Influence of MgO/MgCl2 molar ratio on phase stability of magnesium oxychloride cement

Formation, solution and phase change of hydration products in MgO-MgCl₂-H₂O system was studied with thermodynamics method, and resistance to water immersion and phase change of magnesium oxychloride cement with different MgO/MgCl₂ molar ratio was experimented. The results show that pH value of immersion solution of cement paste has a remarkable influence on phase stability of hydration products. A higher pH value leads to a lower solubility and a better phase stability of hydration products. When the solution pH value is higher than 10.37, the precipitation of much Mg(OH)₂ crystal induces a worse phase stability of hydration products. With the increasing MgO/MgCl₂ molar ratio (lower than 6), the more amount of MgO in the hydration products enhances the alkalinity of solution and the phase stability is improved. However, when the MgO/MgCl₂ molar ratio is higher than 6 and the excessive MgO exsits in the hydration products, the cement paste may be damaged by the excessive crystallization stress of a great deal of Mg(OH)₂ formation. Funded by the National Natural Science Foundation of China(No. 50078019)     

Influence of CG With High Content of Metallic Particles as a Cement Admixture on Cement Strength

1　IntroductionCopperresidueisanindustrialby productdis chargedbycopperrefineries ,butitusuallycontainssomeusefulmetals .Forexamplecopperresiduecontains 4 %to5 %copper ,1.0 1g tgold ,2 4g tsilverand 4 2 %magne tite ,alltheseexceedthecontentforacomprehensiveutili zation .Inordertoextracttheseusefulmetals ,thecoolingtechnologyofremovingresidueisintroducedbyslowlycooling (4 8h)sothatsomemetallicionsgrow .Afteronemoreflotationforcopperandmagneticbyextractionofiron ,thedischargedresidueiscalledcopp…     

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.     

Preparation and microstructure characterization of poly-sialate-disiloxo type of geopolymeric cement

In order to investigate the influence of three key molar ratios (n(SiO₂)/n(Al₂O₃), n(K₂O)/n(Al₂O₃) and n(H₂O)/n(K₂O)), a total of nine potassium poly-sialate-disiloxo (K-PSDS) geopolymeric cement matrices were designed according to orthogonal design principle. Subsequently, XRD, ESEM-EDXA and MAS-NMR techniques were employed to further characterize the microstructure of the most fully reacted geopolymeric cement matrix. The experimental results show that n(K₂O)/n(Al₂O₃) has the most significant effect on compressive strength amongst the three ratios. The highest compressive strength (20.1 MPa) can be achieved when n(SiO₂)/n(Al₂O₃)=6.5, n(K₂O)/n(Al₂O₃)=0.8 and n(H₂O)/n(K₂O)=10.0. The FTIR spectra of nine PSDS geopolymeric cement matrices also indicate that geopolymeric cement matrix with the highest strength is the most fully reacted one and possesses the largest amount of geopolymeric cement products. The microscopic analysis reveals that PSDS geopolymeric cement matrix possesses structural characteristics similar to gel substances in having a wide range of Si endowments, but predominantly the framework molecular chains of Si partially replaced by 4-coordinated Al tetrahedral.     

氯氧镁水泥相稳定性的研究

为在生产和广泛使用氯氧镁水泥时提高氯氧镁水泥相稳定性,运用热力学方法对MgO-MgCl_2-H_2O体系水化产物的生成、溶解和相转化进行分析,并对不同MgO/MgCl_2摩尔比的氯氧镁水泥的耐水性和相变规律进行试验研究.结果表明,水泥石所处的溶液pH值对水化产物相稳定有较大影响,pH升高水化产物的溶解度减小,相稳定性提高;当pH值大于10.37时,溶液析出大量Mg(OH)_2使水化产物相稳定性变差.当摩尔比小于6时,随着摩尔比的增大,水化产物内部过量MgO增多使体系碱度提高,水化产物稳定性改善;当摩尔比大于6 时,水化产物中含过量MgO,由于较多Mg(OH)_2生成产生过大的结晶应力,引起水泥石结构破坏.     

活性MgO固化淤泥水稳特性试验研究

将绿色、低碳、环保的活性MgO引入淤泥固化处理,通过对比活性MgO、MgO-粉煤灰及传统固化剂水泥固化淤泥试样,分析不同浸水时间下试样外观、质量、应力-应变关系和无侧限抗压强度等性质,综合评价活性MgO基材料固化淤泥的水稳特性.结果表明：活性MgO固化淤泥水稳性显著优于未固化淤泥,且水稳性随掺入MgO质量分数和标准养护龄期的增加而增强;粉煤灰的加入可以显著改善活性MgO固化淤泥的抗压强度和水稳性;浸水时间增加对固化淤泥无侧限抗压强度呈削弱效应,提高掺入活性MgO质量分数可以减小固化淤泥破坏应变;整体上,活性MgO-粉煤灰固化淤泥水稳性优于活性MgO,活性MgO优于传统固化剂水泥.微观分析表明：Mg（OH）₂和水化硅酸镁M-S-H等胶结物质是活性MgO和活性MgO-粉煤灰固化淤泥水稳性增强的最主要原因.     

水泥固化温州污染土的力学性质和微观结构特性

在不同水泥掺量和龄期条件下对NaCl、油脂、Pb（NO₃）₂污染的温州软土进行水泥固化处理后,土体的强度得到改善。为进一步得出水泥固化处理对于不同污染土的处理效果,对水泥固化稳定不同的污染土进行了无侧限抗压强度试验和微观结构研究。分析了不同污染物类型、污染物掺入量、水泥掺入量以及养护龄期对水泥固化污染土强度特性的影响以及不同污染物浓度下水泥固化土微观结构的差异。试验结果表明：NaCl在一定范围内促进了水泥固化土早期强度的提高;油脂使水泥固化土的强度明显降低,压缩性增大;Pb（NO₃）₂掺入到土体中后,水泥固化土的强度总体上略有降低,掺入量与强度之间大致呈线性关系。随着水泥掺入量及龄期的增加,水泥固化污染土的强度会有显著提高。扫描电镜（SEM）结果分析得出：由于污染物的作用,污染物浓度的增加使固化土中孔隙增多,结构变得疏松。     

磷酸镁水泥的研究

用氧化镁（MgO）和磷酸二氢铵(NH     

Preparation and characteristic research of anhydrous magnesium chloride with dehydrated ammonium carnallite

Taking the saline lake bischofite and NH4Cl that was removed with the ammonia method and continuously followed by filtration as raw materials with a molar ratio of 1 ： 1 of MgCl2 to NH4Cl, ammonium carnallite was synthesized. And then the ammonium carnallite was dehydrated to some extent at 160 ℃ for 4 h. Ammonium carnallite reacted with ammonia at 240 ℃ for 150 min and the ammonation ammonium carnallite was produced. Finally, the ammonation ammonium carnallite was calcined at 750 ℃ into anhydrous magnesium chloride containing only 0.1% （mass fraction） of MgO. On the other hand, dehydrated ammonium carnallite was mixed with the solid ammonium chloride at mass ratio 1 ： 4 at high temperature and with the differential pressure of NH3 above 30.5 kPa. The dehydrated ammonium carnallite of mixture was dehydrated at 410 ℃, and then calcined at 700 ℃ into anhydrous magnesium chloride with only 0. 087% （mass fraction） of MgO. X-ray diffraction and electron microscopy analysis results prove that anhydrous magnesium chloride obtained by both methods hasn＇t mixed phases,the particle is large and even has good dispersion, which is suitable for preparation of metal magnesium in the electrolysis.     

稻草纤维增强泡沫混凝土物理力学性能试验研究

为了研究稻草纤维增强泡沫混凝土的性能,以普通硅酸盐水泥为主要胶凝材料,硅灰、偏高岭土和粉煤灰为辅助胶凝材料,稻草纤维为增强材料,采用物理发泡法制备纤维增强泡沫混凝土;通过全因子试验,研究在不同水胶比和发泡剂掺量下,稻草纤维掺量对泡沫混凝土的密度、吸水率、抗压强度、抗折强度、劈裂抗拉强度和抗冻性能的影响。结果表明：对于不同水胶比和发泡剂掺量,泡沫混凝土的密度、抗压强度和劈裂抗拉强度均随纤维掺量的增加呈现出先增加后降低的变化规律;抗压强度随密度增加呈幂函数增加关系;劈裂抗拉强度随抗压强度的增加呈指数函数增加关系;当水胶比为0.45时,抗折强度随纤维掺量的增加先增加后降低,当水胶比为0.50时,抗折强度随纤维掺量的增加而增加;纤维的掺入增大了泡沫混凝土的泡孔尺寸和吸水率,降低了其抗冻性能。     

Mechanical Properties and ITZ Microstructure of Recycled Aggregate Concrete Using Carbonated Recycled Coarse Aggregate

The effect of carbonation treatment and mixing method on the mechanical properties and interfacial transition zone (ITZ) properties of recycled aggregate concrete (RAC) was investigated. Properties of recycled concrete aggregate (RCA) were tested firstly. Then, five types of concretes were made and slump of fresh concrete was measured immediately after mixing. Compressive strength and splitting tensile strength of hardened concrete were measured at 28 d. Meanwhile, the microstructure of RAC was analyzed by backscattered electron (BSE) image. It was found that the water absorption ratio of carbonated recycled concrete aggregate (CRCA) was much lower when compared to the untreated RCA. Comparatively, the apparent density of CRCA was not significantly modified. The concrete strength results indicate that the mix CRAC-2 prepared with CRCA by adopting two-stage mixing approach shows the highest compressive strength value compared to the other mixes. The microstructural analysis demonstrate that the mix CRAC-2 has a much denser old ITZ than the untreated RAC because of the chemical reaction between CO₂ and the hydration products of RCA. This study confirms that the ITZ microstructure of RAC can be efficiently modified by carbonation treatment of RCA and encourages broadening the application of construction and demolition wastes.     

Influence of elevated curing temperature on the properties of cement paste and concrete at the same hydration degree

Three different curing temperatures(20 ℃, 40 ℃, and 60 ℃) were set, so that the nonevaporable water(w_n) contents of plain cement pastes cured at these three temperatures were measured to determine the hydration degree of cement. Tests were carried out to compare the pore structure and strength of cement paste, as well as the strength and permeability of concrete under different temperature curing conditions when their cements were cured to the same hydration degree. The experimental results show that either at a relatively low hydration degree(w_n=15%) or high hydration degree(w_n=16.5%), elevated curing temperature has little influence on the hydration products of cement paste, while it has a negative influence on the pore structure and compressive strength of cement paste. However, this negative effect is weaker at high hydration degree. The large capillary pore(100 nm) volumes of cement pastes remain almost the same at high hydration degree, regardless of curing temperatures. As for the concrete, elevated curing temperature also has negative influence on its compressive strength development, at both low hydration degree and high hydration degree. And this negative effect is stronger than that on cement paste's compressive strength at the same hydration degree. On the whole, elevated curing temperature has little influence on the resistance of concrete to chloride ion penetration.     

Long-term Performance of Moderate Heat Portland Cement with Double-expansive Sources

The long-term performance of moderate heat Portland cement with double-expansive sources （DE cement） in the system of high MgO clinker and gypsum was studied by XRD, SEM/EDAX and test methods for strength and expansion of cement. Results indicate that the periclase particle, whose size was 5-7.5μm in DE cement clinker containing 4.8 % MgO, existed individually. The periclase hydration in hardened DE cement paste started at about 60 days and completed up to 2 000 days, and ettringite in the paste was stable from 3 days to 2 000 days. Under the conditions of 4.5%-5.0 % MgO in clinker and 2.8%-3.4 %SO3 in cement, ettringite expansion and brucite expansion in DE cement paste had a continuity, entirety and stability. At the ages of 90, 365,730 and 2 000 days the expansion of the paste reached 0.07%-0.11%, 0.16%-0.21%, 0.21%-0.27 %, and 0.29%-0.38%, respectively. The results suggest that by using this cement in mass concrete it may compensate its temperature shrinkage and autogenous shrinkage to some extent.     

PB-g-PS胶乳改性水泥砂浆的性能研究

PB-g-PS胶乳常用于改性水泥砂浆,聚丁二烯对苯乙烯的重量比的范围从70/30,50/50到30/70.保持水灰比为0.5的常数,在20°、相对湿度为90%的空气中养护24 h,接下来在20°的水中养护27 d.这项研究考察了聚丁二烯与苯乙烯的重量比和聚灰比对PB-g-PS胶乳改性水泥砂浆的流动度,抗压、抗折强度及吸...     

Effects of EVA Latex on the Properties of Glass-fiber/Magnesium-oxychloride Cement Composites

1 IntroductionMagnesiumOxychloride Cement (MOC) or Sorel Ce-ment ,discovered by Sorel in1867 ,is a kind of air-curi-ng colloid material and has many advantages over othercements,including excellent fire retardance ,a lowheatconductivity coefficient , good abradability and corrosionresistance against organic solvents and sulphides . Fur-thermore ,it requires no maintenance under wet condi-tions . For many years , GF/MOC composites have beensuccessfully applied in many fields . However , G…     

Influence of the aggregate volume on the electrical resistivity and properties of portland cement concretes

The electrical resistivity of concretes with various aggregate volume fractions (V _a) of 0%–70% at water/cement (W/C) ratios of 0.4 and 0.5 during 1 day was monitored. It is found that the addition of normal aggregate to cement paste leads to a regular increase in concrete resistivity at each hydration stage and the electrical resistivity has a deeper increase for the lower W/C at a fixed aggregate volume fraction. The number of normalized resistivity (NR) of concrete to its paste matrix was introduced, which is only a function of aggregate volume fraction (V _a). The quantitative relationships give an alternative method for the prediction of aggregate volume in the concrete. A logarithmic relation is established between the elastic modulus of concrete at 7 days or 28 days and the electrical resistivity of concrete at 1 day. The equations are obtained, the compressive strength of concrete at 7 days or 28 days can be determined by the electrical resistivity of concrete at 1 day and the used aggregate content in the concrete. The quantitative relationships give a non-destructive test (NDT) method for prediction of concrete elastic modulus and compressive strength.     

Pozzolanic Activity of Burned Coal Gangue and Its Effects on Structure of Cement Mortar

The pozzolanic activity of coal gangue burned at different burning temperatures was investigated. The burned coal gangue was mixed with portland cement in different proportions （ 20% - 60% ）. The pozzolanic activity of coal gangue burned and the hydration products were examined, the compressive strengths of the pastes of the mixtures were tested, and the mechanism of the reaction was discussed. The experimental results slum, that the coal gangue burned at 750 ℃ has the optimum pozzolanic activity, and the burned coal ganguc with SiO2 and Al2 O3 is in an active form. When the coal gangue burned at 750℃ is mixed into portland cement, the content of calcium hydroxide in paste is significantly reduced, while the contents of hydrated calcium silk.ate and hydrated calcium aluminate are increased accordingly, hence resulting in the improvement of the microstructure of mortar. The compressive strength of cement paste decreases with increasing the content of burncd coal gangue. The decease in strength is small in the range of 20% - 30% coal gangue substitution and significant in 30%- 60% substitution.     

CA砂浆强度影响因素及强度机理研究

为制得满足高速铁路建设的CA砂浆,制备了不同配比的CA砂浆,通过考查CA砂浆的强度并利用扫描电子显微镜观测微观结构,研究砂的级配、砂灰比、乳化沥青与水泥比等因素对CA砂浆固化体不同龄期强度的影响．研究表明：选择粒径为0．15—0．3mm与0．3—0．6mm的组合砂能有效地降低高强型CA砂浆材料的分离度,而对CA砂浆28...     

煅烧煤矸石用作水泥高活性混合材的研究

将经煅烧等过程进行活化处理的煤矸石细粉与磨至一定比表面积的水泥熟料及天然生石膏混合均匀,制成了活化煤矸石粉掺量比例不同的多组混合水泥,并对其胶砂强度性能、标准稠度用水量、胶砂流动度进行了实验检测.同时,还应用SEM对混合水泥硬化浆体的微观结构进行了观察.结果表明,掺入经活化处理的煤矸石粉配制的混合水泥具有较好的强度性能;随掺量增加,水泥浆体的流变性能变差,但对凝结时间并无明显影响.     

Size effect on cubic and prismatic compressive strength of cement paste

A series of compression tests were conducted on 150 groups of cement paste specimens with side lengths ranging from 40 mm to 200 mm. The specimens include cube specimens and prism specimens with height to width ratio of 2. The experiment results show that size effect exists in the cubic compressive strength and prismatic compressive strength of the cement paste, and larger specimens resist less in terms of strength than smaller ones. The cubic compressive strength and the prismatic compressive strength of the specimens with side length of 200 mm are respectively about 91% and 89% of the compressive strength of the specimens with the side length of 40 mm. Water to binder ratio has a significant influence on the size effect of the compressive strengths of the cement paste. With a decrease in the water to binder ratio, the size effect is significantly enhanced. When the water to binder ratio is 0.2, the size effects of the cubic compressive strength and the prismatic compressive strength of the cement paste are 1.6 and 1.4 times stronger than those of a water to binder ratio of 0.6. Furthermore, a series of formulas are proposed to calculate the size effect of the cubic compressive strength and the prismatic compressive strength of cement paste, and the results of the size effect predicted by the formulas are in good agreement with the experiment results.