Pore Structure of Calcium Silicate Hydrate in Hydrated Tricalcium Silicate

Adsorption of N₂ and water vapor was studied in completely hydrated tricalcium silicate and in fully hydrated tricalcium silicate from which Ca(OH)₂ had been extracted. Compared with results obtained using N₂, water vapor adsorption led to increased values for small-pore volume, peak shifts to smaller sizes, and decreased values for large-pore volume. Marked hysteresis was observed in the case of water vapor adsorption; the resorption branch apparently represents the true pore structure. Extraction of Ca(OH)₂ from the paste increased the calculated volume of small pores strikingly, suggesting that adsorption is hindered by Ca(OH)₂; this tendency is more obvious in water vapor adsorption. The adsorption measurements indicate the existence of two kinds of pores, i.e. a wider intergel-particle pore and a smaller pore existing within the gel particle. The latter pore was further classified into intercrystallite and intracrystallite pores.     

Proposed Structure for Calcium Silicate Hydrate Gel

Calcium silicate hydrate gel is a very poorly crystalline material that is the main product of reaction of portland cement with water. Evidence is presented for a disordered layer structure, in which most of the layers are structurally imperfect ones of jennite (Ca₉Si₆O₃₂H₂₂), and others are similarly related to 1.4-nm tobermorite (Ca₅Si₆O₂₆H₁₈), both structures being modified by omission of many of their silicate tetrahedra. The evidence comprises conditions of formation, silicate anion type, Ca/Si ratio, H₂O/Ca ratios and densities for various drying conditions, thermogravimetric curve, X-ray powder and selected-area electron diffraction patterns, and results of analytical electron microfcopy.     

Developments in TEM Nanotomography of Calcium Silicate Hydrate

This investigation was designed to explore the possibility of using transmission electron microscope (TEM) tomography on cement‐based systems gain a greater understanding of their nanostructure and pore network. The preliminary results show a clearly a well‐defined pore network at the nanoscale, with pore size approximately 1.7–2.4 nm in diameter and spaced around 5–8 nm apart. A comparison of small angle X‐ray scattering data with 2‐D TEM images analyzed with the Fourier slice theorem documents an excellent structural correlation.     

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。     

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。     

Synthesis of Calcium Silicate Hydrate in Highly Alkaline System

Synthesis of calcium silicate hydrate (C‐S‐H) was conducted over the range of 50°C–90°C and C/S ratio of 0.86–2.14 in the highly alkaline Na₂O–CaO–SiO₂–H₂O system for silicon utilization in high alumina fly ash. Structural change in C‐S‐H formed in the highly alkaline system was investigated using XRD and ²⁹Si MAS NMR spectra. X‐ray photoelectron spectroscopy was used to confirm the amount of sodium ions in C‐S‐H. Conversion of Si may reach 99% under optimum conditions. A higher degree of polymerization of silicate was obtained at lower temperature and C/S ratio. Na⁺ was confirmed to exist as Na–OSi and Na–OH. The amount of Na⁺ is the least at C/S ratio of 1.43, which conform to the prediction of topological constraint theory. High Ca/Si ratio leads to the increasing in Na⁺ combined in the interlayer. Increasing in the Na⁺ concentration in the system also increases the amount of Na⁺ combined in the interlayer and reduces the polymerization. Ion exchange was proven to be an effective way to remove Na⁺ combined in the interlayer of C‐S‐H.     

Structures and Phase Relations of Aluminum-Substituted Calcium Silicate Hydrate

The structures of aluminum-substituted calcium silicate hydrate (C-S-H) forming in a series of aqueous suspensions formulated with colloidal silica, reactive alumina, and lime and aged for 1 year have been studied using ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy (with and without cross polarization (CP)), solution pH, electron microscopy, and X–ray diffraction. As in earlier work dealing with the nature of C-S-H in the system CaO-SiO₂H₂O₂ two aluminum-substituted C-S-H phases, having distinctly different anionic structures on the atomic level (Q₂ versus Q₁Q₂), were found to extend into the system CaO-Al₂O₃-SiO₂-H₂O₂ X-ray diffraction patterns of the two aluminum-containing C-S-H phases are nearly identical, suggesting that their intermediate-range order is very similar, but MASNMR spectra show that these two phases have markedly different silicate structures on the atomic-level scale. Both C-S-H structures can accommodate approximately 5 mol% of Al₂O₃ in tetrahedral and possibly octahedral coordination as well.     

Nanostructure of Calcium Silicate Hydrate Gels in Cement Paste

High-resolution electron microscopy study of calcium silicate hydrate (C-S-H) gels in ordinary portland cement (OPC) and a slag/OPC blend has been performed. Nanocrystalline regions on the scale of ∼5 nm or less in C-S-H are found in both cement pastes, and they are formed after a curing time as brief as 7 d. A change in the d -spacing of the nanocrystalline regions with time is observed for the first time, which is believed to correspond to the development of C-S-H with time. The nanoheterogeneous nature of C-S-H is demonstrated and correlated to the strong Ca:Si ratio fluctuations that are observed.     

Aqueous Solubility Relationships for Two Types of Calcium Silicate Hydrate

There are many published values for the concentrations of lime and silica in the aqueous phase which is in contact with calcium silicate hydrate. These have been collected, carefully analyzed, and found often to be apparently inconsistent. When, however, they are interpreted as having come from an aqueous phase which is in near equilibrium with one or another of two possible modifications of calcium silicate hydrate, almost all the data are rationalized. Some important insights emerge for the understanding of the complex processes which occur during the hydration of tricalcium silicate.     

Morphological Forms of Tobermorite in Hydrothermally Treated Calcium Silicate Hydrate Gels

The morphology of the calcium silicate hydrate phase, tobermorite, generally thought to be the bonding phase in ordinary portland cement, was studied. Tobermorite was hydrothermally synthesized at pH 12.6 at 150°C. The powder was characterized by XRD and SEM analysis. Modeling of the morphology was performed using the SHAPE and ATOMS programs. Two morphologies were produced: fibers and platelets. Computer calculations of equilibrium particle shape suggest that growth occurs on the (110)-type planes for both morphologies.     

Structure Control of Calcium Silicate Hydrate Gels for Dye Removal Applications

The structure of calcium silicate hydrate (C‐S‐H) gels was modified by hydrothermal reaction with aqueous acetic acid solvent, and then the C‐S‐H gels were used for dye removal from aqueous solution. With increasing acetic acid concentration, the Ca:Si molar ratio decreased and the length of the silicate anion chain structure of the C‐S‐H gels increased. The silicate anion chain length affects the number of available silanol groups on the surface of the C‐S‐H gel: the longer the silicate anion chain length, the greater the number of negative charges and the higher the surface potential. C‐S‐H gels with a long silicate anion structure exhibited higher adsorption capacity for methylene blue than gels with a short silicate anion structure. The enhanced adsorption capacity of the C‐S‐H gels is related to the higher number of silanol groups in the bridging silica tetrahedra of the intermediate anion chain structure compared with those in the end units of silica tetrahedra.     

湿度对水化硅酸钙力学性能影响的分子动力学模拟

水化硅酸钙(C-S-H)是水泥水化产物中最重要的组成成分,是水泥基材料的主要胶凝相。C-S-H层间水对其纳米结构和力学性能会产生显著影响。利用分子动力学研究了不同湿度C-S-H在结构和力学性能方面的差异。通过原子径向分布函数和浓度分布、弹性常数以及应力应变关系分析了湿度对C-S-H结构和弹性性质以及拉伸、压缩、剪切力学性能和变形性能的影响。结果表明:湿度增加会导致C-S-H中Si、Ca原子近程范围内的O原子集聚增多,还会导致C-S-H层间距离增大,分层更加明显,同时会降低C-S-H的弹性性质;湿度的增加会降低C-S-H拉伸、压缩、剪切力学性能和变形性能;湿度对抗拉与抗剪强度影响较大,对抗压强度影响较小,对拉伸时的变形性能影响最大,对压缩时的变形性能影响最小。     

玻璃粉的火山灰反应及对水化硅酸钙组成的影响

在扫描电子显微镜的背散射模式下采用能谱仪研究了两种细度钠钙玻璃粉的火山灰反应特征及其对水化硅酸钙组成的影响。结果表明:火山灰反应主要在玻璃颗粒的初始边界内进行,并形成一个明显的反应环;玻璃火山灰反应释放大部分的Na,并消耗Ca(OH)2、降低内部和外部水化产物硅酸钙的钙硅比(摩尔比);玻璃所释放出的Na+导致孔溶液碱度增加,促进单硫型钙钒石(AFm)的溶解,使更多的Al进入水化硅酸钙中,提高了水化硅酸钙的铝硅比;钙硅比的降低,增强了水化硅酸钙对Na+的固定能力,使内部水化产物和外部水化产物的钠硅比显著提高。     

聚乙烯醇对溶液沉淀法制备的水化硅酸钙纳米结构的影响

通过溶液沉淀法制备了初始钙硅比为1.0～1.7的掺杂PVA的水化硅酸钙(C-S-H-PVA)系列样品,研究了C-S-H-PVA的纳米结构特征.结果表明:PVA的加入增加了C-S-H的层间距,影响了C-S-H的层间结构;C-S-H-PVA样品的Fourier变换红外光谱中出现了CH2/CH伸缩振动峰,29Si核磁共振(NMR)谱中出现了1个Q1峰和2个Q2峰,而且PVA的加入使Q1、Q2峰向更负方向偏移,使Q2/Q1的比值以及硅酸盐链的平均链长(MCL)增长,说明PVA能增加C-s-H结构中的硅氧四面体聚合度.随着钙硅比的增加,C-S-H-PVA样品的29Si NMR谱中Q1、Q2峰的化学位移向正方向偏移,Q2/Q1的比值以及MCL减小,这一变化规律与未掺PVA的C-S-H样品的变化规律类似.     

水热合成制备水化硅酸钙-聚氨酯纳米复合材料的结构分析

本文以生石灰、纳米二氧化硅和聚氨酯为原料,采用水热合成法制备了纯水化硅酸钙和水化硅酸钙-聚氨酯纳米复合材料,并利用扫描电子显微镜、能谱仪、X射线衍射仪、红外光谱仪和热分析仪对其进行表征测试。结果表明:水化硅酸钙和水化硅酸钙-聚氨酯复合材料的微观组织形貌具有明显差异,随着钙硅摩尔比的增大,出现了Tobermorite相,且随着聚氨酯的加入,该相的结晶度增强;聚氨酯可以嵌入水化硅酸钙层间,使其基底层间间距增大1.038 nm;聚氨酯的加入可以提高水化硅酸钙的热稳定性,且对钙硅比较高的水化硅酸钙改性更有效。     

纳米水化硅酸钙的制备及对水泥水化影响的研究进展

随着纳米技术的不断发展,纳米材料逐步开始应用于传统混凝土材料中,以提高混凝土的各项服役性能。纳米水化硅酸钙(纳米C-S-H)是一种新型的早强纳米复合材料,可通过晶核效应加快水泥早期水化速率,显著提高水泥基材料的早期力学性能,从而提高施工效率,满足特殊施工要求。本文系统总结了纳米C-S-H的制备方法,及纳米C-S-H对水泥基材料早期和长期性能的影响规律,探讨了其对于水泥水化过程和水化产物的影响机制,其中重点介绍了采用聚合物分散纳米颗粒制备的C-S-H/PCE(聚羧酸型减水剂,简称PCE)纳米复合材料。     

Investigations of Calcium Silicate Hydrate Structure Using Silicon-29 Nuclear Magnetic Resonance Spectroscopy

Silicon-29 nuclear magnetic resonance has been used to probe the effects of temperature and the addition of reactive silica on the silicate structure of amorphous calcium silicate hydrate (C-S-H), produced by the hydration of alite (Ca₃SiO₅). Both an increase in the temperature of formation and the addition of reactive silica increase the degree of silicate polymerization in the C-S-H. However, in all cases the data are consistent with the formation of linear chain silicates and the absence of any cross-linking. The initial formation of a hydrated orthosilicate is observed, and it is suggested that this species may form during the adsorption of water onto the surface of alite during storage.     

水化硅酸钙的晶型调控及其对吸/放湿性能的影响

为改善水化硅酸钙的孔隙结构、改善其湿度调节性能,以硅灰、氢氧化钙和水为原料,采用水热反应方法合成出预期类型的水化硅酸钙晶体,并对反应产物的微观结构和吸/放湿性能进行细致表征.研究结果表明,200℃合成样品的吸/放湿率随反应时间的延长呈现先升后降的趋势,在4 h时达到最大值,原因在于CSH(B)型水化硅酸钙的形成及其向托贝莫来石的晶型转化;90℃下CSH(B)晶型在较长时间内保持稳定,样品吸/放湿率受水热时间的影响不明显.微观结构对比分析认为,CSH(B)与托贝莫来石均以片状晶体形式存在,而托贝莫来石晶体更为完整、粗大;细小晶体所构成的堆聚结构中存在较大量的介孔,最可几孔径峰位于16 nm附近,但其含量随CSH(B)向托贝莫来石的晶型转变过程而明显减小,表现为CSH(B)型水化硅酸钙的吸/放湿能力明显高于托贝莫来石.研究成果对于水化硅酸钙基建筑功能材料的研发具有一定理论指导意义和工艺价值.     

Synthesis of Calcium Silicate Hydrate with Ca/Si = 2 by Mechanochemical Treatment

An amorphous, C-S-H-like phase with Ca/Si = 2 was synthesized from amorphous precipitated silica, calcium oxide, and water by mechanochemical treatment using a vibration mill at room temperature. The product was studied by XRD, ²⁹Si MASNMR, TEM, analytical TEM, and TGA-DTA. After 14 h of treatment, the starting materials react to form an amorphous phase (C-S-H-like phase). The XRD pattern of C-S-H-like phase resembles the C-S-H formed hydrothermally or by cement hydration except it has no reflection at 0.182 nm. The ²⁹Si NMR results revealed a silicate anion structure of C-S-H-like phase consisting mainly of a mixture of a monomer and a dimer. On heating, the C-S-H-like phase decomposed into β-dicalcium silicate below 1000°C.