粉煤灰水泥基材料的水化产物

用热重仪-差示扫描量热仪、扫描电子显微镜、透射电子显微镜、X射线能量散射、高分辩电子显微镜、压汞仪,测定了粉煤灰水泥基材料水化产物的形貌特征、微观结构、化学组成及水泥石的孔结构,讨论了水化产物性质及水泥石孔结构随粉煤灰掺量的变化规律。结果表明：粉煤灰的大量掺加,可以改善凝胶的化学组成。水化后期,粉煤灰与Ca(OH)2及由熟料水化生成的高n(Ca)／n(Si)的水化硅酸钙(C—S—H)凝胶发生二次水化反应,生成低n(Ca)／n(Si)的C—S—H凝胶,此种凝胶的固碱能力强,可减少碱-集料反应的危害性。同时,二次水化产物能够填充那些对水泥石强度和耐久性极为不利的孔隙空洞,使水泥石的结构更加致密,优化水泥石的孔结构,对提高水泥基材料的耐久性作用极大,为进一步提高工业废渣利用的技术水平奠定了基础。     

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.     

基于分子动力学水分和离子在水化硅酸钙纳米孔道中的传输特性研究

水化硅酸钙凝胶是水泥水化产物中最基本的粘结相,水分子和离子在凝胶孔中的传输从根本上决定着水泥混凝土材料的服役寿命.采用分子动力学方法系统地研究了水分子、氯离子和钠离子在1nm、2nm、3nm和4nm的水化硅酸钙凝胶孔中的传输过程.基于径向分布函数和均方位移的离子轨迹分析发现在纳米孔道中离子和水分子展现出异于毛细水的分子结构和动力学特性:水分子有序性排布、离子大量在界面吸附和扩散速度急剧下降.这种分子结构与动力学的特性是因为水化硅酸钙界面处硅链中的非桥接氧会与水分子形成稳定的氢键连接,而钠离子可以形成Na-O化学键,同时表面的钙离子也可以与氯离子形成CaCl2团簇体.此外,随着孔径的增大,离子和水分子的扩散系数逐渐由0.15×10-9m2/s、0.7×10-9m2/s增大到1.3×10-9m2/s、3×10-9m2/s,这很接近于实验测得的毛细水的扩散系数,说明在纳米尺度上,孔径的约束和界面化学键作用是决定离子和水分子传输的关键因素.     

软X射线显微术及其在水泥水化研究中的应用

与其它显微术相比,软X射线显微术具有许多独特优势。本文通过对软X射线显微术成像原理和特点的介绍,说明了其除了在生物学领域具有独特的优势外,在其它领域也有广泛用途。在此主要介绍软X射线显微术在水泥碱骨料反应机理研究以及水泥中钙分布分析的优势。最后简要介绍了合肥国家同步辐射实验室(NSRL)软X射线显微术的研究进展。     

Hydrothermal Treatment of a Silica Sand Complex with Lime

The feasibility of using relatively low-purity silica as the starting material for hydrothermal solidification was investigated from the consideration of effective utilization of natural resources. Hydrothermal solidification was performed at 140°–180°C for periods ranging from 2 to 40 h using a mixture of 80% silica sand (containing 67% quartz, 22% clay minerals (mica, clinochlore, and kaolinite), and 7% feldspar) and 20% Ca(OH)₂. Within the range of experimental conditions, the flexural strength of the solidified bodies increased with increased treatment temperature and treatment time, reaching values of up to 20 MPa. The flexural strength was proportional to the amount of, but independent of the type of, reaction products. Hydrogarnet and C-S-H were formed in the initial stages of the reaction; the amount of these phases tended to decrease, and 1.1 nm tobermorite formed as the reaction further progressed. These results indicated that hydrogarnet and C-S-H were precursors of 1.1 nm tobermorite.     

Aqueous Solubility Diagrams for Cementitious Waste Stabilization Systems: II, End-Member Stoichiometries of Ideal Calcium Silicate Hydrate Solid Solutions

Solubility in the fully hydrated CaO–SiO₂–H₂O system can be best described using two ideal C-S-H-(I) and C-S-H-(II) binary solid solution phases. The most recent structural ideas about the C-S-H gel permit one to write stoichiometries of polymerized C-S-H-(II) end-members as hydrated precursors of the stable tobermorite and jennite minerals in the form of 5Ca(OH)₂·6SiO₂·5H₂O and 10Ca(OH)₂·6SiO₂·6H₂O, respectively. For thermodynamic modeling purposes, it is more convenient to express the number of basic silica and portlandite units in these stoichiometries using the coefficients n _Si and n _Ca. Thermodynamic solid-solution aqueous-solution equilibrium modeling by applying the Gibbs energy minimization (GEM) approach shows the best generic fits to the available experimental solubility data at solid 0.8 < Ca/Si < 2.0 if both stoichiometry and thermodynamic constants of the end-members are normalized to n _Si= 1.0 ± 0.3. Recommended stoichiometries and thermodynamic data for the C-S-H end-members provide a reliable basis for the subsequent multicomponent extension of the ideal C-S-H solid solution model by incorporation of end-members for the (radio)toxic elements or trace metals.     

粉煤灰预脱硅液动态水热合成硅酸钙晶须的影响因素

以分析纯试剂模拟粉煤灰预脱硅液,采用动态水热法合成硅酸钙晶须,研究了反应体系碱浓度、反应温度、反应时间、溶液中铝硅(Al2O3与SiO2)和钙硅(CaO与SiO2)质量比对产物物相组成及其形貌的影响. 结果表明,控制碱浓度80 g/L以下,反应温度220℃,反应时间6 h,铝硅质量比0.06以下,钙硅质量比0.9,可制备出纯度83%以上的硅酸钙晶须,晶须长度可达20 mm.     

Pair distribution function analysis of nanostructural deformation of calcium silicate hydrate under compressive stress

Despite enormous interest in calcium silicate hydrate (C–S–H), its detailed atomic structure and intrinsic deformation under an external load are lacking. This study demonstrates the nanostructural deformation process of C–S–H in tricalcium silicate (C₃S) paste as a function of applied stress by interpreting atomic pair distribution function (PDF) based on in situ X‐ray scattering. Three different strains in C₃S paste under compression were compared using a strain gauge, Bragg peak shift, and the real space PDF. PDF refinement revealed that the C–S–H phase mostly contributed to PDF from 0 to 20 Å whereas crystalline phases dominated that beyond 20 Å. The short‐range atomic strains exhibited two regions for C–S–H: I) plastic deformation (0‐10 MPa) and II) linear elastic deformation (>10 MPa), whereas the long‐range deformation beyond 20 Å was similar to that of Ca(OH)₂. Below 10 MPa, the short‐range strain was caused by the densification of C–S–H induced by the removal of interlayer or gel‐pore water. The strain is likely to be recovered when the removed water returns to C–S–H.     

Revisiting metastable immiscibility in SiO2–Al2O3: Structure and phase separation of supercooled liquids and glasses

Understanding and controlling liquid–liquid phase separation in aluminosilicates is crucial for optimizing glass properties. However, the metastable nature of aluminosilicates’ phase separation has made it difficult to study experimentally, and uncertainty persists regarding the compositional and temperature extents of the miscibility gap. Here, we present new experimental evidence that suggests a consolute temperature between 1440 and 1590°C and endmember compositions of 7 and 62 mol.% Al₂O₃ for the phase-separated glasses. Using containerless melt processing, deeply supercooled liquids over the 0–60 mol.% Al₂O₃ range are probed with in situ small- and wide-angle X-ray scattering, which simultaneously reveals changes in nanoscale density heterogeneity and atomic structure. Correlations between phase separation and atomic coordination environments are compared for liquids and glasses. Pair distribution function analysis shows mean O–(Si + Al) coordination increases with Al₂O₃ content and decreases with temperature.     

Hydration kinetics and microstructure development of normal and NaAlO2-activated Al-doped β-C2S pastes

Sodium aluminate (NaAlO₂) can be used to accelerate hydration of Portland cements. Here, we compare the use of NaAlO₂ solutions with deionized water on the hydration of Al-doped β-C₂S. The microstructure development of hydration product C-S-H obtained from hydrated Al-doped β-C₂S was investigated. NaAlO₂ significantly improved the hydration kinetics of Al-doped β-C₂S due to the enhancement in the precipitation of calcium hydroxide and stimulation of C-(A)-S-H nucleation and increased the early-age mechanical strength of Al-doped β-C₂S pastes. NaAlO₂ promoted the incorporation of Al in the C-(A)-S-H structure and accelerated the formation of C-(A)-S-H phases containing tetra-, penta- and hexa-coordinated Al in its composition. The alkali cations modified the electrostatic equilibrium of the C-(A)-S-H, thus promoting the development of the nano-mechanical properties.     

C3S and C2S hydration in the presence of Na2CO3 and Na2SO4

This study analyses the behavior of calcium silicates C₃S and C₂S hydrated in two alkaline media, Na₂CO₃ and Na₂SO₄. The silicates were synthesized with laboratory reagents and hydrated in water, to which solid‐state alkaline activators with 4 wt% Na₂CO₃ or 4 wt% Na₂SO₄ were added. Two‐ and 28‐day mechanical strength values were determined and the reaction products were characterized with XRD, SEM/EDX, and ²⁹Si and ²³Na MAS NMR. The findings showed that the presence of Na₂CO₃ hastened hydration kinetics and stimulated early‐age mechanical strength development in both silicates. The most significant effect of sodium sulfate, however, was observed in the 28‐day material in both silicates, in which it raised strength by stimulating the precipitation of C–S–H gels with a high percentage of Q² units.     

Calcium Silicate Hydrates Studied by Small-Angle Neutron Scattering (SANS)

Small-angle neutron scattering (SANS) was used to study calcium silicate hydrate (C-S-H) formed via pozzolanic reaction between calcium oxide and ultrafine silica in water or polymer solutions at a temperature of 20°C. The SANS profile of this product was consistent with a structure that consisted of platelets with maximum diameters of ∼20 nm (± 5 nm) in the x – y plane, which was similar to a structure that had been deduced in previous work via X-ray diffractometry. The presence of two different types of superplasticizer in solution (at a concentration of 10 g/L) had no significant effect on its formation kinetics or its SANS profile.     

Effect of Silicate Anion Structure on Fixation of Chloride Ions by Calcium Silicate Hydrates

Calcium silicate hydrates, CaO–SiO₂-H₂O (C-S-H), were studied as a chloride fixation material. C-S-H of two different CaO/SiO₂ ratios were synthesized and burned with calcium chloride in a temperature range from 600° to 1000°C. Minerals with a chemical composition of CaO·SiO₂·CaCl₂ and 9CaO·6SiO₂·CaCl₂ were identified by X-ray diffraction analysis. Comparing the diffraction intensity, it was found that the most efficient chloride fixation was attained when burned at 800°C. Changes in the morphology of silicate anion associated with burning and fixation of the chloride were studied in terms of chloride fixation capability using the trimethylsililation technique. It was confirmed that some silicate anions formed a glassy infinite chain where the chloride ions were fixed as a solid solution.     

盐湖地区混凝土的长期腐蚀产物与腐蚀机理

运用XRD,DTA—TG,SEM—EDAX和IR方法研究了盐湖地区暴露20a的实际混凝土工程的腐蚀产物,发现了混凝土在盐湖卤水腐蚀条件下3种类型的化学腐蚀。首次报道了盐湖卤水中的碱金属离子对水化硅酸钙(CSH)的两种腐蚀产物,即水化硅酸钙水硅酸镁CSH—MSH的过渡产物含水硅酸钙镁凝胶(C1—xMx)1．15～1．70SH(x＝0．22～0．39)以及Na离子取代Ca的碱硅凝胶产物(C1—xNx)1.85～1．60SH(x＝0．39～0．63)。此外,还提出了水泥的3种水化产物即氢氧钙石(CH)、水化铝酸钙(C3AH6)和水化硅酸钙(CSH)在盐湖地区的化学腐蚀机理。     

Highly Reactive β-Dicalcium Silicate: II, Hydration Behavior at 25°C Followed by 29Si Nuclear Magnetic Resonance

The hydration behavior at 25°C of highly reactive β-dicalcium silicate synthesized from hillebrandite (Ca₂(SiO₃)(OH)₂) was studied over a period of 7 to 224 d using ²⁹Si magic-angle spinning nuclear magnetic resonance (MAS NMR). The hydration product, C-S-H, contains Q² and Q¹ silicate tetrahedra, the chemical shifts of which are independent of the water/solid (w/s) ratio and curing time. Until the reaction is completed, the amounts of Q¹ and Q² formed are independent of the w/s ratio, being determined only by the degree of reaction. The ratio Q²/Q¹ increases as the reaction progresses and as the curing time becomes longer. From the values of Q²/Q¹, it appears that the hydrate is a mixture of dimers and short single-chain polymers. The Ca/Si ratio of the hydrate is high, taking values close to 2.0, but the Ca/Si ratio does not influence the Q²/Q¹ ratio. It was also found that the NMR peak intensities allow quantitative assessment similar to XRD.     

上海地区不同商品粉煤灰的化学活性

对上海地区多种商品粉煤灰的化学活性作了研究与比较。测试结果表明 ,常温下不同灰样在FA Ca(OH) 2 H2 O系统中的反应率有一定差别 ,随龄期延长 ,差别逐渐减小 ,在 180天差值不超过 3 5 %。Ⅱ级磨细灰的化学活性优于Ⅱ级分选灰和Ⅰ级分选灰。但在高温 (80℃ )下 ,分选灰的反应速率常数大于磨细灰。酸溶法不适用于高钙灰反应率的测定。强度测试表明商品高钙灰的活性不低于低钙灰 ,Ⅰ级混烧高钙灰活性最高。     

Hydrothermal Solidification of Kaolinite–Quartz–Lime Mixtures

The strength development of hydrothermally solidified kaolinite–quartz–lime systems with kaolinite as the aluminum source was studied. The starting materials were mixed so that the Ca/(Si + Al) atomic ratio was in the range 0.23 to 0.25, and the Al/(Si + Al) ratio was between 0 to 0.50. Specimens were formed by uniaxial pressing and hydrothermal treatment under saturated steam pressure at 200°C for 2 to 20 h. For quartz-rich systems with Al/(Si + Al) = 0 and 0.05, strength development by the formation of calcium silicate hydrates, such as C–S–H and tobermorite (Ca₅(Si₆O₁₈H₂)·(4H₂O), was observed. On the other hand, in the case of kaolinite-rich systems with Al/(Si + Al) = 0.24 to 0.50, strength development by the formation of hydrogarnet (Ca₃Al₂(SiO₄)(OH)₈) was recognized, resulting in flexural strengths between 15 to 20 MPa. It is proposed that strength development is related to the formation of mesopores (∼0.04 μm) that accompanied formation of the hydrogarnet.     

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.