C3A掺量对C3A-C3S浆体微结构的影响

采用X射线衍射(X-ray Diffraction,XRD)、29Si核磁共振(29Si Nuclear Magnetic Resonance,29Si NMR)、27Al核磁共振(27Al Nuclear Magnetic Resonance,27Al NMR)和BET氮吸附等方法研究了标准养护条件下C3A掺量对C3S-C3A复合浆体微结构的影响.结果表明:C3A的加入可以促进C3S-C3A复合浆体的水化,释放的Al3+可以进入C-S-H凝胶硅氧链上的桥硅氧四面体位置,形成Al掺杂C-S-H凝胶;随C3A掺量增大,复合浆体中C-S-H凝胶的Al[4]/Si和MCL增大;随养护龄期增大,Al掺杂C-S-H凝胶硅氧链上的Al[4]逐渐脱出,进入浆体并转化为Al[6],导致相同C3A掺量下的复合浆体中Al[4]相对含量下降,Al[6]相对含量增长;C3A的加入在降低复合浆体平均孔径的同时,也提高了浆体的孔隙率.     

卤水侵蚀对水泥粉煤灰浆体微结构影响

采用XRD、29Si和27Al MAS NMR测试技术,研究了粉煤灰掺量和侵蚀龄期对卤水侵蚀下水泥-粉煤灰浆体水化产物相组成、含铝相产物迁移与转变、C-S-H凝胶微结构变化的影响规律.研究结果表明:卤水侵蚀导致浆体Ca (OH)2含量降低,AFm和TAH向AFt转变,同时生成大量Friedel盐,C-S-H凝胶中Al[4]脱出;随粉煤灰掺量增加,浆体中AFt、AFm和TAH生成量降低,C-S-H的MCL和Al[4]/Si增大,Friedel盐生成量先增后减;侵蚀早期,水泥-粉煤灰浆体结构疏松,AFt生成量较纯水泥高,后期浆体致密性提高,抑制卤水侵蚀,AFt生成量较少,C-A-S-H脱铝作用减弱.     

养护制度对高掺花岗岩石粉UHPC性能的影响

研究了三种养护制度条件下,高掺(22％)花岗岩石粉UHPC力学性能(3 d、28 d和180 d抗压/抗折强度)和胶凝浆体C-S-H凝胶微结构(水化程度、平均分子链长(MCL)和直链中的[SiO4](Q2))的变化规律.结果表明:同一龄期抗压/抗折强度、胶凝浆体水化程度、C-S-H凝胶MCL和Q2相对含量变化规律由高到低依次为压蒸养护、蒸汽养护和标准养护;随着养护温度和压力增加,其后期强度和胶凝材料水化程度增幅逐渐降低,且不利于后期C-S-H凝胶MCL发展.高掺花岗岩石粉的UHPC胶凝浆体中常温下的水化产物主要包括C-S-H凝胶、AFt和Ca(OH)2,随着养护温度和压力的升高,AFt和Ca(OH)2相对含量降低;至210℃2 MPa时,XRD图谱中两相消失且出现Tobermorite衍射峰,即高温高压环境促进了C-S-H凝胶向Tobermorite晶体转化.     

C-S-H凝胶对Pb(Ⅱ)的吸附固化作用

以分析纯Ca(NO3)2·4H2O、Na2SiO3·9H2O、Pb(NO3)2为原料,通过溶液反应法,制备纯净的C-S-H凝胶和掺杂Pb的C-S-H凝胶,研究了C-S-H凝胶在形成过程中对Pb的吸附固化作用及掺杂Pb后C-S-H凝胶的结构变化.结果表明,可溶铅盐由2％增至6％时,C-S-H凝胶对Pb均有良好的吸附固化作用,俘获Pb总量增大.XRD图谱发现掺杂Pb的C-S-H凝胶,其主峰位置发生偏移,出现明显的Ca(OH)2衍射峰.红外测试表明含Pb C-S-H凝胶硅氧四面体的链接方式发生变化,Q2伸缩振动峰向低波数方向偏移,Q1伸缩振动峰吸收强度增加.     

C-S-H凝胶对Cd2+、Zn2+和Cu2+的吸附固化作用

以分析纯Ca(NO3)2·4H2O、Na2SiO3·9H2O和重金属硝酸盐为原料,通过溶液反应法分别制备钙硅比为0.8和1.8的纯净及含Cd2+、Zn2+和Cu2+的C-S-H凝胶,研究了C-S-H凝胶在形成过程中对三种重金属离子的吸附固化作用.结果表明,两种钙硅比的C-S-H凝胶均能吸附固化Cd2+、Zn2+和Cu2+,重金属离子较易进入低钙硅比C-S-H凝胶夹层结构中,可以置换高钙硅比C-S-H凝胶结构中的Ca2+,在水化液相中形成Ca(OH)2.C-S-H凝胶对Cd2+和Cu2+的固化能力好于对Zn2+的,低钙硅比C-S-H凝胶对Zn2+的吸附固化作用优于高钙硅比C-S-H凝胶.     

Sm3+掺杂碱锌硼硅酸盐玻璃析晶过程的微观结构研究

采用HTEM,NMR以及红外光谱技术,研究适宜半导体微晶掺杂的Sm3 掺杂碱硼硅酸盐基础玻璃在热处理过程中,玻璃结构的变化特点.研究结果表明,含Sm3 基础玻璃失透的主要原因在于在热作用下玻璃中[SiO4]4-以及[BO4]4-四面体结构首先分解形成了富硅相以及富硼相,稀土Sm3 则富集到富硼相中,并进一步析出了硼酸钐晶体;通过优化玻璃的组成能够改进基础玻璃的热稳定性.     

水化硅酸钙纳米尺度微结构的AFM研究

用原子力显微镜的轻敲模式对采用特殊复型技术制备的平滑水泥基材料试样中水化硅酸钙(calcium silicate hydrate,C-S-H)的纳米尺度结构进行了研究.研究发现:水泥水化产物C-S-H形态呈凝胶状,是由一种基本的结构单元--粒径约为5nm的纳米颗粒组成,这些纳米颗粒相互之间会凝聚和自组装成不同的微米构造,微米构造的特征与生长空间有关.纳米微粒表面吸附水后在微粒之间产生一种聚凝连结,随着龄期的增长,纳米微粒的粒径逐渐增大,聚羧酸减水剂的掺入也促进了纳米微粒的长大,这主要是由于纳米微粒之间形成更多较强的结构连结所致.经过溶蚀处理后的C-S-H凝胶样品表面的纳米颗粒粒径明显减小并接近了基本结构单元的尺寸,其主要原因是NH4NO3使C-S-H凝胶体系中的一些未与[SiO4]四面体结合的Ca(OH)2溶解,因此,可以认为纳米颗粒之间主要的结构连结是Ca-O假六方体的共晶连结.     

干湿循环下MgSO4侵蚀对UHPC相组成及其微结构的影响机理

UHPC在海洋工程建设中具有良好的应用前景,但海洋环境中存在的腐蚀性离子和干湿循环作用将会影响UHPC的微观结构,进而影响其耐久性能.通过XRD,SEM-EDS,29 Si和27 Al NMR等方法研究了干湿循环下MgSO4侵蚀和养护制度对UHPC微结构的影响.结果表明:干湿循环下MgSO4侵蚀可促进UHPC浆体中AFt和Mg(OH)2生成,降低C-S-H凝胶的Al[4]/Si和Ca/Si,并促进TAH和硅氧链上脱离的Al向AFt和AFm转化;210℃-2 MPa压蒸养护过程可降低UHPC浆体中的Ca(OH)2和SiO2含量,提高UHPC浆体的水化程度和致密度,并形成Al[4]/Si较高且Ca/Si较低的C-S-H凝胶;压蒸养护可减少侵蚀作用下AFt和Mg(OH)2的生成,有效削弱MgSO4干湿循环侵蚀对UHPC浆体中C-S-H凝胶的脱铝和脱钙作用,并减少Al相转化程度.     

C-S-H凝胶产物的Ca/Si比对碱-骨料反应的影响

采用混凝土加速实验,对添加不同混合材试样的膨胀率和试样中生成的C-S-H凝胶产物进行了研究,测定了不同混合材对试样膨胀率的影响以及所生成的C-S-H凝胶的化学组成,进一步验证C-S-H凝胶的Ca/S i比对碱骨料反应(AAR)的影响。     

水化硅酸钙凝胶的研究进展

水化硅酸钙(C-S-H)凝胶是影响水泥基胶凝材料宏观力学性能与长期耐久性的核心组分,阐明结构和性能间的关联关系有利于实现C-S-H凝胶的合成及推广应用。本文首先对C-S-H凝胶不同阶段的典型结构进行了综述,并引出结构与性能间的内在联系;随后,介绍水热合成法对不同结构C-S-H凝胶的精确调控,并详细阐述了合成要素对其组成、形貌以及结构的影响规律;最后,综述了合成C-S-H凝胶在相关应用领域的基础研究。本文聚焦于C-S-H凝胶构效关系的梳理总结以及C-S-H凝胶工程应用的前景探索,可为充分完善水泥水化机理、开发高性能绿色化建材提供理论参考。     

水泥基材料氯离子结合机理及影响因素研究综述

水泥基材料的氯离子结合能力主要取决于水化硅酸钙(C-S-H)凝胶和Friedel's盐的含量及其稳定性,两者的含量越高、稳定性越好,水泥基材料的氯离子结合能力越强。对水泥基材料氯离子结合能力的分析需考虑多种因素的影响,如水泥种类、矿物掺合料种类、温度、氯离子浓度、阳离子类型、硫酸盐侵蚀和碳化等因素,它们会通过直接影响C-S-H凝胶和Friedel's盐的生成量,或间接影响孔隙液pH值和离子浓度改变C-S-H凝胶和Friedel's盐的稳定性,进而影响其物理吸附能力与化学结合能力,促使氯离子重新结合或释放,导致氯离子结合能力变化显著。本文综述了上述影响因素下,水泥基材料中C-S-H凝胶和Friedel's盐含量、稳定性的变化,及其对氯离子结合能力的影响,并为今后的研究方向提出了建议。     

What causes differences of C-S-H gel grey levels in backscattered electron images?

Backscattered electron (BSE) images of heat-cured concretes show alite grains surrounded by inner C-S-H gel of two distinct grey levels (referred to as two-tone inner C-S-H gel). The lighter rim forms at elevated temperature whereas the darker rim develops during subsequent exposure to moisture at 20 °C. This microstructural feature can potentially be used as an indicator to assess the curing history of a concrete. However, microstructural examinations of room-temperature concretes containing silica fume or which have been exposed to severe conditions (external sulfate, carbonation) also show distinct rims of two-tone inner C-S-H gel.The chemical compositions of the rims were determined by EDX microanalysis in the scanning electron microscope (SEM). Our results show that for heat-cured samples, the different grey levels of the two-tone inner C-S-H are caused by relative differences in microporosity and water content and not by ones in chemical composition. However, in silica-fume blended concrete, sulfate attacked or carbonated specimens the different grey levels of the two-tone inner C-S-H gel were associated with significant differences in chemical composition. This difference allows two-tone inner C-S-H gel arising from heat curing to be distinguished from that arising from these other causes.     

Free-Energy-Based Model of Chemical Equilibria in the CaO–SiO2-H2O System

The chemical equilibrium between the C-S-H gel phase and the aqueous phase in the CaO-SiO₂-H₂O system was modeled using the free-energy-based Thermo-Calc commercial software system. Unlike programs that are based on solubility products, the minimization of free energy allows the composition and solubility of the C-S-H phase to vary, as in real cementitious systems. The C-S-H gel is modeled as a defect tobermorite structure, based on recent compositional data gathered using ²⁹Si NMR. The composition is varied by removal of some of the bridging silica tetrahedra in the SiO layers and by incorporating different amounts of Ca in the interlayer spaces.     

Formation of ASR gel and the roles of C-S-H and portlandite

Experimental investigations of the reactions between silica, alkali hydroxide solution, and calcium hydroxide show that alkali-silicate-hydrate gel (A-S-H) comparable to that formed by the alkali-silica reaction (ASR) in concrete does not form when portlandite or the Ca-rich, Si-poor C-S-H of ordinary portland cement (OPC) paste is available to react with the silica. Under these conditions, we observe either the formation of additional C-S-H by reaction of Ca(OH)₂ with the dissolving silica or the progressive polymerization of C-S-H. The A-S-H dominated by Q³ polymerization forms only after portlandite has been consumed and the C-S-H polymerized. These conclusions are consistent with previously published results and indicate that the ASR gel of concrete forms only in chemical environments in which the pore solution is much lower in Ca and higher in Si than bulk pore solution of OPC paste. These results highlight the similarity between ASR and the pozzolanic reaction and are supported by data for mortar bar specimens.     

A colloidal interpretation of chemical aging of the C-S-H gel and its effects on the properties of cement paste

The properties, structure, and behavior of cement paste, including surface area, drying shrinkage, creep, and permeability are discussed with the assumption that the C-S-H gel is an aggregation of precipitated, colloidal-sized particles that undergoes chemical aging. A basic thesis of this paper is that C-S-H particles bond together over time, increasing the average degree of polymerization of the silicate chains and causing the C-S-H to become stiffer, stronger, and denser. This process occurs slowly at ambient temperatures, but can be greatly accelerated by elevated temperature curing and is also encouraged by drying, which introduces large local strains that may provide a microstructural basis for creep sites. This chemical aging process of C-S-H can thus affect many of the physical properties of cement paste, and there is particular relevance for the complex shrinkage and creep behavior of this material. The effects of a short heat treatment, which causes rapid aging, depend strongly on the moisture of the paste when it is heated. Many of the observations and insights presented here are not new. The primary objective of this paper is to demonstrate, by reporting a variety of published findings in one place, the significant amount of evidence that has been generated over the past 50 years favoring this interpretation. Another objective is to show that the properties and behavior of the C-S-H gel, and of cement paste, do not require a layered microstructure. Separating chemical aging effects from other changes, such as continued hydration, may well lead to a better understanding of the microstructural causes of creep and shrinkage.     

C-S-H/PCE凝胶对掺磷渣水泥的低温促凝早强性能研究

探讨C-S-H/PCE凝胶对掺磷渣硅酸盐水泥的低温促凝早强性能的影响,并通过上清液pH值、Ca²⁺浓度和化学结合水量分析其对水化的影响机理。低温下(8℃) C-S-H/PCE凝胶能有效缩短混凝土终凝时间,并提高早期强度,当掺量5%时,混凝土终凝时间缩短近6 h,16 h、24 h和3 d抗压强度比分别为160%、150%和110%,且不影响2.5 h混凝土工作性能。C-S-H/PCE凝胶能使低温下掺磷渣硅酸盐水泥浆体处于高pH状态,加速早期水泥矿相离子溶解,提高Ca²⁺过饱和度,加速后期析晶过程,提高化学结合水量,从而促进掺磷渣硅酸盐水泥的水化,实现低温促凝早强。     

A multi-technique investigation of the nanoporosity of cement paste

The nanometer-scale structure of cement paste, which is dominated by the colloidal-scale porosity within the C-S-H gel phase, has a controlling effect on concrete properties but is difficult to study due to its delicate structure and lack of long-range order. Here we present results from three experimental techniques that are particularly suited to analyzing disordered nanoporous materials: small-angle neutron scattering (SANS), weight and length changes during equilibrium drying, and nanoindentation. Particular attention is paid to differences between pastes of different ages and cured at different temperatures. The SANS and equilibrium drying results indicate that hydration of cement paste at 20 °C forms a low-density (LD) C-S-H gel structure with a range of gel pore sizes and a relatively low packing fraction of solid particles. This fine structure may persist indefinitely under saturated conditions. However, if the paste is dried or is cured at elevated temperatures (60 °C or greater) the structure collapses toward a denser (less porous) and more stable configuration with fewer large gel pores, resulting in a greater amount of capillary porosity. Nanoindentation measurements of pastes cured at different temperatures demonstrate in all cases the existence of two C-S-H structures with different characteristic values of the indentation modulus. The average value of the modulus of the LD C-S-H is the same for all pastes tested to date, and a micromechanical analysis indicates that this value corresponds to the denser and more stable configuration of LD C-S-H. The experimental results presented here are interpreted in terms of a previously proposed quantitative “colloid” model of C-S-H gel, resulting in an improved understanding of the microstructural changes associated with drying and heat curing.     

Effect of alkalis on fresh C-S-H gels. FTIR analysis

The present study addresses the effect of different concentrations of Na₂O on the structure of a composition of fresh C-S-H gels. The gels were synthesized from laboratory reagents, using calcium nitrate as the source of calcium and a sodium silicate solution for the silicon. A 10-M solution of NaOH was used to maintain a pH of over 13 throughout. The synthesized gels were exposed to different amounts of 8-M NaOH to determine their chemical stability and subsequently characterized with Fourier transform IR spectroscopy. The results showed that the addition of different concentrations of sodium leads to C-S-H gel modification even in the very short term.