Effect of hydration on the dielectric properties of C-S-H gel

Correlation between the Dynamics of Nanoconfined Water and the Local Chemical Environment in Calcium Silicate Hydrate Nanominerals

Calcium silicate hydrates are members of a large family of minerals with layered structures containing pendant CaOH and SiOH groups that interact with confined water molecules. To rationalize the impact of the local chemical environment on the dynamics of water, SiOH- and CaOH-rich model nanocrystals were synthesized by using the continuous supercritical hydrothermal method and then systematically studied by a combination of spectroscopic techniques. In our comprehensive analysis, the ultrafast relaxation dynamics of hanging hydroxy groups can be univocally assigned to CaOH or SiOH environments, and the local chemical environment largely affects the H-bond network of the solvation water. Interestingly, the ordered "ice-like" solvation water found in the SiOH-rich environments is converted to a disordered "liquid-like" distribution in the CaOH-rich environment. This refined picture of the dynamics of confined water and hydroxy groups in calcium silicate hydrates can also be applied to other water-containing materials, with a significant impact in many fields of materials science.

THz Fingerprints of Cement-Based Materials

To find materials with an appropriate response to THz radiation is key for the incoming THz technology revolution. Unfortunately, this region of the electromagnetic spectra remains largely unexplored in most materials. The present work aims at unveiling the most significant THz fingerprints of cement-based materials. To this end transmission experiments have been carried out over Ordinary Portland Cement (OPC) and geopolymer (GEO) binder cement pastes in combination with atomistic simulations. These simulations have calculated for the first time, the dielectric response of C-S-H and N-A-S-H gels, the most important hydration products of OPC and GEO cement pastes respectively. Interestingly both the experiments and simulations reveal that both varieties of cement pastes exhibit three main characteristic peaks at frequencies around ~0.6 THz, ~1.05 THz and ~1.35 THz, whose origin is governed by the complex dynamic of their water content, and two extra signals at ~1.95 THz and ~2.75 THz which are likely related to modes involving floppy parts of the dried skeleton.

Dolado J. Elucidation of Conduction Mechanism in Graphene Nanoplatelets (GNPs)/Cement Composite Using Dielectric Spectroscopy

Understanding the mechanisms that govern the conductive properties of multifunctional cement-materials is fundamental for the development of the new applications proposed to enhance the energy efficiency, safety and structural properties of smart buildings and infrastructures. Many fillers have been suggested to increase the electrical conduction in concretes; however, the processes involved are still not entirely known. In the present work, we investigated the effect of graphene nanoplatelets (1 wt% on the electrical properties of cement composites (OPC/GNPs). We found a decrease of the bulk resistivity in the composite associated to the enhancement of the charge transport properties in the sample. Moreover, the study of the dielectric properties suggests that the main contribution to conduction is given by water diffusion through the porous network resulting in ion conductivity. Finally, the results support that the increase of direct current in OPC/GNPs is due to pore refinement induced by graphene nanoplatelets.

Water as a Probe of the Colloidal Properties of Cement

Cement is produced by mixing mineral phases based on calcium silicates and aluminates with water. The hydration reaction of the mixture leads to a synthetic material with outstanding properties that can be used as a binder for construction applications. Despite the importance of cement in society, for a long time, the chemical reactions involved in its hydration remained poorly understood as a result of the complexity of hydration processes, nanostructure, and transport phenomena. This feature article reviews the recently obtained results using water as a probe to detail the essential features in the setting process. By examining the peculiar physicochemical properties of water, fundamental information on the evolving inorganic colloid matrix can be deduced, ranging from the fractal nanostructure of the inorganic silicate framework to the transport phenomena inside the developing porosity. A similar approach can be transferred to the investigation of a plethora of other complex systems, where water plays the main role in determining the final structural and transport properties (i.e., biomaterials, hydrogels, and colloids).

Dynamics of nano-confined water in Portland cement - comparison with synthetic C-S-H gel and other silicate materials

The dynamics of water confined in cement materials is still a matter of debate in spite of the fact that water has a major influence on properties such as durability and performance. In this study, we have investigated the dynamics of water confined in Portland cement (OPC) at different curing ages (3 weeks and 4 years after preparation) and at three water-to-cement ratios (w/c, 0.3, 0.4 and 0.5). Using broadband dielectric spectroscopy, we distinguish four different dynamics due to water molecules confined in the pores of different sizes of cements. Here we show how water dynamics is modified by the evolution in the microstructure (maturity) and the w/c ratio. The fastest dynamics (processes 1 and 2, representing very local water dynamics) are independent of water content and the degree of maturity whereas the slowest dynamics (processes 3 and 4) are dependent on the microstructure developed during curing. Additionally, we analyze the differences regarding the water dynamics when confined in synthetic C-S-H gel and in the C-S-H of Portland cement.

Molecular Simulation of the Ions Ultraconfined in the Nanometer-Channel of Calcium Silicate Hydrate: Hydration Mechanism, Dynamic Properties, and Influence on the Cohesive Strength

Reactive force field molecular dynamics was utilized to investigate the structure, dynamics, and mechanical nature of different cations solvated in the nanometer-channel of highly disordered calcium silicate hydrate. The local structures of different cations bonded with hydroxyl groups are characterized by the long spatial correlation, bond angel distribution preference, and featured coordinated number, resembling those of the tetra-/penta-/octahedron for cation-oxygen structure in the defective region of the silicate glass. Al atoms in the interlayer region play a role in bridging the defective silicate chains and enhance the connectivity of the silicate skeleton. Dynamically, the mobility of ultraconfined water molecules and cations is significantly influenced by the ionic chemistry: the residence time for water molecules in the hydration shell of Al and Mg ions is longer than that in the environment of Na and Ca ions. Furthermore, uniaxial tension simulation provides insight that while both the stiffness and cohesive strength of the C-S-H gels are significantly improved due to the silicate-aluminate branch structure formation, sodium ions with unstable Na-O connection weaken the loading resistance of the C-S-H gels. During the tensile process, the hydrolytic reaction is also affected by the cationic type: water molecules coordinated with Al and Mg cations at high stress state are likely to decompose, but those aggregated with sodium ions are hard to be stretched broken due to the low failure stress.

Distribution of Water in Synthetic Calcium Silicate Hydrates

Understanding calcium silicate hydrates (CSHs) is of paramount importance for understanding the behavior of cement materials because they control most of the properties of these man-made materials. The atomic scale water content and structure have a major influence on their properties, as is analogous with clay minerals, and we should assess these. Here, we used a multiple analytical approach to quantify water distribution in CSH samples and to determine the relative proportions of water sorbed on external and internal (interlayer) surfaces. Water vapor isotherms were used to explain the water distribution in the CSH microstructure. As with many layered compounds, CSHs have external and internal (interlayer) surfaces displaying multilayer adsorption of water molecules on external surfaces owing to the hydrophilic surfaces. Interlayer water was also quantified from water vapor isotherm, X-ray diffraction (XRD), and thermal gravimetric analyses (TGA) data, displaying nonreversible swelling/shrinkage behavior in response to drying/rewetting cycles. From this quantification and balance of water distribution, we were able to explain most of the widely dispersed data already published according to the various relative humidity (RH) conditions and measurement techniques. Stoichiometric formulas were proposed for the different CSH samples analyzed (0.6 < Ca/Si < 1.6), considering the interlayer water contribution.

Effect of chemical environment on the dynamics of water confined in calcium silicate minerals: natural and synthetic tobermorite

Confined water in the slit mesopores of the mineral tobermorite provides an excellent model system for analyzing the dynamic properties of water confined in cement-like materials. In this work, we use broadband dielectric spectroscopy (BDS) to analyze the dynamic of water entrapped in this crystalline material. Two samples, one natural and one synthetic, were analyzed, and despite their similar structure, the motion of confined water in their zeolitic cavity displays considerably different behavior. The water dynamics splits into two different behaviors depending on the chemical nature of the otherwise identical structural environment: water molecules located in areas where the primary building units are SiO4 relax slowly compared to water molecules located in cavities built with both AlO4 and SiO4. Compared to water confined in regular porous systems, water restricted in tobermorite is slower, indicating that the mesopore structure induces high disorder in the water structure. A comparison with water confined in the C-S-H gel is also discussed in this work. The strong dynamical changes in water due to the presence of aluminum might have important implications in the chemical transport of ions within hydrated calcium silicates, a process that governs the leaching and chemical degradation of cement.

Effect of addition of silica- and amine functionalized silica-nanoparticles on the microstructure of calcium silicate hydrate (C-S-H) gel

In this work we study the influence of adding nano-silica (SiO2, Nyasil™) and aminopropyl (-(CH2)3-NH2,) functionalized silica nanoparticles (Stoga) during the synthesis of calcium-silicate-hydrate (C-S-H gel). Characterization by solid state (29)Si NMR and ATR-FTIR spectroscopy showed that the addition of both particle types increases the average length of the silicate chains in C-S-H gel being this effect slightly more important in the case of Stoga particles. In addition, (13)C NMR and XPS confirmed that the aminopropyl chain remains in the final product cleaved to silicon atoms at the end of the silicate chain of C-S-H gel whereas XRD measurements showed that this result in an increment in the basal distance compared with ordinary CSH. In addition, the dynamics of water within the pores of C-S-H gel was analyzed by broadband dielectric spectroscopy. We observed that water confined in C-S-H formed with the addition of nanoparticles is faster than that in plain C-S-H which can be related to a different porous structure in these materials.

Dynamics of deeply supercooled interfacial water

In this review we discuss the relaxation dynamics of glassy and deeply supercooled water in different types of systems. We compare the dynamics of such interfacial water in ordinary aqueous solutions, hard confinements and biological soft materials. In all these types of systems the dielectric relaxation time of the main water process exhibits a dynamic crossover from a high-temperature non-Arrhenius temperature dependence to a low-temperature Arrhenius behavior. Moreover, at large enough water content the low-temperature process is universal and exhibits the same temperature behavior in all types of systems. However, the physical nature of the dynamic crossover is somewhat different for the different types of systems. In ordinary aqueous solutions it is not even a proper dynamic crossover, since the water relaxation decouples from the cooperative α-relaxation of the solution slightly above the glass transition in the same way as all secondary (β) relaxations of glass-forming materials. In hard confinements, the physical origin of the dynamic crossover is not fully clear, but it seems to occur when the cooperative main relaxation of water at high temperatures reaches a temperature where the volume required for its cooperative motion exceeds the size of the geometrically-confined water cluster. Due to this confinement effect the α-like main relaxation of the confined water seems to transform to a more local β-relaxation with decreasing temperature. Since this low-temperature β-relaxation is universal for all systems at high water content it is possible that it can be considered as an intrinsic β-relaxation of supercooled water, including supercooled bulk water. This possibility, together with other findings for deeply supercooled interfacial water, suggests that the most accepted relaxation scenarios for supercooled bulk water have to be altered.

Cause of the fragile-to-strong transition observed in water confined in C-S-H gel

In this study, the rotational dynamics of hydration water confined in calcium-silicate-hydrate (C-S-H) gel with a water content of 22 wt.% was studied by broadband dielectric spectroscopy in broad temperature (110-300 K) and frequency (10(-1)-10(8) Hz) ranges. The C-S-H gel was used as a 3D confining system for investigating the possible existence of a fragile-to-strong transition for water around 220 K. Such transition was observed at 220 K in a previous study [Y. Zhang, M. Lagi, F. Ridi, E. Fratini, P. Baglioni, E. Mamontov and S. H. Chen, J. Phys.: Condens. Matter 20, 502101 (2008)] on a similar system, and it was there associated with a hidden critical point of bulk water. However, based on the experimental results presented here, there is no sign of a fragile-to-strong transition for water confined in C-S-H gel. Instead, the fragile-to-strong transition can be explained by a merging of two different relaxation processes at about 220 K.

Water-induced changes in the charge-transport dynamics of titanate nanowires

The temperature dependence of dielectric processes in humid titanate nanowires was investigated via broadband dielectric spectroscopy under quasi-isosteric conditions in the temperature range of 150-350 K. It was found that the dynamic parameters obtained from low-temperature measurements cannot describe the dielectric behavior of the system above 273 K, implying changes in the dynamics of the corresponding dielectric processes. The calculated activation energies and pre-exponential factors counterintuitively increase linearly with the amount of adsorbed water, and a compensation effect was also found to apply to all contributions in the TiONW spectra.

Dynamic behavior of hydration water in calcium-silicate-hydrate gel: a quasielastic neutron scattering spectroscopy investigation

The translational dynamics of hydration water confined in calcium-silicate-hydrate (C-S-H) gel was studied by quasielastic neutron scattering spectroscopy in the temperature range from 280 to 230 K. The stretch exponent β, the self-diffusion constant D, the average translational relaxation time {τ}, and the temperature dependence of confinement radius α extracted from the elastic fraction of immobile water molecules p(Q) were obtained from the analyses of the low-Q spectra according to the relaxing cage model. Measurements were made using C-S-H of three different water contents, 10%, 17%, and 30%. Among the three samples of C-S-H gel with different water contents, the values of β decrease with increasing water contents, while α increases. The values of D and {τ} are insensitive to temperature for the two lower water contents, as opposed to the 30% case where a slight variation is observed. The trend for violation of the Stokes-Einstein relation is only visible in the case of 30% water content.