29Si Magic-Angle-Spinning Nuclear Magnetic Resonance Study of Hydrated Cement Paste and Mortar

This paper presents ²⁹Si magic-angle-spinning nuclear magnetic resonance measurements that trace the cement hydration process in cement paste and mortar specimens made from ordinary portland cement, type I. These specimens were moist-cured for 3, 7, 14, and 28/31 d at temperatures ranging from 21° to 80°C. Compressive strength for all tested specimens was also determined. The results show that the degree of hydration ( Q ¹+ Q ²) and the compressive strength increase with curing times and temperatures. However, at 80°C, the compressive strength decreases while the degree of hydration increases.     

Si NMR Study of the Hydration of Ca3SiO5 and ß-Ca2SiO4 in the Presence of Silica Fume

²⁹Si magic-angle spinning nuclear magnetic resonance (MASNMR) was used to study the room-temperature hydration of C₃S, ß-C₂S, and reactive ß-C₂S mixed with different amounts of silica fume (SF) that had been hydrated up to nine months and longer. The overall CaO:SiO₂ molar ratios of the mixes were 0.12, 0.20, 0.35, 0.50, and 0.80. NMR spectroscopy was used to quantify the remaining starting materials and the resulting hydration products of different species. A broad peak assigned to Q³, appearing in both the fourier transform (FT) and the cross-polarization (CP) modes, increased in intensity with increased SF content and with age. This Q³ species was attributed to two sources: (1) the surface hydroxylation of SF and (2) the cross-linking of dreierketten (chains of silicate tetrahedra arranged in a repeating three-unit conformation) in the calcium silicate hydrate (C-S-H) structure. A Q⁴ species also appeared in the CP spectra of samples with large SF additions after extended hydration and was attributed to cross-polarization by adjacent hydroxylated Q³ species at the surface of amorphous SiO₂.     

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.     

Relationship Between Structure and Glass Transition Temperature in Low-silica Calcium Aluminosilicate Glasses: the Origin of the Anomaly at Low Silica Content

The anomalous behavior of the glass transition temperature ( T _g) in low silica calcium aluminosilicate glasses has been related to the structural modifications observed by neutron and X-ray diffraction. The diffraction data indicate that Al and Si are in tetrahedral sites and that Ca atoms are in distorted octahedral sites. By subtracting the correlation functions for glasses at constant SiO₂ or constant Al₂O₃ content, we have shown that Si and Al atoms are introduced in a different way within the glass structure. Si is present in various Q ⁿ sites, while Al resides in Q³ and Q⁴ sites for glasses with high CaO content and enters fully polymerized Q⁴ sites with increasing SiO₂ or Al₂O₃ content. The higher proportion of Al in Q³ positions at high CaO content yields a depolymerization of the network. The lower connectivity will contribute to a decrease of the viscosity, which may be at the origin of the decrease of T _g for glasses at low silica content.     

Utilization of fly ash with silica fume and properties of Portland cement-fly ash-silica fume concrete

This paper reports the normal consistency, setting time, workability and compressive strength results of Portland cement-fly ash-silica fume systems. The results show that water requirement for normal consistency was found to increase with increasing SF content while a decrease in initial setting time was found. Workability, measured in term of slump, was found to decrease with silica fume content (compared to blends without silica fume). However, it must be noted that despite the reduction in the slump values, the workability of Portland cement-fly ash-silica fume concrete in most cases remained higher than that of the Portland cement control concrete. Furthermore, the utilization of silica fume with fly ash was found to increase the compressive strength of concrete at early ages (pre 28 days) up to 145% with the highest strength obtained when silica fume was used at 10 wt%. Moreover, scanning electron micrographs show that utilization of fly ash with silica fume resulted in a much denser microstructure, thereby leading to an increase in compressive strength.     

MAS NMR Studies of Partially Carbonated Portland Cement and Tricalcium Silicate Pastes

²⁹Si, ²⁷Al, and ¹H MAS NMR studies of partially carbonated mature ordinary Portland cement (OPC) and tricalcium silicate (C₃S) pastes have been carried out. The water-to-solid ratios ( W/S ) have been varied between 0 and 1 at hydration temperatures of 23^o and 90^oC. Various Q ⁿi units with n =0, 1,2,3, and 4, and a Q³ (1Al) group have been identified using ²⁹Si NMR. Cross-polarization experiments, in addition, have made it possible to assign the OH groups. Two types of fourfold- and one type sixfold-coordinated aluminum have been distinguished using ²⁷Al NMR. In C₃S pastes for w/s >0.7, progressive carbonation leads to a nearly perfect three-dimensional network consisting of Q³ and Q⁴only. In contrast, in OPC pasted only about 40% of the highly polymerized silicate units are formed, partially copolymerized with AlO₄ tetrahedra.     

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.     

Phase Composition of Hydrated DSP Cement Pastes

Cement pastes densified with small particles (DSP) containing up to 48% silica fume by weight of cement, and hydrated to up to 180 d at room temperature, have been analyzed using TMS-GPC, TGA, and ²⁹Si NMR to quantitatively estimate the amount of unreacted cement, Ca(OH)₂, and residual silica fume, respectively. Using a mass balance approach, the CaO/SiO₂ and H₂O/SiO₂ molar ratios of the C-S-H in the samples were calculated. For samples containing silica fume, the values of CaO/SiO₂ lie between 0.9 and 1.3, depending on the degree of hydration and silica fume content, whereas for samples without silica fume they were 1.6. Silicate polymerization analysis using TMS-GPC suggests that the molecular structure of the C-S-H is similar to that formed in conventional hydration. No cross-linking species were found, but the fraction of higher polymers (above octamer) increases as the CaO/SiO₂ ratio decreases.     

Curing requirements of silica fume and fly ash mortars

The curing requirements of silica fume and fly ash mortars were investigated in this study. Silica fume and fly ash mortar specimens were moist cured for periods of 0, 3, 7, 14 and 28 days. After each of the five periods, the moist curing was interrupted by oven-drying the specimens at a temperature of 110°C for 3 days. The specimens were later tested for compressive strength and absorptivity. In this study, it was also determined whether the losses in strength and impermeability of silica fume and fly ash mortars due to an interruption in curing could be regained by recuring. The test results clearly indicate that the curing requirement of silica fume mortar is less than that of plain cement mortar, while in the case of fly ash mortar it is hogher than that of plain cement mortar.     

Silicon-29 Magic Angle Spinning Nuclear Magnetic Resonance Study of Calcium Silicate Hydrates

The reaction products formed in a series of fully "equilibrated," roomtemperature-hydrated, fumed colloidal silica plus lime water mixtures were examined using ²⁹Si magic angle spinning nuclear magnetic resonance. The data suggest that two structurally distinct calcium silicate hydrate (C-S-H) phases exist in the system CaO–SiO₂–H₂O. The more silica-rich C-S-H (Ca/Si = 0.65 to 1.0) consists predominantly of long chains of silica tetrahedra (Q² middle units) similar to those found in 1.4-nm tobermorite. The studied more lime-rich C-S-H (Ca / Si = 1.1 to 1.3) consists of a mixture of dimer (Q¹) and shorter chains (Q¹ end units and Q² middle units) similar to that reported for synthetic jennite. No monomer units (Q⁰) were detected.     

Slag–Portland Cement Based DSP Paste

The partial replacement of Portland cement and silica fume by ground granulated blast furnace slag in DSP cement pastes has been investigated under different curing regimens. The influence of long-term hydration at 25°C, as well as hydrothermal curing (at 80°C) and strong drying (at 200° and 400°C), on the composition and microstructures of slag–Portland cement DSP materials was investigated by nsing QXRD, TGA, MIP, and TMS-GPC methods in this study. Slag is found to be an additive improving intrinsic properties of the DSP paste. CaO/SiO₂ molar ratios of C-S-H phase in the DSP cement past have been calculated.     

石膏、硅灰对硅酸盐胶凝材料早期抗压强度的影响

为了研究石膏、硅灰对硅酸盐胶凝材料早期强度的影响,分别测试了石膏、硅灰不同掺量下的胶凝材料的4 h、1 d、28 d的抗压强度。利用X射线衍射仪和扫描电子显微镜分析了水化产物的微观结构特征。研究表明,在一定的试验范围内,胶凝材料的抗压强度随石膏的增加而变大,掺量为0.75%时最佳,4 h和1 d的抗压强度分别达到5.8 MPa和63.4 MPa;硅灰掺量从0%增长到15%,胶凝材料的各龄期抗压强度均随掺量的增加而呈增长趋势;硬化浆体的微观结构特征表明,一定的试验范围内,石膏使体系中的AFt数量增加,硅灰使体系中的C-S-H凝胶增多,且硅灰未水化的细小颗粒体有效填充硬化浆体的孔隙。     

Effects of densified silica fume on microstructure and compressive strength of blended cement pastes

Some experimental investigations on the microstructure and compressive strength development of silica fume blended cement pastes are presented in this paper. The silica fume replacement varies from 0% to 20% by weight and the water/binder ratio (w/b) is 0.4. The pore structure by mercury intrusion porosimetry (MIP), the micromorphology by scanning electron microscopy (SEM) and the compressive strength at 3, 7, 14, 28, 56 and 90 days have been studied. The test results indicate that the improvements on both microstructure and mechanical properties of hardened cement pastes by silica fume replacement are not effective due to the agglomeration of silica fume particles. The unreacted silica fume remained in cement pastes, the threshold diameter was not reduced and the increase in compressive strength was insignificant up to 28 days. It is suggested that the proper measures should be taken to disperse silica fume agglomeration to make it more effective on improving the properties of materials.     

海水环境下矿物掺合料对硫铝酸盐水泥的水化影响

研究了海水环境下掺入硅灰、粉煤灰、矿渣对硫铝酸盐水泥抗压强度、化学收缩和水化产物的影响规律.结果表明:当硅灰的掺量为2.5%时,水泥浆体的抗压强度比空白组高.矿渣掺量为10%的水泥浆体28 d抗压强度明显超过掺入硅灰和粉煤灰时的强度,60 d强度高于空白组.掺入2.5%硅灰后,水泥浆体的化学收缩增大;在水化早期,粉煤灰和矿渣的火山灰活性很低,导致水泥浆体的化学收缩降低.掺入10%硅灰加快了硫铝酸盐水泥3 d水化反应,钙矾石生成量增多,水泥浆体早期强度比掺其它掺合料有所提高,但体积过快膨胀会破坏其内部结构,对水泥浆体的强度发展不利.     

Creep Behavior of Uranium Carbide-Based Alloys

Compression creep tests were performed on fully dense specimens of UC_1.01, UC_1.05, UC_1.01.+ 4 wt% W, and U_0.9Zr_0.1C_1.01+ 4 wt% W. Steady-state creep rates were measured from 1400° to 1800°C in a vacuum of 1.33 × 10^-3 N/m² (1 × 10^-5 torr) at stresses of 4.55 to 69.0 MN/m² (660 to 10,000 psi). The data for UC_1.01 could best be fit by an expression of the form ɛ= 1773σ^6.024 exp (106.5/RT) , where σ is the steady-state creep rate (h^-l), σ is the applied stress (MN/m²), and the creep activation energy is given in kcal/mol. The stress dependence for creep of UC_1.05 decreased with decreasing temperature because of second-phase precipitation; therefore, a unique creep activation energy could not be established for this U/C ratio. At all temperatures, the creep strength of UC_1.05 exceeded that of UC_1.01. For example, at 1700 ° C steady-state creep rates for UC_1.05 are ∼1/4 those for UC_1.01, but at 1400°C the creep rates are ∼ 3 orders of magnitude less. At 1700°C, creep rates for UC alloys are ∼4 orders of magnitude lower than those for unalloyed UC_1.01.     

Microstructural Evolution on Firing Soda–Lime–Silica Glass Fluxed Whitewares

Density and microstructural evolution of porcelains containing 0–25 wt% soda–lime–silica (SLS) waste glass fired over a range of temperatures from 600° to 1400°C have been investigated. After firing 3 h at 1100°C, batches containing 6.25 wt% SLS glass and 18.75 wt% nepheline syenite flux system attained open-pore closure and a bulk density of 2.40 g/cm³, comparable to results from commercial porcelain after firing at 1200°C. SLS glass softens and melts, conferring early densification and overfiring on porcelains fired at normal commercial firing temperatures. The microstructural evolution examined using XRD, SEM/energy dispersive spectroscopy (EDS), and TEM/EDS revealed formation of a variable composition plagioclase, rounded wollastonite particles, sodium silicates, and tridymite in batches containing SLS glass, in addition to primary and secondary mullites, partially dissolved quartz, and a glassy matrix as found in the waste-free batch. Ca²⁺ and Na⁺ from the SLS glass migrate to regions containing the products of clay decomposition to form plagioclase, limiting the extent of mullite crystallization. The presence of a solution rim surrounding quartz and different glass compositions around wollastonite crystals indicate that the system is not in equilibrium, although phases predicted by the Na₂O–CaO–SiO₂ equilibrium diagram were formed.     

Saturation Factors for Calcium Hydroxide and Calcium Sulfates in Fresh Portland Cement Pastes

Aqueous extracts from five cement pastes at w/c = 0.5 were analyzed for Ca²⁺, SO^2–₄, Na⁺, and K⁺. Degrees of saturation with respect to portlandite, gypsum, and syngenite were determined at ages from 6 min to 3 h, taking into account ionic association effects. Portlandite saturation is usually attained within a few minutes of mixing, but rates of saturation vary. The maximum degree of saturation is 2 to 3 times the equilibrium value and is attained within 2 h. Initial super-saturation with respect to gypsum, and in some cases syngenite, occurs at very early ages and declines to the saturation level within 12 min. Results are interpreted by use of a solution-precipitation model for cement hydration. The calculated critical size of portlandite nuclei at early ages is ∼0.1 μm. Slow growth of portlandite nuclei suggests a surface poisoning phenomenon.     

Properties of portland cement-silica fume pastes II. Mechanical properties

Compressive strength, Young's modulus, and microhardness are measured as a function of porosity for pastes of silica fume-cement blends made with silica fume contents of 0, 10 and 30 per cent at w/(c⁺sf) ratios of 0.25 and 0.45. Porosities are lower for pastes without silica fume addition, but strengths are greater at w/ (c⁺sf) of 0.25 at 180 days curing. At a w/(c⁺sf) of 0.45, however, strengths are marginally greater with large additions of silica fume. Modulus of elasticity is greater without addition. It is shown that silica fume addition at a w/(c⁺sf) of 0.45 results in a relatively low E_o but high S_o. This is due to a combination of low-density dispersed hydrate of low CaO/SiO₂ and low Ca(OH)₂ content, leading to a relatively homogeneous composite. Correlations between log mechanical property and non-evaporable water content are good for all specimens, with and without silica fume.     

Effect of Temperatures up to 90°C on the Early Hydration of Portland–Blastfurnace Slag Cements

Isothermal conduction calorimetry has been used to monitor the early hydration of Portland–blastfurnace slag (BFS)-blended cements. Portland:BFS composite cements with ordinary Portland cement replacements from 0 to 90 wt% were studied at curing temperatures from 12° to 90°C. Peak II, principally associated with alite (Ca₃SiO₅) hydration, was accelerated with increasing temperature for all blends. Peak S, associated with BFS hydration, was particularly noticeable at 40° and 60°C. At higher curing temperatures, peak S merged with peak II, indicating thermal activation of BFS. Novel plots of total heat output against percentage replacement show that BFS contributes to the heat of hydration, even at temperatures below its thermal activation.     

In Situ Synthesis of Ultrafine ZrB2–SiC Composite Powders and the Pressureless Sintering Behaviors

Ultrafine ZrB₂–SiC composite powders have been synthesized in situ using carbothermal reduction reactions via the sol–gel method at 1500°C for 1 h. The powders synthesized had a relatively smaller average crystallite size (<200 nm), a larger specific surface area (∼20 m²/g), and a lower oxygen content (∼1.0 wt %). Composites of ZrB₂+20 wt% SiC were pressureless sintered to ∼96.6% theoretical density at 2250°C for 2 h under an argon atmosphere using B₄C and Mo as sintering aids. Vickers hardness and flexural strength of the sintered ceramic composites were 13.9±0.3 GPa and 294±14 MPa, respectively. The microstructure of the composites revealed that elongated SiC grain dispersed uniformly in the ZrB₂ matrix. Oxidation from 1100° to 1600°C for 30 min showed no decrease in strength below 1400°C but considerable decrease in strength with a rapid weight increment was observed above 1500°C. The formation of a protective borosilicate glassy coating appeared at 1400°C and was gradually destroyed in the form of bubble at higher temperatures.