only before 3 d, on the contrary to that from 3 to 28 d. However, the 20 ℃ curing stage from 28 to 180 d at variable temperature regime, is beneficial to the increase of AI/Si ratio which is still lower than that at constant temperature regime of 20 ℃ for the same age. A nonlinear relation exists between the Al/Si ratio and temperature variation or mean chain length （MCL） of C-S-H gels, furthermore, the amount ofAl3＋ which can occupy the bridging tetrahedra sites in C-S-H structure is insufficient in hardened Portland cement pastes.
基金Funded by the National Basic Research Program of China(No.2009CB623203)the National Natural Science Foundation of China(No.51008072)
文摘Using the tomography image, a method to characterize the 3D spatial distributions of increased porosity was proposed, and the increased porosity distributions of cement pastes with different leaching degrees were given using the current method. The leaching processes of CH/C-S-H and the contribution of CH/C-S-H leaching to porosity evolution were discussed. The proposed method can be applied to all cement- based materials with any leaching degrees. From the quantitative increased porosity results, we find that the CH leaching finished quickly on the sharp CH leaching front.
基金Funded by the Major State Basic Research Development Program of China(973 Program)(No.2009CB623201)
文摘The effect of curing regime on the distribution ofAl3＋ coordination in hardened cement pastes within 28 d were investigated by 29Si and 27Al magic angle spinning （MAS） nuclear magnetic resonance（NMR） with deconvolution technique. The results indicate that the tetrahedral coordination Al3＋ incorporated in C-S-H structure mainly originate from the AP＋ incorporated in the alite and belite phases in the Portland cement. The curing regime of constant temperature of 20 ℃ is beneficial to the octahedral coordination Al3＋ transforming to tetrahedral coordination AP＋ incorporated in C-S-H structure. However, at curing regime of variable temperature, the temperature rising process is more advantageous to the transformation from ettringite to monosulphate, substitution of Al3＋ for Si4＋ in the C-S-H structure and the formation of the third aluminate hydrate （TAH） than that at constant temperature of 20 ℃. The high temperature of 60 ℃ in the holding temperature process promotes the decomposition of ettringite, and enhances the consumption of the Al3＋ incorporated in C-S-H phases and the Al3＋ in TAH for the monosulphate forming. The temperature decreasing promotes the transformation from monosulphate to ettringite, and increases the consumption of the Al3＋ incorporated in C-S-H phases, and then increases the quantity of the TAH.
基金Project(2009CB623201) supported by the National Basic Research Program of ChinaProjects(50902106, 51272193) supported by the National Natural Science Foundation of ChinaProject(NCET-10-0660) supported by New Century Excellent Talents in Universities of China
文摘Micro computed tomography (Micro-CT) was applied to obtain three-dimensional images of the microstructure of cement paste (water-to-cement mass ratio of 0.5) at different ages. By using the Amira software, component phases of the cement paste such as pores, hydration products, and unhydrated clinker particles were segmented from each other based on their 3D image grey levels; their relative contents were also calculated with the software, and the data are 61.2%, 0% and 38.8% at the beginning of hydration and 11.8%, 78.5% and 9.7% at 28 d age, respectively. The hydration degree of cement paste at different ages was compared with the experimental data acquired by loss on ignition (LOI) tests. The results show that the calculated and measured data reasonably agree with each other, which indicates that micro-CT is a useful and reliable approach to characterize the micro structure evolution of hydrating cement paste.
基金the National Natural Science Foundation of China(Nos.51461135001 and 51741804)the Natural Science Foundation of Hunan Province,China(No.2017JJ2066)the Scientific Research Project of Education Department,Hunan Province,China(No.17A054)
文摘Four cellulose ethers（CEs） were compared for their effects on the pore structure of cement paste using mercury intrusion porosimetry. The experimental results show that the total pore volume and porosity of cement pastes containing the four cellulose ethers are significantly higher than that of the pure cement pastes and the total pore volume and porosity of cement pastes containing HEC（hydroxyethyl cellulose ether） or low viscosity cellulose ethers are low in four CEs. By changing the surface tension and viscosity of liquid phase and the strengthening of liquid film between air voids in cement pastes, CEs affect the formation, diameter evolution and upward movement of air voids and the pore structure of hardening cement paste. For the four CEs, the pore volume of cement pastes containing HEC or low viscosity cellulose ethers is higher with the diameter of 30-70 nm while lower with the diameter larger than 70 nm. CEs affect the pore structure of cement paste mainly through their effects on the evolvement of the small air voids into bigger ones when the pore diameter is below 70 nm and their effects on the entrainment and stabilization of air voids when the pore diameter is above 70 nm.
基金supported bythe National Fundamental Scientific Research Project (PR China),relevant to"Basic researchin Environmentally Friendly Concrete (2009CB623201)"research Project 50802067 supported by National Natural Science Foundation of Chinasupported by"the Fundamental Research Funds for the Central Universities"
文摘This paper is aimed to study the factors influencing rheological properties of cement paste with fly ash,including the quantity of cement replaced by fly ash,types of the fly ash,degree of cement hydration and the microstructure of cement paste. By changing the water-cement ratio,mixing different amounts of fly ash,adding different types fly ash,the rheological parameters of cement paste are measured with a rheometer,and the needle penetration are tested by Vical needle. A ultrasound velocity test was aslo employed to monitor hydration of paste at early ages. Combinations of different factors on the rheological properties of cement paste are studied. The objective is to reveal the effect mechanism of material nature and proportion of each component on the rheological properties of cement paste with fly ash.
基金Funded by the Scientific Research Foundation of the Graduate School of Southeast University (YBJJ1113)the National Basic Research Program of China (No.2009CB623200)the National Natural Science Foundation of China (No.51178103)
文摘The microstructure characteristics and meso-defect volume changes of hardened cement paste before and after carbonation were investigated by three-dimensional （3D） X-ray computed tomograpby （XCT）, where three types water-to-cement ratio of 0.53, 0.35 and 0.23 were considered. The high-resolution 3D images of microstructure and filtered defects were reconstructed by an XCT VG Studio MAX 2.0 software, The meso- defect volume fractions and size distribution were analyzed based on 3D images through add-on modules of 3D defect analysis. The 3D meso-defects volume fractions before carbonation were 0.79%, 0.38% and 0.05% corresponding to w/c ratio=0.53, 0.35 and 0.23, respectively. The 3D meso-defects volume fractions after carbonation were 2.44%, 0.91% and 0.14% corresponding to w/c ratio=0.53, 0.35 and 0.23, respectively. The experimental results suggest that 3D meso-defects volume fractions after carbonation for above three w/c ratio increased significantly. At the same time, meso-cracks distribution of the carbonation shrinkage and gray values changes of the different w/c ratio and carbonation reactions were also investigated.
基金Funded by the National Natural Science Foundation of China(No.50902107)National Science and Technology Supporting Program (No.2011BAJ04B02)the Fundamental Research Funds for the Central Universities (No.2011-YB-03)
文摘Hydration heat effect of cement pastes and mechanism of hydroxypropyl methyl cellulose ether （HPMC） and expanded perlite in cement pastes were studied by means of hydration exothermic rate, hydration heat amount, FTIR and TG-DTG. The results show that HPMC can significantly delay the hydration induction period and acceleration period of cement pastes. As mixing amount increased, hydration induction period of cement pastes enlarged and accelerated period gradually went back. At the same time, the amount of hydration heat gradually decreased. Expanded perlite had worse delay effects and less change of hydration heat amount of cement pastes than HPMC. HPMC changed the structure of C-S-H during cement hydration. The more amount of HPMC, the more obvious effect. However, EXP had little influence on the structure of C-S-H. At the same age, the content of Ca （OH）2 in cement pastes gradually decreased as the mixing amount increase of HPMC and expanded perlite, and had better delay effect than that single-doped with HPMC or expanded perlite when HPMC and expanded nerlite were both dooed in cement pastes.
基金Funded by the National Natural Science Foundation of China(No.51578004)the Program for Changjiang Scholars and Innovative Research Team in University(PCSIRT)(No.IRT1146)the Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD)。
文摘Cement pastes containing 0%, 15%, 25% and 35% fly ash were prepared. After being cured for 90 days, all fly ash blended cement pastes were crushed and ground into powders with a particle size less than 80 μm and then the powders were immersed in alkali solutions. Adsorption characteristics of K^+ and Na^+ ions in the pastes were investigated. Meawhile, the desorption characteristics of the adsorbed alklai ions and the inherent K^+ and Na^+ ions in the pastes were also investigated. Results showed that the contents of K^+ and Na^+ ions adsorbed by the pastes increased with increasing the substitution levels of fly ash and/or the concentrations of alkali solutions. Each paste was characterized by having the same adsorption capacity for K^+ or Na^+ that was essentially independent of alkali concentration. Adsorption mechanism of K^+ and Na^+ ions by the pastes is believed to be an effect of charge compensation of the C-S-H gel. Adsorption-desorption of the adsorbed K^+ and Na^+ ions in the pastes is reversible. The inherent K^+ and Na^+ ions in the pastes entered rapidly into the de-ionized water during the first 120 minutes, and then they were released at a relatively slow rate. A steady-state alkali partition was reached at about 720 minutes. Some K^+ and Na^+ ions which were originally "bound" by the hydration products were considered to be released into de-ionized water. Leaching tests showed that there was no significant effect of fly ash on the retaining of available alkalis in the pastes. A part of the released alkali ions exists in the pore solutions and the other part may be physically adsorbed by the hydration products.
基金Funded by the National Fundamental Scientific Research Project of China (No.2009CB623201)National Natural Science Foundation of China (No.50802067)
文摘The dominant factors during early hydration process of cement paste containing 10% metakaolin replacement （MK10） and 10% kaolin replacement （K10） are investigated in comparison to neat cement paste （NCP）, and X-ray Diffraction （XRD） analysis is employed to identify the crystalline phases of all specimens. Thermogravimetric （TG） and Differential Scanning Calorimetry （DSC） are used to identify the phase constituents. The amount of acid-insoluble residue （AIR） of all specimens is used to evaluate the unreacted materials. The results indicate that, after the first day, MK act as nuclei for the formation of C-S-H during hydration of C3S and C2S, densifying the microstructure of cement paste. Its contribution is mainly due to the fine nature of the MK. From 3 days to 7 days, more and more MK reacts with CH to form C-S-H, making the microstructure denser. The strength contribution is mainly due to the chemical activity of MK.
基金Funded by the National Nature Science Foundation of China(No.51108282)Natural Science Foundation of Hebei Province of China(No.E2011210025)+1 种基金Excellent Youth Scholars of University Science and Technology Research of Hebei Province(No.Y2011111)Hebei Key Discipline Construction Project
文摘The chemical and physical interactions in the interfacial transition zone （ITZ） between three different types of coarse aggregates （limestone, granite and basalt） and cement paste were investigated. The results show that all the aggregates are chemically active. Significant amounts of Ca2＋, K＋, and Na＋ are absorbed by all the aggregates from the cement solution, granite and basalt also absorb significant amounts of OH- and release significant amounts of Si4＋ into cement solution. The XRD, EDXA and pore structure results of the ITZ also show that more clinkers participate in the cement hydration in the ITZ of granite and basalt, and more hydrates are generated, hence resulting in a denser ITZ structure with a lower content of maeropores. Although the limestone has the least activity, the connection between it and cement paste is tight, due to its rough surface and higher water absorption. Whereas the granite with smooth surface and lower water absorption has a loose connection with cement paste, many pores and cracks are visible, which is very detrimental to the concrete durability.
基金Funded by the National Natural Science Foundation of China(No.51678081)Natural Science Foundation of the Jiangsu Higher Education Institution of China(No.18KJB560001)。
文摘To further strengthen the protective effect of aerogel cement paste (ACP) coating on self-compacting concrete (SCC) in tunnel fire under the optimal mix proportion,the effect of curing temperature (from 5 to 80 ℃,based on site construction curing temperature and surrounding rock temperature) on fire insulation of ACP was investigated.The mechanical properties,thermal conductivity and porosity of ACP were tested.The microstructure of ACP was characterized by means of SEM,XRD and TG/DTG.The results reveal that 50 ℃ is the optimal curing temperature for ACP with good mechanical properties and fire insulation.Relatively high curing temperature can facilitate hydration and pozzolanic reactions,contributing to the generation of more stable substances (such as C-S-H gels,tobermorite and thenardite,etc).ACP under excessive low curing temperature brings inhomogeneous microstructure with coarse pores,leading to producing wider and longer microcracks when it is exposed to tunnel fire.The microcracks make the heat convection and thermal radiation more significant and thus accelerate the damage of ACP under fire.In case of the less than 7% difference of thermal conductivity,dense microstructure and stable substances are more conducive to strengthening fire insulation of ACP.In practical engineering applications,the thickness of protective layer of ACP can be further optimized when ACP is cured under about 50 ℃.
基金Funded by the National Natural Science Foundation of China(Nos.U1301241 and U1234211)the Postdoctoral Science Foundation of China(No.2015M580042)
文摘The rheologicalbehaviors of fresh cement paste with polycarboxylate superplasticizer were systematically investigated.Influentialfactors including superplasticizer to cement ratio（Sp/C）,water to cement ratio（w/c）,temperature,and time were discussed.Fresh cement pastes with Sp/Cs in the range of 0 to 2.0% and varied W/Cs from 0.25 to 0.5 were prepared and tested at 0,20 and 40 °C,respectively.Flowability and rheologicaltests on cement pastes were conducted to characterize the development of the rheologicalbehavior of fresh cement pastes over time.The exprimentalresults indicate that the initialflowability and flowability retention over shelf time increase with the growth in superplasticizer dosage due to the plasticizing effect and retardation effect of superplasticizer.Higher temperature usually leads to a sharper drop in initialflowability and flowability retention.However,for the cement paste with high Sp/C or w/c,the flowability is slightly affected by temperature.Yield stress and plastic viscosity show similar variation trends to the flowability under the abovementioned influentialfactors at low Sp/C.In the case of high Sp/C,yield stress and plastic viscosity start to decline over shelf time and the decreasing rate descends at elevated temperature.Moreover,two equations to roughly predict yield stress and plastic viscosity of the fresh cement pastes incorporating Sp/C,w/c,temperature and time are developed on the basis of the existing models,in which experimentalconstants can be extracted from a database created by the rheologicaltest results.
