Hydration mechanism of silica fume-sulphoaluminate cement

Setting time and strength of sulphoaluminate rapid hardening cement （SAC） incorporated in the presence and absence of silica fume （SF） were determined. Combined with the techniques of＂ isothermal calorimeter, XRD and FSEM, the hydration kinetics of the two systems and the effect mechanism of SF on SAC were investigated. The experimental results showed that SF was proved to be beneficial for SAC system, in terms of setting time and late strength gain. Evidence of accelerator effect of silica fume was found during the first 8 hours of hydration. The formation of AFt was accelerated and the microstructure of the hydration products grew denser with incorporation of SF. SF was proved to play the role of dispersion and setting control at early age and had a greater contribution to later strength due to the increment of crystal nucleation point and the pozzolanic activity. Therefore, SF can be used to not only control the hydration kinetics of SAC, but also develop the late strength and improve the microstructure.     

粉煤灰-脱硫石膏水泥基材料水化活性及微结构

采用DTA—TG、XRD、SEM以及宏观水化收缩和强度试验等手段研究了粉煤灰一脱硫石膏一水泥三元复合胶凝体系的水化过程、活性效应及微观结构等,根据试验结果总结了复合胶凝材料的水化动力学过程。结果表明,粉煤灰一脱硫石膏水泥石的钙矾石吸热峰强于基准样;在各组分相互活性激发和外掺激发剂作用下,粉煤灰一脱硫石膏水泥石中2次水化效应明显;SEM、XRD表明水泥石早期有明显的钙矾石生成,同时粉煤灰颗粒的表面侵蚀现象明显,进一步说明复合胶凝体系的早期活性得到有效激发,硬化后综合性能得到有效保证。且宏观收缩及强度试验也从侧面印证了微观试验结果。粉煤灰一脱硫石膏水泥基复合胶凝材料体系的研发可大量消耗燃煤电厂的工业废渣,具有显著的“绿色”效应。     

Effects of LiAl-layered double hydroxides on early hydration of calcium sulphoaluminate cement paste

As a 3D micro-nano material, layered double hydroxides have been widely used in many fields, especially for reinforced composite materials. In this paper, LiAl-LDHs was obtained by a hydrothermal method. In order to investigate the effects of LiAl-LDHs on the early hydration of calcium sulphoaluminate (CSA) cement paste, compressive strength, setting time and hydration heat were tested while X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scaning electron microscopy (SEM) and differential scanning calorimetry (DSC) analysis were employed. The results indicated that LiAl-LDHs could significantly improve the early compressive strength and shorten the setting time of calcium sulphoaluminate cement paste with 3wt% concentration. Besides, the hydration exothermic rate within 5 h was accelerated with increasing LiAl-LDHs content. Moreover, the addition of LiAl-LDHs did not result in the formation of a new phase, but increased the quantity of hydration products providing higher compressive strength, shorter setting time and denser microstructure.     

The Pore Structure and Hydration Performance of Sulphoaluminate MDF Cement

1　IntroductionReducingporositycouldimprovethestrengthofma terials ,butitisnotenoughthatthechangeofthestrengthofthematerialsisonlysubjectedtothechangeofporosi ty .Thereforetheporedistributionconceptisintroduced .Weeventhinkthattherelationbetweentotalporos…     

Early hydration of composite cement with thermal activated coal gangue

Composite cement samples were prepared by mixing clinker, gypsum with burnt coal gangues which was calcined at various temperatures. The mechanical strength and Ca(OH)₂ content in the cement paste were tested, and the paste composition and microstructure were analyzed by thermogravimetry-differential thermal analysis (TG-DSC), X-ray diffraction(XRD), scanning electronic microscopy (SEM) and pore structure analysis. Results demonstrate that the thermal activated coal gangue could accelerate the early hydration of cement clinker obviously, which promotes the gangue hydration itself. The early hydrated products of the cement are C-S-H gel, Ca(OH)₂ and AFt. The cement with 30% (in mass) the gangue exhibits higher mechanical strength, and among all the cement samples the one with the gangue burnt at 700 °C displays the highest hydration rate, mechanical strength, the most gel pores and the lowest total porosity.     

硅酸盐-硫铝酸盐水泥混合体系的试验研究

研究了不同比例的硅酸盐、硫铝酸盐水泥混合体系的凝结时间、水泥砂浆的强度性能,并对一定混合比例的OPC-SAC水泥进行了XRD、SEM和水化量热测试。结果表明,硅酸盐水泥与硫铝酸盐水泥混合,SAC中的C4A3-S矿物与OPC中的C3S矿物在共同水化过程中有相互促进的作用,会使混合水泥水化和凝结加速;混合水泥的强度性能与两种水泥的混合比例有关。本研究可对硅酸盐-硫铝酸盐水泥混合体系的应用提供借鉴。     

低水胶比掺膨胀剂的水泥水化特性

应用XRD、TG-DTA和温度测量等方法，研究了低水胶比条件下普硅水泥和膨胀水泥的水化特点。结果发现，膨胀水泥水化各龄期中AFt的含量和CH的早期含量明显增加，水化后期CH量则与普硅水泥石基本相当。膨胀水泥硬化浆体的后期膨胀主要依靠凝胶状AFt吸水肿胀而形成驱动力。膨胀刑的组分对膨胀水泥的水化热有明显影响，如果膨胀剂含有硫铝酸盐类矿物，则可能造成体系水化热的升高。     

Carbonation resistance of sulphoaluminate cement-based high performance concrete

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. Funded by the National Natural Science Foundation of China (No.50872043)     

Preparation and performance research of cement-based grouting materials with high early strength and expansion

Main performance of the cement grouting materials made up by Portland cement(PC) and sulphoaluminate cement(SAC) was investigated in this program, a kind of expanding agent(EA) which was mainly constituted by metakaolin and alunite was utilized for the compensation of the shrinkage, the hydration products and micro structure of the grouting materials were researched by X-ray diffraction(XRD) and scanning electron microscopy (SEM). The results showed that a high expansion rate of the grouting materials could be reached as the expanding agent mixed in 6% of PC mass; the addition of SAC in the S2(PC:SAC:EA=34:6:2.25) brought a further improvement of the expansion rate of the grouting materials, the analysis of XRD and SEM showed that due to the reaction of expanding agent and SAC in the grouting materials, more ettringite crystal was generated, which resulted in a higher early strength, the addition of SAC played an expansion and strength reinforcement role in the grouting materials.     

Influence of Nano-SiO_2 Addition on Properties of Sulphoaluminate Cement Based Material

The influences of different nano-SiO_2(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.     

The hydration characteristics and expansion machanism of expansive cement at low<Emphasis Type="Italic">W/B</Emphasis> ratio

The hydration characteristics and expansion impetus of three kinds of cement paste under free-and confined-curing conditions were investigated, which were respectively mixed with three different kinds of expansive agent at low W/B ratio. The results show that the hydration products of pure cement paste and paste mixed with expansive agent are same, but the amount of hydration products, un-hydrated C₃S and C₂S are obviously different at the same hydration age. At 3 d age, the amount of CH in pure cement paste is less than that of paste mixed with expansive agent, but it is reverse when at 28 d age. The amount of AFt at 3d and 28d age in pure cement paste is less than those of paste mixed with expansive agent. Regardless of under free- or confined-curing condition, the amount of ettringite produced varies little since 3d age. The joint effect of the tumefaction of gel-ettringite due to water absorption and the expansive pressure on the pore caused by the crystalloid ettringite is the cause of the volume expansion of cement paste, and the former effect is much greater than the latter. LU Lin-nu : Born in 1972 Funded by 973 High-tech Project of China (No. 2001CB610704-2) and the Natural Science Foundation of Hubei Province(No. 2002AB075)     

Dominant factors on the early hydration of metakaolin-cement paste

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.     

Hydraulicity of Wet-milling Ultra-fine Grouting Cement

The physical and mechanical properties of wet-milling ultra-fine grouting cement were studied,and its microstructure was abserted through modern instrwnentation analysis such as scanning electronic micoscoty (SEM),X-ray diffraction and Hg-intrusion micromeritics ,The experimertal results indicate that wet-milling ultra-fine cement possesses high rheological properties and groutability,It can be filles densely in cracks of rock and hydrate fully,which may endow hydrated cemetnt with high mechanical strength Main hydration products of wetmilling ultra-fine cement are poorly crystalline G-S-H(I),acicular AFt and plank-shape Ca(OH)2,The dense crystal-netwonk structure can be formed in the rock gaps filled with cement psate ,but some weak regions exist owing to Ca(OH)2,The features of micro-pore structure of hydrated wet-milling ultra-fine cement are few big harmful pores,abundant harmless micro pores and little most possible pore radius.     

Comparison of hydration properties between cement-GGBS-fly ash blended binder and cement-GGBS-steel slag blended binder

The hydration properties of cement-GGBS-fly ash blended binder and cement-GGBS-steel slag blended binder were compared. The experimental results show that the hydration rate of cement-GGBS- steel slag blended binder is higher than that of cement-GGBS-fly ash blended binder within 28 days, but lower than the latter after 28 days. The hydration of cement-GGBS-steel slag blended binder tends to produce more Ca（OH）2 than the hydration of cement-GGBS-fly ash blended binder, especially at late ages. Cement-GGBS- steel slag mortar exhibits higher strength than cement-GGBS-fly ash mortar within 28 days, but at late ages, it exhibits similar compressive strength with eement-GGBS-fly ash mortar and even slightly lower bending strength than cement-GGBS-fly ash mortar. Cement-GGBS-steel slag paste has finer early pore structure but coarser late pore structure than cement-GGBS-fly ash paste. Cement-GGBS-steel slag paste can get satisfied late pore structure and cement-GGBS-steel slag mortar can get satisfied late strength as compared with pure cement paste and pure cement mortar, respectively.     

Carbonation of sulphoaluminate cement with layered double hydroxides

The increasing importance of the ecologically minded production of building materials makes it necessary to develop reasonable alternatives to the CO2-intense production of ordinary Portland cement （OPC）. The development of new or modified concrete is an important part of existing strategies to improve performance and minimize life-cycle costs. Therefore, we investigated carbonation resistance properties of sulphoaluminate cement （SAC） concrete incorporating layered double hydroxides （LDHs）. X-ray diffraction （XRD） and IR-spectroscopy were employed to characterize the component and structural changes of LDHs and cement paste before and after carbonation test. Carbonation resistance of concrete was experimentally evaluated. Finally, carbonation of Portland cement and SAC concrete was compared. The experimental results show that carbonation depth decreases remarkably with the addition of LDHs, especially the calcinated LDHs. Carbonation depth of SAC concrete is smaller than that of PC concrete regardless of curing time.     

Influence of ultra-fine fly ash on hydration shrinkage of cement paste

1INTRODUCTION Hydrationshrinkageisalsoknownaschemicalshrinkage[1].Thesolidvolumeincreasesafterce menthydrating,buttheabsolutevolumeofcementwatersystemreduces.Usuallythetotalamountofvolumeshrinkageofcementwatersystemis7%9%[1,2].Withthedevelopmentofthetechno logyofcementandconcrete,concretewithhighstrengthandhighperformanceisthedevelopingtendencynow.However,nowadays,comparedwithordinaryconcrete,thehydrationshrinkageofhighperformancecementconcreteincreasesobvi ouslybecauseoftheaccelerationof…     

Mechanical properties and microstructure of sulfur aluminate cement composites reinforced by multi-walled carbon nanotubes

The effect of multi-walled carbon nanotubes (MWCNTs) on the mechanical properties and microstructure of sulfur aluminate cement (SAC) composites was investigated. The dispersed MWCNTs were added into SAC in various weight contents.The results of mechanical properties of the MWCNTs/SAC composites indicated that the addition of 0.08 wt% MWCNTs can improve the SAC compressive strength, flexural strength, and bend-press ratio by 15.54%, 52.38%, and 31.30% at maximum, respectively. The degree of SAC hydration and porosity and pore size distribution of the matrix were measured by X-ray diffraction (XRD), thermal analysis (TG/DTG), and mercury intrusion porosimetry (MIP). Results show that the addition of MWCNTs in SAC composites can promote the hydration of SAC and the formation of C-S-H gel, reduce the porosity and refine the pore size distribution of the matrix. The microstructure was characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It is found that the MWCNTs have been dispersed homogeneously between the hydration products of SAC paste and act as bridges and networks between cracks and voids, which prevents the development of the cracks and transfers the load.     

Effects of citric acid on hydration process and mechanical properties of thermal decomposed magnesium oxychloride cement

In order to make full use of salt lake magnesium resources and improve the strength of the thermal decomposed magnesium oxychloride cement(TDMOC), the effects of citric acid on the hydration process and mechanical properties of TDMOC was studied. The hydration heat release at initial 24 h and strengths at 3, 7, and 28 days of TDMOC specimens were conducted. The hydration products and paste microstructure were analyzed by XRD, FT-IR and SEM, respectively. The results showed that citric acid can not only reduce the 24 h hydration heat release and delay the occurring time of second peak of TDMOC, but also produce more 5Mg(OH)2·MgCl 2·8H2O and less Mg(OH)2 in hydration process of TDMOC. More perfect and slender crystals were observed in the microstructure of the TDMOC pastes with citric acid. The results demonstrated that citric acid as an additive of TDMOC can decrease the hydration heat release and increase the compressive strength and flexural strength of TDMOC. The possible mechanism for the strength enhancement was discussed.     

Hydration kinetics of phosphorus slag-cement paste

Hydration characteristics of Portland cement paste with phosphorus slag powder incorporated and hydration kinetics was investigated with SEM, X-ray diffraction, DTA-TG and calorimeter II 80. Results showed that phosphorus slag powder could reduce total amount of hydration products yet had little influence on the type of hydration products. The total amount of heat of hydration was decreased by 49.11% and the final setting was postponed by 2.28 h when phosphorus slag powder substituted 35% Portland cement by mass. The accelerating stage of this composite paste was controlled by catalysis, decreasing stage controlled by both catalysis and diffusion while stabilizing stage by diffusion alone. Hydration resistance and activation energy were reduced and hydration speed was accelerated.     

Microstructure and Composition of Hydration Products of Ordinary Portland Cement with Ground Steel-making Slag

1　IntroductionSteel makingslagisthewasteofsteel makingindus tryandnearlysixteenmilliontonssteel makingslagisproducedinChinaperyear[1] .Justasflyashandblastfurnaceslag ,itisoneofthreekindsofdominantindustrywastesinourcountry .Eventhoughsteel makingslagce menthasdevelopedformorethantwentyyearsinCh ina[2 ,3 ] ,comparedwithothertwowastes ,thestudyandap plicationonsteel makingslagincementandconcreteareinsufficientyet.Moststeel makingslagcementsarepre paredbyinter grindingprocess ,sotheparticlesiz…