混合材抑制碱-硅酸反应(ASR)的新机理

目前研究表明混合材对碱一硅酸反应（ASR）有很好的抑制作用，但对混合材抑制ASR机理的认识需进一步完善。本文采用TEM／EDS、FT-IR和^29Si MAS NMR等现代测试方法对不同粉煤友掺量的水泥基材料主要水化产物C-S-H凝腔化学组成和结构进行研究，指出了低Ca／Si比的C-S-H凝腔在抑制ASR中的重要作用（能够结合孔溶液中的碱，减少可发生ASR的有效碱量），并分析了低Ca／Si比的C-S-H凝胶的持碱机制，对混合材抑制ASR的机理有了新的认识，完善了混合材抑制ASR的机理。研究结果不仅对于建立比较完整的碱一集料反应预防理论是十分必要的，而且对于抑制和防止碱集料反应具有一定的指导意义。     

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。     

碱矿渣水泥混凝土的抗渗性能试验研究

抗渗性是混凝土重要的耐久性指标之一.通过对比研究了不同配制参数的碱矿渣混凝土与普通硅酸盐水泥混凝土的抗压强度及抗渗性能,并结合扫描电子显微镜、氮吸附对两种水泥的微观结构进行分析.结果表明:碱矿渣混凝土水化产物主要为低Ca/Si比的C-S-H凝胶,结构密实,抗渗性能优于普通硅酸盐混凝土;碱矿渣混凝土的抗渗性和强度随着水灰比的增大而降低,碱当量在3％ ～4％、水玻璃模数在1～1.5、矿渣细度在350～460 m2/kg范围内变化对碱矿渣混凝土的抗渗性和强度影响不大.     

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.     

抑制碱-集料反应的工艺措施

碱-集料反应（AAR）在国内外已引起了大量的混凝土建筑物破坏，损失巨大，因此研究抑制AAR的时策非常必要、以冀东水泥厂为例，通过回转窑碱含量平衡计算和快速砂浆棒膨胀试验得出，就水泥而言，降低混凝土AAR的主要工艺措施有二方面：一是选择低碱的原燃材料，以降低水泥中的总碱量；二是改变碱的存在形式，即在总碱量难以控制时设法减少可溶性碱的存在，以有效抑制AAR的危害性．     

电场与硫酸盐侵蚀共同作用下混凝土的劣化及其机理

通过试验模拟电场与硫酸盐的共存环境,测试了不同配比的混凝土在这两种因素共同作用后的外观、抗压强度等宏观性能变化,并借助能谱分析、扫描电子显微镜和X射线衍射等方法,探讨了混凝土的劣化机理。结果表明:在电场作用下,混凝土孔溶液中的Ca2+发生定向迁移从而导致Ca2+浓度降低;Ca(OH)2分解溶出Ca2+来维持其平衡;此外,水化硅酸钙(C-S-H)凝胶的Ca/Si比下降,表明C-S-H凝胶也分解析出Ca2+,这对强度产生不利影响。同时,大量SO42–因电场作用从阴极溶液进入混凝土内部,并与水泥水化产物反应生成更多钙矾石和石膏晶体,导致强度进一步降低。低水胶比或掺粉煤灰混凝土的耐蚀性有所提高,但其劣化仍较为明显。因此,当混凝土受到电场和硫酸盐侵蚀共同作用时,应针对性地提出更为合理的混凝土耐久性设计及防范措施。     

碱粉煤灰矿渣混凝土的抗渗性能实验研究

抗渗性是混凝土重要的耐久性指标之一.通过对比研究了不同固态分散相组成的碱粉煤灰矿渣混凝土的抗渗性能,并结合X-射线衍射(XRD)、傅立叶转变红外光谱(FT-IR)、扫描电子显微镜(SEM)、压汞法(MIP)对碱粉煤灰矿渣水泥石的物相组成和微观结构进行分析.结果表明:碱矿渣混凝土水化产物主要为低Ca/Si比的(I)C-S-H凝胶,体系中随着粉煤灰掺量的增大,水化产物逐渐向聚合度更高的C(N)-A-S-H凝胶结构转变;当粉煤灰掺量不超过30％,粉煤灰能够优化碱粉煤灰矿渣体系孔隙结构,提高碱粉煤灰矿渣混凝土的抗渗性;当粉煤灰掺量大于30％,粉煤灰较低的活性导致水泥石水化产物数量减少,水泥石大孔数量和总孔隙量增大,相应混凝土抗渗性能呈明显下降趋势.     

硅酸盐水泥混凝土的碳化分析

介绍了硅酸盐水泥混凝土的碳化反应和碳化过程,分析了Ca(OH)_2与水化硅酸钙(C-S-H)的碳化作用.Ca(OH)_2发生碳化反应的同时,C-S-H也会发生碳化反应;Ca(OH)_2的碳化产物是方解石,而C-S-H碳化后会转变成无定形硅胶,可能形成稳定性差、结晶度差的球霰石、文石,其分解温度低于方解石的分解温度;C/S低、结晶度差的C-S-H凝胶易于碳化;水泥浆体孔隙溶液中的碱含量越高,碳化速度越快,深度越大.     

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.     

碱矿渣胶凝体系的水化特性及机理分析

通过测定矿渣胶凝材料体系不同龄期的化学结合水含量,结合SEM分析,研究了碱矿渣胶凝材料的水化特性以及水化产物的微观形貌.结果表明:随着水化时间的增加,水化程度呈现不断增长的趋势,水化时间为1d时,水化程度为40.37％;水化初期,小颗粒形状的凝胶体在矿渣周围形成,凝胶间不断组合生长为C-S-H凝胶,随着水化时间的增加,胶凝体系逐渐致密.水化产物的Ca/Si、Ca/(Si+ Al)、Na/(Si+ Al)的质量比比值趋于稳定,表明碱矿渣-钢渣复合胶凝体系已形成稳定的水化产物.     

Alkali sorption by C-S-H and C-A-S-H gels: Part II. Role of alumina

In Part I, it was shown that alkali partition between C-S-H gel and an aqueous phase can be represented by a partition function, R_d, the numerical value of which, at constant temperature, is defined by the Ca/Si ratio. This R_d value is constant or nearly so over wide ranges of NaOH and KOH concentrations up to ∼0.3 M. In the present paper, Al has been introduced to form C-A-S-H gels, and the influence of Al on alkali sorption properties was determined: Approximately 6-7% replacement of Si by Al was used. Microprobe evidence is presented to show that the Al is actually in solid solution. Introduction of Al into C-S-H markedly increases R_d, indicating enhancement of alkali binding. The results underpin and quantify the beneficial effects of alkali binding arising from the introduction of aluminous supplementary cementing materials, such as fly ash, into cement pastes.     

Alkali binding in cement pastes: Part I. The C-S-H phase

The binding of sodium and potassium into cement paste influences the performance of concrete: for example, alkali balances between solid and paste constituents and pore fluid affect the potential for reaction with alkali-susceptible aggregates. However, quantification of the binding potential into paste solids has proven to be difficult, although much empirical data are available from pore fluid analyses. In this study, single-phase homogeneous C-S-H phases have been prepared at Ca:Si molar ratios of 1.8, 1.5, 1.2, and 0.85 and reacted with six alkali hydroxide concentrations, both NaOH and KOH, between 1 and 300 mM, giving a grid of 48 alkali concentrations and Ca:Si ratios. A steady-state alkali partition is attained in less than 48 h. A distribution coefficient, R_d, was calculated to express the partition of alkali between solid and aqueous phases at 20°C. The numerical value of R_d is independent of alkali hydroxide concentration and depends only on Ca:Si ratio. Approximate reversibility is demonstrated, so the R_d values are constants of a C-S-H over wide ranges of alkali concentration. The trend of R_d values indicates that alkali binding into the solid improves as its Ca:Si ratio decreases.     

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.     

碱激发剂浓度及模数对碱矿渣胶凝材料抗压性能及水化产物的影响研究

采用水玻璃(复掺氢氧化钠调整模数)激发粒化高炉矿渣活性制备碱矿渣净浆试样.采用抗压强度测试、X-射线衍射(XRD)、综合热分析(TG-DSC)等技术手段研究了激发剂碱浓度(4%、6%、8%)及模数(0.75、1.00、1.50、2.00)对碱矿渣胶凝材料抗压性能及水化产物的影响.研究结果表明:激发剂模数较低时(0.75和1.00),碱矿渣胶凝材料抗压强度随着碱浓度的增加而呈下降趋势;激发剂模数较高时(1.50和2.00),试件强度在碱浓度为6%时达到最大值.在相同碱浓度下,激发剂模数为1.50时试件抗压强度值最大.碱矿渣胶凝材料主要水化产物为C-S-H凝胶,同时伴有C-A-S-H凝胶生成.另外观测到少量斜方钙沸石(CaAl2Si2O8· 4H2O)的生成.在部分配合比中还观测到水滑石(Mg6Al2(OH)16CO3· 4H2O)的存在.碱浓度较高的碱矿渣胶凝材料中生成了较多的C-S-H水化产物.激发剂模数较高时(1.50和2.00),更有利于碱矿渣中C-S-H水化产物的生成.碱浓度/模数较低时, C-S-H产物结晶度有所提高.相较于C-S-H凝胶结晶度,其生成量对碱矿渣胶凝材料抗压强度的影响更为显著.     

Alkali uptake in calcium alumina silicate hydrate (C-A-S-H)

Uptake of the alkalis K and Na by calcium silicate hydrate (C-S-H) and calcium alumina silicate hydrate (C-A-S-H) of molar Ca/Si ratios = 0.6 to 1.6 and molar Al/Si ratio = 0 or 0.05 has been studied at 20 °C. Alkalis are thought to be bound in the interlayer space of C-A-S-H and show preferred uptake by lower Ca/Si ratios and by higher alkali concentrations. A consequence of alkali uptake into C-A-S-H is a rearrangement of the C-A-S-H structure. Less calcium is present in the interlayer and shorter silica chains are observed for the same molar Ca/Si ratio.No significant difference was observed between sodium and potassium uptake. Equilibration times of 91 days to 1 year or the solid phase being either C-S-H or C-A-S-H had seemingly no effect on alkali uptake.     

X-ray photoelectron spectroscopic investigation of nanocrystalline calcium silicate hydrates synthesised by reactive milling

X-ray photoelectron spectroscopy (XPS) has been used to analyse a series of mechanochemically synthesised, nanocrystalline calcium silicate hydrates (C-S-H). The samples, with Ca/Si ratios of 0.2 to 1.5, showed structural features of C-S-H(I). XPS analysis revealed changes in the extent of silicate polymerisation. Si 2p, Ca 2p and O 1s spectra showed that, unlike for the crystalline calcium silicate hydrate phases studied previously, there was no evidence of silicate sheets (Q³) at low Ca/Si ratios. Si 2p and O 1s spectra indicated silicate depolymerisation, expressed by decreasing silicate chain length, with increasing C/S. In all spectra, peak narrowing was observed with increasing Ca/Si, indicating increased structural ordering. The rapid changes of the slope of FWHM of Si 2p, Δ_Ca-Si and Δ_NBO-BO as function of C/S ratio indicated a possible miscibility gap in the C-S-H-solid solution series between C/S 5/6 and 1. The modified Auger parameter (α′) of nanocrystalline C-S-H decreased with increasing silicate polymerisation, a trend already observed studying crystalline C-S-H. Absolute values of α′ were shifted about − 0.7 eV with respect to crystalline phases of equal C/S ratio, due to reduced crystallinity.     

Tobermorite/jennite- and tobermorite/calcium hydroxide-based models for the structure of C-S-H: applicability to hardened pastes of tricalcium silicate, β-dicalcium silicate, Portland cement, and blends of Portland cement with blast-furnace slag, metakaolin, or silica fume

The purpose of this article is to discuss the applicability of the tobermorite-jennite (T/J) and tobermorite-‘solid-solution’ calcium hydroxide (T/CH) viewpoints for the nanostructure of C-S-H present in real cement pastes. The discussion is facilitated by a consideration of the author's 1992 model, which includes formulations for both structural viewpoints; its relationship to other recent models is outlined. The structural details of the model are clearly illustrated with a number of schematic diagrams. Experimental observations on the nature of C-S-H present in a diverse range of cementitious systems are considered. In some systems, the data can only be accounted for on the T/CH structural viewpoint, whilst in others, both the T/CH and T/J viewpoints could apply. New data from transmission electron microscopy (TEM) are presented. The ‘inner product’ (Ip) C-S-H in relatively large grains of C₃S or alite appears to consist of small globular particles, which are ≈4-8 nm in size in pastes hydrated at 20 °C but smaller at elevated temperatures, ≈3-4 nm. Fibrils of ‘outer product’ (Op) C-S-H in C₃S or β-C₂S pastes appear to consist of aggregations of long thin particles that are about 3 nm in their smallest dimension and of variable length, ranging from a few nanometers to many tens of nanometers. The small size of these particles of C-S-H is likely to result in significant edge effects, which would seem to offer a reasonable explanation for the persistence of Q⁰(H) species. This would also explain why there is more Q⁰(H) at elevated temperatures, where the particles seem to be smaller, and apparently less in KOH-activated pastes, where the C-S-H has foil-like morphology. In blended cements, a reduction in the mean Ca/Si ratio of the C-S-H results in a change from fibrillar to a crumpled-foil morphology, which suggests strongly that as the Ca/Si ratio is reduced, a transition occurs from essentially one-dimensional growth of the C-S-H particles to two-dimensional; i.e., long thin particles to foils. Foil-like morphology is associated with T-based structure. The C-S-H present in small fully hydrated alite grains, which has high Ca/Si ratio, contains a less dense product with substantial porosity; its morphology is quite similar to the fine foil-like Op C-S-H that forms in water-activated neat slag pastes, which has a low Ca/Si ratio. It is thus plausible that the C-S-H in small alite grains is essentially T-based (and largely dimeric). Since entirely T-based C-S-H is likely to have different properties to C-S-H consisting largely of J-based structure, it is possible that the C-S-H in small fully reacted grains will have different properties to the C-S-H formed elsewhere in a paste; this could have important implications.     

3D Nanotomography of calcium silicate hydrates by transmission electron microscopy

Calcium silicate hydrate (C-S-H), is the principal hydration product of Portland cement that mainly contributes to the physical and mechanical properties of concrete. This paper aims to investigate the three-dimensional structure of C-S-H with Ca/Si ratios of 1.0 and 1.6 at the nanoscale using electron tomography. The 3D reconstructions and selected region of interest analysis confirm that the morphology of both C-S-H materials are foil-like structures. The difference between the two materials is the density of elongated structures. C-S-H with Ca/Si ratio 1.6 is clearly composed of denser particles compared to the other C-S-H material due to overlapping of the foil-like structure. Pore analysis shows that C-S-H 1.0 and C-S-H 1.6 have porosities 69.2% and 49.8% respectively. Pore size distribution also reveals that C-S-H 1.0 has pore size range between 0-250 nm and C-S-H 1.6 between 0-100 nm. The pore network's size of C-S-H 1.0 is significantly larger than 1.6. This study illustrates the capability of using electron tomography to determine the 3D nanoscale structure of cementitious products and to distinguish between C-S-H 1.0 and 1.6.     

Application of Selective 29Si Isotopic Enrichment to Studies of the Structure of Calcium Silicate Hydrate (C-S-H) Gels

Selective isotopic enrichment of SiO₂ with ²⁹Si in a mixture with tricalcium silicate (C₃S) has allowed the Si from this phase to be effectively labeled during the course of the hydration reaction, thus isolating its contribution to the reaction. A double Q₂ signal has been observed in ²⁹SI solid-state MAS NMR spectroscopy of C-S-H gels of relatively low Ca/Si ratio, prepared by hydration or by carbonation of a C₃S paste. The origin of the weaker, downfield peak is discussed and tentatively attributed to bridging tetrahedra of a dreierkette silicate chain structure.