用IR方法研究硅酸钙水化产物的碳化

本文用红外光谱分析研究了水化硅酸钙碳化产物的性质,发现水化βC_2S 的早期碳化产物为方解石和文石,但在后期,文石又转变成方解石,水化 C_3S 的碳化产物不管是早期还是后期均生成方解石,而无其它晶型的碳酸钙生成。水化硅酸钙碳化时生成硅胶。硅酸钙的水化产物 Ca(OH)_2和 C—S—H 凝胶这两者的碳化性能总的来说区别不大,但在碳化初期,Ca(OH)_2的碳化稍优先于 C—S—H 凝胶。     

碳化法制备轻质碳酸钙的粒度分布研究

运用正交实验法对碳化法制备轻质碳酸钙实验中碳化温度、碳化气体流量、Ca(OH)_2浓度、外加剂掺量对轻质碳酸钙粒度分布的影响进行研究。实验结果表明Ca(OH)_2浓度是影响轻质碳酸钙平均粒径的主要因素,碳化反应温度是影响颗粒均匀性系数的主要因素。在碳化温度50℃,碳化气体流量10 L·min~(-1),Ca(OH)_2浓度为20 g·L~(-1),外加剂柠檬酸钠添加量为0.1%的情况下,制得的轻质碳酸钙钙平均粒径为4.623μm,均匀性系数为8.297。     

机械化学预处理污泥在不同温度下催化热解特性

为更好地实现污泥资源化以及无害化处理,得到较高品质燃气,采用机械化学预处理方式将CaO作为催化剂温和注入污泥样品,探究在催化热解中反应温度对产物的影响及机理分析。结果表明,在机械化学作用下,混合料中的CaO与羟基反应生成Ca(OH)_2,有助于热解气产率的提高。混合物料中有机成分随着温度升高发生分解,■、C—O和C—H官能团减弱,O—H官能团消失,热解气产量增高,900℃时达到最大,为32.10%,其中可燃气体(H_2,CO,CH_4)产量上升至17.22 mol/kg,增幅明显;随着热解反应的进行,混合料中Ca(OH)_2物质吸收CO_2生成大量方解石,方解石高温分解导致热解气中的CO和CO_2气体质量升高。热解半焦表面孔隙结构特性分析发现,高温有助于混合料分解产生更多的可燃气体。     

油酸在CaCl2和Ca(OH)2反应体系中对CaCO3晶型和形貌的调控

分别在CaCl2碳化体系中(以CaCl2、CO2、NH3·H2O和油酸为原料)和Ca(OH)2碳化体系中(以Ca(OH)2、CO2和油酸为原料)采用微孔分散碳化法于室温下制备了微纳米CaCO3,并采用XRD、TEM和SEM等表征手段重点研究了油酸对CaCO3晶型和形貌的影响和调控.结果表明:在不添加油酸的CaCl2反应体系中,反应初期生成方解石相,随着反应的进行,球霰石相的含量逐渐增加,而油酸的加入使得该体系整个碳化过程中产物均为球霰石相.另外,油酸的加入还可以加速反应过程,使得球形颗粒的形成时间缩短.而Ca(OH)2反应体系中,整个碳化过程中产物均为方解石相,油酸的加入对晶体类型没有明显影响.     

加速碳化对水化硅酸钙显微结构的影响

废弃水泥石等固体废弃物碳酸化不仅能够永久固碳,还可实现固体废弃物的再利用,减少对环境的污染。水化硅酸钙(C-S-H)是最主要的可碳化成分之一。合成了钙硅(C/S)比为1.50的C-S-H,研究了加速碳化对其显微结构的影响。用Rietveld全谱拟合的方法和热重-质谱联用的方法对碳化产物进行定量分析,用扫描电镜、N2吸附和29Si固体核磁共振对碳化前后的显微结构进行表征。结果表明:在99.9%CO2,0.2 MPa压力下加速碳化2 h之后,生成了3种不同晶型的碳酸钙和硅胶,碳酸钙从300℃开始分解,文石和球霰石具有较低的分解温度,结晶良好的方解石分解温度较高;多孔结构硅胶具有更高的吸附能力,但C-S-H碳化后的平均孔径从10.33 nm减小到6.69 nm,比表面积由85.6 m2/g减小到67.7 m2/g,这是由于大量的结构致密的碳酸钙晶体堆积造成的;C-S-H双层硅氧链之间的Ca–O层逐渐脱去与CO2反应,硅氧四面体被质子化或与邻近的硅氧四面体链接,形成了聚合度更高的Q3和Q4结构。     

碱式碳酸钙的制备

碱式碳酸钙Ca_3(OH)_2(CO_3)_2(1)或Ca_6(OH)_2(CO_3)_5(11)具有一定的结晶结构,是由Ca(OH)悬浮液与CO_2(预定温度2—50℃)下碳化,最终碳化Ca/碳酸盐mol比3:2或6:5,上还(1)、(11)型产物用于造纸、分散染料、及用作塑料、橡     

钙硅比对水化硅酸钙加速碳化的影响

废弃水泥石、钢渣等碳酸化固定CO2不仅可以缓解温室效应还可以实现废弃物的再利用,同时制备出性能优良的建材制品。为了研究废弃水泥石矿物组成的碳酸化机理,探讨了钙硅比对水化硅酸钙加速碳化的影响。结果表明:随着钙硅比增加,水化硅酸钙(C-S-H)碳化率逐渐降低,高钙硅比的C-S-H具有相对粗大的孔结构使得早期的碳化速率增加;碳化产物中文石、球霰石、方解石在不同钙硅比时所占比例不同,钙硅比≤0.67时文石占较大比例,钙硅比≥1.00时方解石为主要碳化产物,钙硅比=0.83时球霰石含量最大;加速碳化条件下形成的碳酸钙分解温度分成两部分,在400~620℃范围内文石和球霰石都分解,方解石在650~800℃范围内分解。     

钙基固硫剂对高硫煤固硫效果的研究

为降低高硫煤燃烧过程中SO_2等有害气体的排放,以木坦坝高硫煤为原料,分别利用CLS-2型库伦测硫仪和X射线衍射仪（XRD）分析钙剂固硫剂固硫率和煤灰中矿物组成,研究了3种钙基固硫剂（CaO、Ca(OH)_2、CaCO_3）在不同钙硫物质的量比情况下与木坦坝高硫煤混合燃烧过程中的演变行为和固硫效果。结果表明,Ca(OH)_2固硫效果优于其它两种固硫剂;800℃为Ca(OH)_2固硫作用的最佳温度,CaO固硫能力最强的温度点是900℃,CaCO_3固硫效率随温度增加而提升;钙硫物质的量比超过2.5之后固硫效果减弱,最佳钙硫物质的量比为2.5;XRD分析得出Ca(OH)_2作为固硫剂时,分解更多的CaO参与反应,故固硫效果佳。     

合成超微细CaCO3的非稳态碳化反应过程-Ⅱ.碳化速率与传质模型

在合成超微细CaCO3的非稳态体系中,通过电导率仪、pH计、SEM以及化学分析等手段,跟踪测定了添加剂Na5P3O10存在时Ca(OH)2悬浮液的碳化过程.结果表明,Ca(OH)2的碳化分为两个阶段:初期的恒速反应阶段和末期的变速反应阶段.在恒速阶段,碳化的表观反应级数为-2,碳化反应速率随Na5P3O10浓度的增加而减小.在变速阶段,碳化的表观反应级数也随Na5P3O10浓度的增加而减小.提出了在恒速反应阶段碳化过程由Ca2+和CO 反应速度控制,CO2吸收的影响可以忽略,变速反应阶段由Ca(OH)2溶解和Ca2+及CO 反应速度控制的传质模型.     

PE／Ca（OH）2／Mg（OH）2复合材料的阻燃性能初探

采用氧指数测试、水平燃烧性能测试、扫描电镜分析及热重分析等方法研究了不同配比 Ca(OH)_2/Mg(OH)_2的加入对聚乙烯(PE)复合材料[PE 用量为65%(质量分数,下同),Mg(OH)_2和 Ca(OH)_2总用量为35%]的氧指数、水平燃烧级别和燃烧后炭层表面形貌等燃烧性能的影响。结果表明,PE 复合材料氧指数达到24.5%,且其最快分解温度(T_(max))比纯 PE 滞后9.2℃,并促进致密炭层的形成。Ca(OH)_2与 Mg(OH)_2并用对 PE 阻燃性能有一定的协同增效作用,有望实现用廉价的 Ca(OH)_2对 Mg(OH)_2的替代。     

Polymorph and morphology of CaCO3 in relation to precipitation conditions in a bubbling system

Simulating the typical carbonation step in a mineral CO2 sequestration,precipitated calcium carbonate (PCC) was prepared by bubbling CO2 gas into a rich Ca solution.These carbonation reactions were conducted at three pH ranges,namely 10.0-9.0,9.0-8.0,and 8.0-7.0,in which temperature and CO2 flow rate are additional experimental variables.The PCC obtained in experiments was examined by Fourier transform infrared spectroscopy (FrIR) and X-ray diffraction (XRD).It was found that supersaturation determined by pH value and flow rate of CO2 has significant influence on polymorph of PCC.Vaterite was preferably formed at high supersaturation,while dissolution of metastable vaterite and crystallization of calcite occurred at low supersaturation.High temperature is a critical factor for the formation of aragonite.At 70 ℃,vaterite,calcite and aragonite were observed to coexist in PCC because transformation from vaterite to aragonite via calcite occurred at this temperature.Scanning electron microscopy (SEM) technology was performed on prepared PCC,and various morphologies consistent with polymorphs were observed.     

Investigation of the carbonation front shape on cementitious materials: Effects of the chemical kinetics

Carbonation depth-profiles have been determined by thermogravimetric analysis and by gammadensitometry after accelerated carbonation tests on ordinary Portland cement (OPC) pastes and concretes. These methods support the idea that carbonation does not exhibit a sharp reaction front. From analytical modelling, this feature is explained by the fact that the kinetics of the chemical reactions become the rate-controlling processes, rather than the diffusion of CO₂. Furthermore, conclusions are drawn as to the mechanism by which carbonation of Ca(OH)₂ and C-S-H takes place. Carbonation gives rise to almost complete disappearance of C-S-H gel, while Ca(OH)₂ remains in appreciable amount. This may be associated with the CaCO₃ precipitation, forming a dense coating around partially reacted Ca(OH)₂ crystals. The way in which CO₂ is fixed in carbonated samples is studied. The results indicate that CO₂ is chemically bound as CaCO₃, which precipitates in various forms, namely: stable, metastable, and amorphous. It seems that the thermal stability of the produced CaCO₃ is lower when the carbonation level is high. It is also shown that the poorly crystallized and thermally unstable forms of CaCO₃ are preferentially associated with C-S-H carbonation.     

Influence of relative humidity on the carbonation of calcium hydroxide nanoparticles and the formation of calcium carbonate polymorphs

A consolidating product based on nanoparticles of slaked lime (Ca(OH)₂) dispersed in isopropyl alcohol was exposed under different relative humidities (RH), 33%, 54%, 75% and 90% during 7, 14, 21 and 28 days. The characterization of the calcium hydroxide nanoparticles and the formed calcium carbonate polymorphs have been performed by Micro Raman spectroscopy, Transmission Electron Microscopy (TEM), Environmental Scanning Electron Microscopy (ESEM) with Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Diffraction (XRD). Precipitation and transformation of calcium carbonate polymorphs strongly depend on the relative humidity (RH). Higher RH (75%–90% RH) gives rise to amorphous calcium carbonate and monohydrocalcite, calcite, aragonite and vaterite, faster carbonation and larger particles sizes with higher crystallinity compared to lower RH (33%–54% RH) that gives rise mainly to portlandite and vaterite, slower carbonation and smaller particle sizes with lower crystallinity.     

Synthesis of single phase aragonite precipitated calcium carbonate in Ca(OH)2-Na2CO3-NaOH reaction system

The formation behavior of precipitated calcium carbonate polymorphs was investigated in three different supersaturation levels. Because the most easily adjustable and influential variable determining supersaturation is the ion concentration of the major reactants — Ca²⁺ and CO3 ₃ ²⁻ — the supersaturation can be adjusted by changing the ion concentration of these two ions. At high supersaturation, free energy is necessary for a decrease in nucleation, promoting the formation of a sphere-shaped vaterite, while aragonite and calcite were seen to co-exist at medium supersaturation. At low supersaturation, aragonite was mainly formed by mixing with some calcite. Hence, we considered that lower supersaturation was necessary to obtain a single phase aragonite. Furthermore, we found that the solubility of Ca(OH)₂ was decreased with the addition of NaOH by a common ion effect. Thus, it is possible to perform an experiment at a lower Ca²⁺ concentration. The aragonite was synthesized by adding the Na₂CO₃ solution to the Ca(OH)₂ slurry containing several concentrations of NaOH solution at 75°C and under the addition rate of Na₂CO₃ at 3 ml/min. The formation yield of calcite decreased when the NaOH concentration was increased. In conclusion, in the case of the reaction of the 2.5 M NaOH solution over 210 minutes, single-phase aragonite with an aspect ratio of 20 was obtained.     

Progressive Changes in the Structure of Hardened C3S Cement Pastes due to Carbonation

The structures of partially carbonated hardened C₃S cement pastes have been investigated by a combination of ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy and analytical transmission electron microscopy, supported by X-ray diffraction and thermogravimetric analysis. Progressive changes in structure are reported for thin slices for a paste carbonated in pure CO₂ for times from 1 to 16 h, and the results are compared with those for a paste carbonated for 2 months in air. C-S-H gel of reduced Ca:Si ratio and increased silicate polymerization was formed during the early stages of carbonation. The morphology of the original C-S-H was, in the main, retained. A cross-linked silica-rich gel formed at later times in paste carbonated in CO₂ but not up to the time of 2 months in air. Calcium carbonate took the form of microcrystals of vaterite and calcite which formed dense masses between gel fibrils and around partially reacted CH crystals, possibly accounting for the observed slowing in the rate of reaction of CH with time.     

Synthesis of Aragonite by the Carbonation Process

The conditions necessary for synthesizing aragonite by the carbonation process were investigated. Aragonite formation was found to have no relation to the pH value, the mole ratio of MgCl₂/Ca(OH)₂, or the magnesium/calcium ion concentration in a solution. The synthesis of aragonite requires a concentration of magnesium ions in the appropriate range (∼0.1–0.26 mol/L) and a concentration of calcium ions below a certain range (less than ∼0.16–0.25 mol/L). An excess of calcium ions or magnesium ions favors calcite formation. Needlelike aragonite with a large aspect ratio was successfully synthesized in the present study.     

Reaction of Hydraulic Calcium Silicates with Carbon Dioxide and Water

The carbonation of wetted powders of beta-dicalcium silicate (β·2CaO·SiO₂=β-C₂S) and tricalcium silicate (3CaO·SiO₂= C₃S) was studied as a function of reaction conditions. The water/solids ratio is an important parameter and there is an optimum value for each silicate. Relative humidity and the partial pressure of CO₂ also strongly affect the reaction. The rate of carbonation can be conveniently represented by plotting the degree of carbonation against the logarithm of time. C-S-H and calcite are the initial reaction products. Subsequently, carbonation of the C-S-H produces silica gel, whereas aragonite may form if the system is allowed to dry out.     

文石晶须制备中氯化镁的影响

采用碳酸化法制备了文石CaCO3晶须，分析了合成晶须过程中MgCl2的作用机理，运用扫描电镜和X射线衍射对产物进行了表征。结果表明：由MgC2形成的Mg(OH)2胶状沉淀作为晶种促进晶须成核，并抑制了文石晶须向方解石转变。在确定的工艺条件下，利用回收的MgCl2溶液制备了文石CaCO3晶须，降低了文石晶须的制备成本。     

Microstructure of Mature Alite Pastes

Tricalcium silicate and Fe-bearing alite pastes hydrated 4 years and portland cement paste hydrated 33 days were studied by SEM, TG, and XRD. SEM studies of the effect of water, NaCl, and MgSO₄ solutions on the microstructure of alite are reported. The characteristics of the attack were clarified by examining both sides of the fracture plane. The Ca(OH)₂ phase present in the fresh fracture surface is very unstable toward water, NaCl, and MgSO₄ solutions. When exposed to MgSO₄ solution the Ca(OH)₂ reacts, producing Mg(OH)₂ and CaSO₄·2H₂O. The C-S-H gel phase is more resistant to the various reactants. The presence of calcite, aragonite, and Ca(OH)₂ and their proportion in the surface layer depend on the characteristics of the environment.     

γ-C2S和 β-C2S的碳化与水化活性研究

在常温常压下,γ-C2 S的水化活性很低,β-C2 S的水化活性相对较高;但在一定湿度和二氧化碳浓度下,γ-C2 S与β-C2 S都能快速地发生碳化反应.作者在实验室条件下制备了纯的γ-C2 S和β-C2 S,在比表面积相近的条件下,通过XRD、SEM、TG-DSC、游离氧化钙含量和烧失量的测试与分析,对γ-C2 S与β-C2 S在不同温度下的水化活性和常温常压下的碳化反应进行了研究.结果表明:γ-C2 S和β-C2 S的碳化产物主要是方解石和少量的球霰石,γ-C2 S碳化后还有少量的文石生成,并且γ-C2 S的碳化活性略高;在140℃时,γ-C2 S的水化活性较β-C2 S低很多,当温度升高到200℃时,γ-C2 S的水化活性大幅提升,β-C2 S的水化活性也有一定提升,两者的水化活性差距显著缩小.