Effects of Temperature on the Preparation of Magnesium Carbonate Hydrates by Reaction of MgCI2 with Na2CO3

Homogeneous(unseeded)precipitation of magnesium carbonate hydrates by the reaction of MgCl2 with Na2CO3 in supersaturated solutions between 273 and 363K was investigated.The compositions,morphologies and filtration characteristics of the precipitates were studied in detail.The magnesium carbonate hydrates obtained at 313K and in the range of 343-363K showed good morphologies and filtration characteristics.Magnesium oxides(MgO)with high purity(97.6%-99.4%)were obtained by calcining magnesium carbonate hydrates at 1073K.

Structure, Formation, Properties, and Application of Calcium and Magnesium Silicate Hydrates System—A Review

In order to expand the advantages of strong durability and high compressive strength of calcium silicate hydrates(C-S-H),at the same time to make up for the poor early mechanical strength of magnesium silicate hydrates (M-S-H),we present the features and advantages of C-S-H and M-S-H and a comprehensive review of the progress on CaO-MgO-SiO_(2)-H_(2)O.Moreover,we systematically describe natural calcium and magnesium silicate minerals and thermodynamic properties of CaO-MgO-SiO_(2)-H_(2)O.The effect of magnesium on C-S-H and calcium on M-S-H is summarized deeply;the formation and structural feature of CaO-MgO-SiO_(2)-H_(2)O is also explained in detail.Finally,the development of calcium and magnesium silicate hydrates in the future is pointed out,and the further research is discussed and estimated.

Effect of Borax on Hydration and Hardening Properties of Magnesium and Pottassium Phosphate Cement Pastes

Magnesium and potassium phosphate cement （MKPC） sample were prepared by mixing dead burnt MgO powder, potassium phosphate and different dosages of retarder borax to investigate the effect of borax on its hydration and hardening characteristics. The pH value, fluidity, hydration temperature and strength development of MKPC paste were investigated, and the mineralogical composition and microstructural morphology of its hydration products were analyzed. The experimental results indicated that, within a certain dosage, borax caused an endothermal effect for MKPC paste, which decreased the early hydration rate of MKPC paste, increased the fluidity of MKPC paste. Thus, strength and micro-morphology of hardened MKPC are affected. It can be concluded that borax in MKPC paste retards the early hydration rate of MKPC paste by forming a film onto surface of MgO, decreasing the temperature and increasing the pH value of the system. As borax dosage varying, different factors may dorminate the effects.

Influence of MgO/MgCl_2 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.

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）z.MgC12.SH20 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.

Effect of Magnesium on the C-S-H Nanostructure Evolution and Aluminate Phases Transition in Cement-Slag Blend

The microstructural study was conducted on cement and cement-slag pastes immersed in different concentrations of Mg（NO3）2 solutions utilizing ^29Si, ^27Al NMR spectroscopy and XRD techniques. The results show that the hydration of both the cement and cement-slag pastes is delayed when the pastes are cured in Mg（NO3）2 solutions as compared to the pastes cured in water. Moreover, Mg^2＋ ions also exhibit an decalcifying and dealuminizing effect on the C-A-S-H in cement and cement-slag pastes, and thereby decrease Ca/Si and Al[4]/Si ratios of the C-A-S-H. The dealuminization of C-A-S-H is mitigated for cement-slag paste as compared to pure cement paste. The depolymerized calcium and aluminum ions from C-A-S-H gel mainly enter the pore solution to maintain the pH value and form Al^[6] in TAH, respectively. On the other hand, Mg^2＋ ions exert an impact on the intra-transition between Al^[6] species, from AFm and hydrogarnet to hydrotalcite-like phase. NO3^-ions are interstratified in the layered Mg-Al structure and formed nitrated hydrotalcite-like phase（Mg1-xAlx（OH）2（NO3）x·nH2O）. Results from both ^27Al NMR and XRD data show that ettringite seems not to react with Mg^2＋ ions.

Effects of H_3PO_4 and Ca(H_2PO_4)_2 on mechanical properties and water resistance of thermally decomposed magnesium oxychloride cement

The effects of H3PO4 and Ca（H2PO4）2 on compressive strength, water resistance, hydration process of thermally decomposed magnesium oxychloride cement （TDMOC） pastes were studied. The mineral composition, hydration products and hydration heat release were analyzed by XRD, FT-IR, SEM and TAM air isothermal calorimeter, etc. After being modified by H3PO4 and Ca（HzPO4）2, the properties of the TDMOC are improved obviously. The compressive strength increases from 14.8 MPa to 48.1 MPa and 37.1 MPa, respectively. The strength retention coefficient （Kn） increases from 0.38 to 0.99 and 0.94, respectively. The 24 h hydration heat release decreases by 10% and 4% and the time of hydration peak appearing is delayed from 1 h to about 10 h. The XRD, FT-IR and SEM results show that the main composition is 5Mg（OH）z＇MgCIz＇8H20 in the modified TDMOC pastes. The possible mechanism for the strength enhancement was discussed. The purposes are to extend the potential applications of the salt lake magnesium resources and to improve the mechanical properties of TDMOC.

Crystallization Characteristic of Periclase in Clinker and Effect of Mg2+ on Hydrate of Cement Pastes

Crystallization characteristic of periclase in clinker and effect of Mg^2+ on hydrate of cement pastes were investigated. Morphologies and relative content of periclase were characterized with scanning electron microscopy and X-ray diffraction. Derivative thermogravimetry analysis and backscattered electron imaging were used to characterize the effect of Mg^2+ on hydrate of cement pastes. The experimental results show that in ample space, periclase forms octahedron structure, and subhedral or anhedral crystal is formed in limited space. Due to the accelerated burning temperature and prolonged holding time, coarse pericalase crystals are formed. Mg(OH)2 particle thickness increases due to faster crystal growth rate along c axis at later age. Mg^2+can substitute Ca^2+ in C-S-H or C-A-H to form magnesium silicate hydrate(M-S-H) or magnesium aluminate hydrate(M-A-H), and the substitution extent for C-A-H is higher than that for C-S-H. Cured in 80 ℃ water, the decalcification rate of C-A-H in pastes is higher than that cured in 50 ℃ water. M-A-H with an atomic Mg/Al ratio of 2 is formed through substitution of Ca by Mg in C-A-H.

Prepartion of Flake-Like Basic Magnesium Carbonate from the Ammonium Sulfte Leaching Liquid of Asbestos Tailing

The flake-like basic magnesium carbonate was prepared from the ammonium sulfate leaching liquid of asbestos tailings. The method of X-ray diffraction, Fourier transform infrared spectra, TA-DTG and SEM analysis had been used to research the influence of the reaction temperature on the phase, structure and morphology of the product during the preparation of the basic magnesium carbonate. The results indicates that the product was needle-like magnesium carbonate hydrate at the lower reaction temperature (from 35 ℃ to 65℃), with the increase of the reaction temperature (75 ℃ to 95 ℃ ), the needle-like magnesium carbonate hydrate (MgCO3·xH2O) transformed to flake-like basic magnesium carbonate [Mg5(CO3)4(OH)2·4H2O], the reaction temperature plays an important role in the preparation of flake-like basic magnesium carbonate, the preparation of flake-like basic magnesium carbonate was a nucleation-dissolution-recrystallization growth mechanism process.

Gas chromatographic thermodynamics on hydration processes of magnesium chloride with low water

The dehydration and hydration processes of magnesium chloride hydrates were studied by means of frontal chromatography analysis, calorimetry, thermogravimetry and chemical analysis. The mathematical imitation for the adsorption isotherms of MgCl 2·4H 2O and MgCl 2·2H 2O at different temperatures indicates that Boltzmann Function is the ideal equation to describe those adsorption isotherms. Its adsorption heat is -13.06 kJ/mol and -16.11 kJ/mol, respectively. The adsorption equilibrium constants are also given. From the data obtained, there is a thermodynamical possibility to use partial dehydrated magnesium chloride hydrates as an absorbance to clean water vapor contained in bischophite dehydration equipment and let the protection gas HCl recycle in the fluid bed reaction system.

磷镁物质的量比对天然水镁石制备的磷酸镁水泥性能的影响

本工作采用无需煅烧的天然水镁石作为镁质原料制备磷酸镁水泥(MPC),主要研究了磷镁物质的量比(P/M)对MPC工作性能、力学性能、物相组成及微观形貌的影响规律。结果表明:随P/M减小,MPC浆体流动度逐渐减小,凝结时间逐渐延长,抗压强度先提高后降低。P/M为1/4时,MPC综合性能较优,此时流动度为110 mm,凝结时间为7 min,1 d和28 d抗压强度分别为20.0 MPa和43.7 MPa。微观试验结果表明,MPC水化产物为MgKPO_(4)·6H_(2)O、Ca_(10)(OH)_(2)(PO_(4))_(2)。MPC水化28 d后,水镁石未完全反应。随着P/M减小,硬化体结构密实度增加,总孔隙率减小。

粉煤灰对轻烧氧化镁制备磷酸镁水泥性能影响的研究

文中分析了粉煤灰(FA)对轻烧氧化镁制备磷酸镁水泥(MPC)性能的影响。随着FA替代率的增加,MPC的凝结时间逐渐降低,流动度逐渐提高,抗压强度先升高后降低。当FA替代率为10%时,MPC的各项性能较佳,凝结时间为42.0 min,流动度为200 mm,28 d抗压强度为82.6 MPa。

泵送碱式硫酸镁水泥混凝土的制备和性能

碱式硫酸镁水泥混凝土是新开发出来不久的混凝土材料,具有高抗拉强度、抗冲击和耐腐蚀等优点,但由于其较小的塌落度,限制了其工程应用。本文试验研究了不同掺量萘系减水剂对泵送碱式硫酸镁水泥混凝土塌落度和强度的影响,确定强度等级C30、C40、C50和C60的萘系高效减水剂适宜掺量分别为2.0%、1.2%、1.5%和1.8%。分别研究了掺粉煤灰型和矿渣型泵送碱式硫酸镁水泥混凝土基本力学性能及在海水浸泡、淡水浸泡1280 d后的强度保留率并进行机理分析,发现粉煤灰型泵送碱式硫酸镁水泥混凝土抗海水腐蚀性较好,适合东部沿海地区应用;随着海水暴露时间的延长,泵送碱式硫酸镁水泥混凝土的Df值呈现幂函数的降低关系。本文还对大气中暴露2400 d的钢筋泵送碱式硫酸镁水泥混凝土小梁进行观测,未有锈涨现象。本文为碱式硫酸镁水泥混凝土在工程领域的应用具有重大意义。

氯化镁熔盐水合物的电化学性质研究

熔盐水合物一般是指水盐物质的量比小于或等于阳离子水合数或配位数的无机盐熔体,对其性质进行研究有助于深入了解无机盐结晶水合物的脱水过程和脱水机理。测定了不同组成的MgCl_(2)-H_(2)O体系以液态存在的最低温度及其在不同温度下的电导率,并采用循环伏安法研究了MgCl_(2)-H_(2)O体系和MgCl_(2)-KCl-H_(2)O体系的电化学性质。结果表明:在相同温度下,MgCl_(2)-H_(2)O体系稀溶液的电导率随着含水量的降低先增大后逐渐减小,当含水量相同时,MgCl_(2)-H_(2)O体系电导率随着温度的升高逐渐增大;在浓溶液和熔盐水合物中,相同温度下电导率随着含水量的降低逐渐减小;对于MgCl_(2)-H_(2)O体系,当达到氧化或还原电位时,阳极和阴极电流均随着含水量的增加而明显增大;在MgCl_(2)熔盐水合物中加入KCl后,熔盐水合物体系中Cl-浓度增大,Cl-置换了水合镁离子中的部分水分子,使氢的还原更加容易;在MgCl_(2)·6H_(2)O脱水过程中,Cl-的加入可以抑制MgCl_(2)·6H_(2)O水解反应的发生,从而更有利于MgCl_(2)·6H_(2)O的脱水。

添加剂对六水氯化镁制备氧化镁及氧化镁水化产物的影响

为解决盐湖镁资源的大规模利用问题,以盐湖六水氯化镁为原料,分别添加硫酸镁、氯化钠和氨水,考察不同添加剂与用量对煅烧制备MgO的影响,并进行水化反应得到不同形貌的氢氧化镁。利用XRD、SEM、激光粒度分析检测方法对MgO及其水化产物进行表征。结果表明:引入SO_(4)^(2-)使六水氯化镁溶液的pH降低,不利于MgO的生长,融合为大颗粒形貌。Na^(+)与SO_(4)^(2-)相比,对六水氯化镁制备MgO的破坏程度强。Na^(+)使MgO的晶粒尺寸变大,特征粒径达D50=28.69μm,畸变程度变差,制备的MgO水化后仍存在未完全水化的MgO,衍射峰I001/I101的值为0.723,(101)生长面显露得更多,导致六方片层变厚,产物相互粘结,片层堆积。引入氨水后,当其添加量(物质的量之比)为0.2时,可制备片状形貌的MgO,特征粒径D50仅为1.78μm。MgO水化时,产物全部转化为Mg(OH)2,I001/I101的值为0.777,结晶完整,呈六方片穿插的花簇状,为其在吸附净化等领域提供了潜在的应用前景。

Mg(OH)_(2)对磷酸镁水泥水化过程及性能的影响

磷酸镁水泥(MPC)的镁质原料主要为重烧MgO,由于制备重烧MgO需要高温煅烧,其使用成本和碳排放量过高。氢氧化镁(Mg(OH)_(2))的来源丰富,制取成本低廉且无污染、安全环保,如能大量采用对缓解使用成本和碳排放量过高这一问题具有重要意义。因此,本工作使用Mg(OH)_(2)代替部分重烧MgO,通过X射线衍射、热分析、扫描电镜和能谱分析等测试手段,研究其对MPC工作性能、水化过程、强度发展和微观结构的影响。结果表明,Mg(OH)_(2)的掺入加快了MPC的水化过程,浆体的流动度降低,凝结时间缩短,本工作通过水玻璃热处理改性等缓凝手段改善其工作性能。随着Mg(OH)_(2)的掺入,磷酸镁水泥的水化温度和pH值显著降低,水化28 d后的K-鸟粪石(K-struvite)结晶数量增多,抗压强度随Mg(OH)_(2)含量的增加而降低,当Mg(OH)_(2)掺量在50%以内时,其后期强度下降在15%的范围内,随Mg(OH)_(2)掺量增大,更多的水化产物是由Mg(OH)_(2)反应形成的,当Mg(OH)_(2)掺量为66.6%时,重烧MgO的反应率由平均值22%降低至7%。总而言之,采用大掺量Mg(OH)_(2)制备磷酸镁水泥具有可行性且能确保材料的后期力学性能满足工程应用的要求,对降低磷酸镁水泥使用成本和碳排放具有潜在的应用前景。

水泥结合骨料-基质协同低导莫来石浇注料性能研究

针对高温工业炉的节能降耗和安全长寿服役要求,进一步降低炉衬耐火材料的导热系数和保障服役性能是当前的研究热点。本文选择水泥结合莫来石质浇注料为研究对象,提出利用高比表面积水合硅酸镁(M-S-H)调控铝酸钙水泥的水化行为,提高浇注料早期结合强度的思路,通过采用轻质莫来石化骨料和在基质中引入引气剂的方式协同降低导热系数。研究发现,高比表面积M-S-H有利于促进水泥水化形成针柱状十水铝钙(CAH 10),大幅提高浇注料结合强度。在轻量化莫来石骨料制备浇注料中引入M-S-H和0.01%(质量分数)引气剂时,1250℃热处理后材料中1~100μm的孔隙显著增加,1000℃时导热系数仅为0.599 W·(m·K)^(-1),较致密浇注料降低18.9%。

镁质胶凝材料综述:研究进展与低碳路径探讨

镁质胶凝材料具有轻质、高强、低碱、耐高温和低二氧化碳排放的特点,已被广泛应用于预制建筑、道路修复和核废料固定等领域。本文系统综述了镁质胶凝材料近年来的主要研究成果;重点介绍了氯氧镁水泥、硫氧镁水泥、水化硅酸镁水泥和磷酸镁水泥的原材料组成、水化机理、相转变和微观结构等,总结了其在碳固存、力学和耐久性等方面的研究进展,对比了上述四种水泥的性能及应用现状。最后,结合镁质胶凝材料的优势以及不足,提出了未来研究的方向和思路。我国是世界上镁资源最丰富的国家,镁质胶凝材料具有独特的性能,作为菱镁产业的重要一环,必将在低品位菱镁尾矿建材资源化利用方面大有可为。

菱镁矿尾矿中钙质量分数对硫氧镁水泥胶凝性能的影响

为探究尾矿中杂质因子(钙)在硫氧镁水泥体系中的影响,以菱镁矿尾矿为原料制备硫氧镁水泥并将其作为研究对象。首先,以菱镁矿尾矿为基础,向其中掺加白云石,改变尾矿中的钙质量分数;然后,对硫氧镁水泥进行凝结时间测定和单轴抗压强度试验;最后,采用热重及差热、X射线衍射、扫描电镜等分析方法研究钙质量分数对硫氧镁水泥水化产物、力学性能、凝结时间以及微观形貌的影响。研究结果表明:硫氧镁水泥主要由未完全反应的氧化镁(MgO)和水化产物3Mg(OH)_(2)·MgSO_(4)·8H_(2)O(318相)、氢氧化镁(Mg(OH)_(2))、二水硫酸钙(CaSO_(4)·2H_(2)O)以及碳酸镁(MgCO_(3))组成;菱镁矿尾矿中钙质量分数增加,硫氧镁水泥水化产物中318相晶体逐渐减少,Mg(OH)_(2)和CaSO_(4)·2H_(2)O晶体逐渐增加,致使硫氧镁水泥试块的力学强度逐渐降低;菱镁矿尾矿中钙质量分数增加将导致煅烧产物中活性MgO的质量分数及活性度降低,游离氧化钙的质量分数升高;钙质量分数增加使硫氧镁水泥的水化反应速率明显升高,凝结时间大幅缩短,同时导致硫氧镁水泥水化反应不能完全进行,水化产物生成量减少。

甘蔗渣灰对磷酸钾镁水泥性能与水化的影响

甘蔗渣灰(SCBA)是蔗糖生产过程中产生的废弃物。由于SCBA产量丰富且具有火山灰活性,已被逐渐应用于建筑材料。本工作研究了不同掺量及粒径的SCBA对磷酸钾镁水泥(MKPC)凝结时间、流动度、抗压强度、体积稳定性、质量损失、水化进程、水化产物和微观结构的影响。结果表明,由于SCBA的稀释效应延缓了MKPC的pH值上升,从而延缓了MKPC的早期水化,延长了其凝结时间,降低了其早期(24 h之前)强度。但随着中后期pH值的上升,SCBA发挥填充作用和成核效应,MKPC的水化产物生成量增多且SCBA中的元素会渗入K-鸟粪石晶体中或形成一些新的无定形磷酸铝、硅酸钾和钙质类的胶凝水化产物,致使结构更致密,从而提升了试件中长期的强度。MKPC的早期水化速率随着SCBA掺量的增加逐渐降低,早期水化产物K-鸟粪石的生成量减少,导致早期收缩减小;中后期50~100 nm与小于50 nm的连通孔的比例增加,加快了自由水的蒸发和迁移,使得质量损失和干燥收缩增大。小粒径SCBA的掺入量为20%时MKPC的综合性能较好。本研究为SCBA作为MKPC替代胶凝材料提供了理论基础,并进一步促进了农副产品在新型胶凝材料中的应用。