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.     

Thermal Reactions of Pyrophyllite Studied by High-Resolution Solid-state 27Al and 29Si Nuclear Magnetic Resonance Spectroscopy

Water is lost in two overlapping steps from well-crystallized pyrophyllite from Coromandel, New Zealand. The pyrophyllite structure survives the loss of the first 30% of the total water content, but the loss of a further 60% water in the second step results in the formation of pyrophyllite dehydroxylate, with corresponding changes in both the ²⁹Si and ²⁷Al solid-state NMR spectra. Detailed analysis of the ²⁹Si chemical shift of the dehydroxylate has allowed the silicate layer structure of this phase to be refined. A similarly detailed interpretation of the ²⁷A1 spectra is not possible because of electric field gradient effects which result in the loss of ∼90% of the A1 spectral intensity due to the formation of five-coordinate A1 on dehydroxylation. The loss of further water from the dehydroxylate on further heating results in the formation of mullite and cristobalite and is accompanied by changes in the ²⁹Si and ²⁷Al spectra which can be accounted for in terms of coordination changes in the structural regions which contained the residual hydroxyls.     

Analysis of Local Cation Arrangement in Mullite Using 29Si Magic-Angle Spinning Nuclear Magnetic Resonance Spectra

Local cation arrangements for mullites with various chemical compositions were examined using ²⁹Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra and the X-ray Rietveld method. The structure of mullite was refined using the Rietveld method, and the relations between chemical composition and lattice constants were revised. ²⁹Si NMR spectra of mullites showed a main resonance at −86 ppm accompanied by weak resonances at −94, −90, and −81 to −82 ppm. These resonances were assigned to each local cation arrangement of TO₄ tetrahedra in mullite by calculating the chemical shift according to Sherriff and Grundy's method. The resonances at −81 to −82 ppm were assigned to the arrangement of the T(Si),T(Al)–O^*_c–T^* (Al) cluster, which has been considered to be rather unusual, and their population was found to increase with increase in the Al₂O₃ content in mullite.     

29Si and 27Al MASNMR Study of Stratlingite

Strätlingite (2CaO·Al₂O₃·SiO₂·8H₂O) is a complex calcium aluminosilicate hydrate commonly associated with the hydration of slag-containing cements or other cements enriched in alumina. Strätlingite can coexist with the hydrogarnet solid solution [hydrogarnet (3CaO·Al₂O₃·6H₂O)-katoite (3CaO·Al₂O₃·SiO₂·4H₂O)] and calcium silicate hydrate (C-S-H). Since Strätlingite is present in many blended cements, the knowledge of strätlingite's characteristic silicate anion structure and how aluminum is accommodated by the structure is important. Phase pure Strätlingite samples have been synthesized from oxides in the presence of excess water and from metakaolinite, calcium aluminate cement, CaO, NaOH, and water. The samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) and then further examined using ²⁹Si, with and without cross-polarization (CP), and ²⁷Al solid-state magic angle nuclear magnetic resonance spectroscopy (MASNMR). For the most part, NMR data for these strätlingites corroborate structural information available in the literature. The aluminum atoms are both tetrahedrally and octahedrally coordinated, and the silicon atoms exist predominantly as Q₂, Q₂(1Al), and Q₂(2Al) species. The presence of alkali affects the structure of strätlingite in subtle ways, significantly reducing the Al^IV/A1^VI ratio.     

Outstanding Problems in the Kaolinite-Mullite Reaction Sequence Investigated by 29Si and 27Al Solid-state Nuclear Magnetic Resonance: I, Metakaolinite

Structural models previously proposed for metakaolinite are examined in light of the most recent published experimental data and new information obtained by solid-state high resolution ²⁹Si and ²⁷AI NMR. A new model for metakaolinite is proposed, consisting of anhydrous regions of distorted AI-0 tetrahedra containing randomly distributed isolated residual hydroxyls associated with A1-0 configurations of regular octahedral and tetrahedpal symmetry. Such a structure, which can readily be formed from kaolinite by the removal of hydroxyls in certain sequences, accounts for the lack of a well-defined X-ray pattern and the persistence of ∼10% residual hydroxyls in metakaolinite and is consistent with the most recent density data, bond lengths, and the new ²⁹Si and ²⁷AI NMR data.     

Structure and Hydration Kinetics of Silica Particles in Rice Husk Ash Studied by 29Si High-Resolution Nuclear Magnetic Resonance

The structure, surface fractal dimension, and global hydration kinetics of silica particles obtained from rice husk ashes (RHAs) were studied with ²⁹Si-NMR spectroscopy and spin–lattice relaxation techniques. Silica particles presented an amorphous fraction higher than 93%, with traces of silica-organic bonding and crystal-like domains. Fe-impurities are located preferentially on the surface of the particles. From the effect of these paramagnetic ions on the spin–lattice relaxation of Q³ and Q⁴ silicate groups, the surface of the particles was characterized as being effectively two-dimensional ( D =1.9±0.1). The hydration kinetics of the particles during the reaction with lime and water was monitored from 8 to 706 days. The process can be described by a power law, with the characteristic exponent higher than those measured for other cements. Also, Johnson–Mehl–Avrami expressions reproduce equally well the experimental data, with parameters compatible with growth habits and morphology known for C–S–H. Two types of Q² tetrahedra were identified in C–S–H, which can be attributed to the bridging and nonbridging silicate groups predicted by the "dreier-kette" structural model of C–S–H.     

Outstanding Problems in the Kaolinite-Mullite Reaction Sequence Investigated by 29Si and 27Al Solid-state Nuclear Magnetic Resonance: 11, High-Temperature Transformations of Metakaolinite

Solid-state ²⁹Si and ²⁷Al NMR spectra of kaolinite fired at 800° to 1450°C, interpreted in light of a newly proposed metakaolinite structure and complementary X-ray diffraction results, lead to the following conclusions about the hightemperature reactions: (1) Removal of the final residual hydroxyl radicals of metakaolinite at ∼9707deg;C triggers the separation of a considerable amount of amorphous free silica and the formation of poorly crystalline mullite and a spinel phase. (2) Mullite and spinel form in tandem, the former originating in the vicinity of AI-0 units of regular octahedral and tetrahedral symmetry randomly distributed throughout the metakaolinite structure. (3) The initially formed mullite is alumina-rich but at higher temperatures progressively gains silica, approaching the conventional 3Al₂O₃· 2SiO₂ composition. (4) The spinel phase contains insufficient Si to be detected by ²⁹Si NMR but has a ²⁷Al NMR spectrum consistent with γ-Al₂O₃. On further heating, the spinel is converted to mullite by reaction with some of the amorpholls silica, the balance of which eventually becomes cristobalite.     

29Si and 27Al MAS-NMR of NaOH-Activated Blast-Furnace Slag

b²⁹Si and ²⁷Al MAS-NMR were performed on NaOH-activated blast-furnace slag to better characterize the amorphous and poorly crystalline phases which occur in this system. The unreacted glass has a mainly dimeric silicate structure represented by a broad ²⁹Si peak (FWHM = 15 ppm) centered at –74.5 ppm [ Q ¹], with aluminum present exclusively in tetrahedral coordination. Upon reaction with 5M NaOH ( w/s = 0.4), three new ²⁹Si peaks with widths of ca. 2 ppm are formed at -78.5 Q ¹, –81.4 [ Q ²(1Al)J, and -84.3 [ Q ²]. Relative peak areas indicate a mostly dimeric silicate structure for the tobermorite-like C─S─H layers, with roughly a third of the bridging sites occupied by aluminum, and less than 10% by silicon. In addition to the tetrahedrally coordinated aluminum substituted in the C─S─H structure, ²⁷Al MAS-NMR reveals the presence of aluminum in octahedral sites, which is attributed to the aluminate phase (C,M)₄AH₁₃. ²⁹Si results indicate rapid initial consumption of the glass, with roughly a third of the glass reacting within the first day and another third consumed over the following 27 days.     

29Si MASNMR Spin-Lattice Relaxation Study of α-, β-, and Amorphous Silicon Nitride

The spin-lattice relaxation times, T ₁, in α-, β-, and amorphous Si₃N₄ have been obtained for the first time, using a multiple-pulse saturation recovery method. The saturation recovery of the ²⁹Si magnetization follows exponential behavior under magic-angle spinning conditions, within the limits of experimental error. A rather wide dispersion of T ₁ values is observed for the phases of Si₃N₄: 284 ± 29 min (–46.686 ppm) and 260 ± 23 min (–48.812 ppm) for the α-phase, 36 ± 4 min for the β-phase, and 11 ± 1 min for the amorphous phase, assuming an exponential recovery. The values obtained for the exponent in the power-law fitting are 0.599(9) (–46.686 ppm) and 0.61(1) (–48.812 ppm) for the α-phase, 0.52(2) for the β-phase, and 0.53(3) for the amorphous phase.     

1H Nuclear Magnetic Resonance Imaging of Water Diffusion in Hardened Cement Pastes

The diffusion of water into hardened cement samples containing different kinds of additives has been studied for the first time using nuclear magnetic resonance imaging (NMRI). The results indicate that NMRI can be used as a research and industrial method to evaluate the properties (permeability, durability, etc.) of the final cement product.     

A Combined 29Si MAS NMR and Selective Dissolution Technique for the Quantitative Evaluation of Hydrated Blast Furnace Slag Cement Blends

Quantification of the C–S–H hydrates and anhydrous material in plain and blended cement systems can be performed by deconvolution of ²⁹Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectra. NMR data are reliable for simple cement systems, but with the incorporation of supplementary cementitious materials, quantification is often uncertain. For example, the overlap of peaks from slag cement and C–S–H in ²⁹Si MAS NMR spectra causes problems with deconvolution. A novel method was developed to address these difficulties. ²⁹Si MAS NMR was combined with a selective dissolution method. The hydrate peaks in slag blends can now be quantified without interference from the slag peak. This new method enables the silica remaining in unreacted slag to be estimated, thus allowing the degree of slag hydration to be quantified. Hydrate Al/Si ratios correlate well with data from analytical transmission electron microscopy (TEM). Analysis of the dissolution residues by TEM and ²⁹Si MAS NMR indicates that they consist of a mixture of unreacted slag, a hydrotalcite-type phase, and small amounts of aluminosilicate gel. The origin of the aluminosilicate gel needs further investigation.     

27Al and 29Si MAS NMR Study of Kaolinite Thermal Decomposition by Controlled Rate Thermal Analysis

The processing of kaolinite under controlled rate thermal analysis conditions (CRTA) provides homogeneously transformed samples at defined dehydroxylation rates. The kaolinite-to-metakaolinite transformation has been monitored by solid-state NMR and X-ray diffraction. The metakaolinite product is an amorphous material with no resolved XRD pattern and is based on a disordered polymerized silicon/aluminum network in which aluminum preferably occupies four- and five-fold structural positions in the ratio 60% Al_VI, 30% Al_V, and 10% Al_VI. This complex dehydroxylation process has been viewed at different scales, with the aluminum NMR tracing a site-by-site destructon of the octahedral sheet and the silicon NMR and XRD following a more rapid destruction of the longrange order.     

基于低场核磁共振技术高温水泥砂浆局部热损伤的研究

为研究高温水泥砂浆在局部水冷作用后的损伤演化特征以及裂纹扩展情况,在水泥砂浆试件中部钻孔,将试件加热到200 ℃及400 ℃后向孔洞注水进行局部冷却。采用低场核磁共振技术(NMR)研究水泥砂浆试件在高温及局部水冷作用下的损伤行为。结果表明:随着热处理温度的升高,当试件局部水冷后,小孔的孔径及含量不断增大,而大孔径变化不是特别明显,试件的损伤度也相应增大;同时,经高温及局部水冷处理后,试件核磁共振成像灰度值的概率密度分布中的峰随温度的增大向右移动,且温度越高,概率密度分布函数对应的灰度值增幅越大,与T₂谱的变化一致。最后,求解了水泥砂浆试件高温及局部水冷后的温度和温度应力的分布,并基于最大拉应力准则求解了200 ℃和400 ℃后局部冷却水泥砂浆试件的裂纹萌发及扩展范围。当温度为200 ℃时局部水冷所产的温度应力不足以引起试件裂纹的萌生。     

基于低场核磁共振技术水泥砂浆反复高温热损伤的研究

为了研究水泥砂浆反复经历高温及局部水冷作用后的损伤演化特征,将水泥砂浆试件中部进行钻孔,将试件加热到400 ℃并且向孔洞注水冷却至室温,重复操作。采用低场核磁共振技术、数字声波仪器、数码显微镜分别研究了水泥砂浆试件在高温及局部水冷作用下的损伤行为。结果表明,随着高温及局部冷却次数增多,试件小孔孔径及数量不断增大,而大孔孔径则减小,试件在经过第一次处理后其性能劣化最明显,且局部注水冷却使得孔洞周围的损伤大于其他部位。同时,随着高温及局部冷却次数增多,概率密度峰值所对应的灰度值增大并向右移动。另外,高温弱化了胶凝材料与砂粒的胶结能力,反复高温处理使得水泥浆与砂粒之间的胶结能力进一步削弱,试件的波速减小,但孔洞中注水冷却产生的温度应力使得胶凝材料与砂粒的胶结能力急剧下降,温度骤降产生的温度应力对胶凝材料与砂粒间的胶结能力的影响远大于反复高温处理。     

不同细度水泥的水化特性及其对水泥砂浆自愈合性能的影响

研究了不同细度水泥的水化热、水化产物和力学性能,以及水泥细度和养护条件对预制裂缝的水泥砂浆强度恢复率和裂缝愈合的影响.结果表明:随着水泥比表面积增大,水泥水化放热量峰值越高,相同龄期下的水化程度越高;预制裂缝试块在自然养护、标准养护和水中养护90 d后的强度恢复率呈现出来相似的规律,均随着细度增加而逐渐减小;比表面积为254 m2/kg和407 m2/kg的水泥制成的砂浆试块,预制裂缝的初始最大宽度都为0.070 mm,前者水中养护90 d后裂缝基本愈合;而后者的裂缝愈合率只有57.1%.     

29Si MAS-NMR Study of the Short-Range Order in Lithium Borosilicate Glasses

²⁹Si MAS-NMR measurements have been made on a series of lithium borosilicate glasses of general composition R Li₂O.B₂O₃· K SiO₂. At low alkali contents ( R < 1), the ²⁹Si resonance envelope is broadened and indicates a distribution of Si sites. As R increases above 1, the FWHM of the ²⁹Si resonance narrows considerably to that representative of a single chemical site. Simultaneously, the average chemical shift of the resonance shifts upfield in agreement with the trends found in the binary lithium silicate glass system. Using the chemical shifts for the individual Q species in the binary system it was found that very good agreement between the chemical shifts of the binary glasses and the ternary glasses examined here could be achieved if a model of proportional sharing of the added oxygen (from lithia) between silicate and borate units was used. In contrast to the ¹¹B NMR studies of these same glasses, the ²⁹Si NMR data are quantitatively best-fit if it is assumed that the proportional sharing of the oxygen from the added lithia begins at R = 0. Models of sharing developed from the ¹¹B NMR studies of these glasses, where proportional sharing above a certain fixed (independent of K ) or variable (dependent on K ) minimum R ₀, have been reexamined and were quantitatively shown through residual analysis to give consistently poorer fits to our data. At present the reasons for the discrepancy between the two sets of NMR data are unknown.     

Effect of Heat Treatment on the Pore Structure and Drying Shrinkage Behavior of Hydrated Cement Paste

The effect of a short heat treatment on hydrated cement paste has been investigated by measuring the weight and length changes of specimens as they undergo various combinations of heating, drying, and resaturation. Heating a cement paste to 60°C coarsens the capillary pore system, decreases the volume of mesopores, and increases the degree of polymerization of the silicates. In addition, the saturated weight of the paste is permanently decreased by a heat treatment. This weight loss can be explained by conversion of bound hydroxyl groups into liquid water during polymerization of the C-S-H gel phase. These experiments help reconcile and interpret published results describing the properties of cement cured at various temperatures, the effects of a short heat treatment on cement paste, and the thermal expansion behavior of saturated and dry cement paste.     

Solid-State27Al and 29Si Magic-Angle Spinning NMR of Aluminosilicate Gels

The environment of the Al and Si atoms of diphasic and single-phase aluminosilicate gels was determined by solid-state ²⁷Al and ²⁹Si magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy, The tetrahedral coordination of Al increased with decreasing Al/Si ratio in single-phase gels but not in diphasic gels. The results showed that ultrahomogeneous mixing on a next-nearest-neighbor level is achieved in single-phase aluminosilicate gels, while in diphasic gels this has successfully been avoided .     

高温循环作用下水泥砂浆的力学性能研究

为了研究高温循环作用下水泥基材料的力学特性,对经高温循环作用后的水泥砂浆试件进行了单轴压缩变形试验.研究结果表明:当循环次数为5次以内时,水泥砂浆的抗压强度随着高温循环次数的增加呈波动变化,无明显增大或减小趋势,但随温度和水灰比的增大而减小;在400℃以内,温度对水泥砂浆的峰值应变影响较小,但峰值应变随着高温循环次数的增加而增大;水泥砂浆的弹性模量随着高温循环次数的增加呈先增大后减小的变化规律.