Determination of the State of Water in Hydrated Cement Phases Using Deuterium NMR Spectroscopy

ACS Sustainable Chem. Eng.

Wang

2018

193

The transport of water molecules and ions in the nanopores of calcium aluminosilicate hydrate (CASH) influences the durability and sustainability of environmentally friendly cement-based materials with industrial waste substitution. In this study, molecular dynamics was utilized to study aqueous NaCl solution capillary transport through the calcium silicate hydrate (CSH) and calcium aluminosilicate hydrate (CASH) gel pore with pore size of 3.2 nm. The invading depth for the NaCl solution advancing frontier with meniscus shape follows a parabolic relation as a function of time, consistent with the classic Lucas−Washburn equation in capillary adsorption theory. As compared with the solution transport in the CSH pore, both water molecules and ions migrate more slowly in the gel pore of CASH, and sodium ions accumulate in the entrance region of the gel pore. The incorporation of Al atoms in the silicate substrate resists the ingress of ions and water. The Al−Si substitution on the CASH interface enhances the charge negativity of solid oxygen atoms, which polarizes the dipole moment of surface water molecules to a larger extent, strengthens the interfacial hydrogen bond, and elongates the residence time of water near the aluminate substrate. In addition, the silicate− aluminate chains in the CASH substrate provide plenty of oxygen sites to associate with the sodium ions by forming a stable Na−O S bond, immobilizing the cations deeply in the vacancy region of the aluminate−silicate channel. The inner sphere adsorption of Na ions on the CASH surface further contributes to the secondary outer sphere adsorption of the Cl ions by forming the Na−Cl ionic pairs. Hopefully, the transport and adsorption mechanism of the ions and water in the CASH gel can help guide the cementitious material substituted by Al-rich industry waste with sustainability and durability.

Section: ■ Introductionmentioning

confidence: 99%

Molecular Dynamics Study on the Structure and Dynamics of NaCl Solution Transport in the Nanometer Channel of CASH Gel

ACS Sustainable Chem. Eng.

Wang

2018

193

“…The evaporation of water and C–S–H dehydration lead to the weight loss of cement paste in the temperature range from 378 to 1273 K (105 to 1000 °C) . The wide range for the C–S–H gel dehydration is attributed to the various compositions of the water molecules including the free water in the capillary pore, the physical bond water near the surface, and the chemical bond water rooted in the calcium silicate layers . This means that the dehydration process is a multistage reaction.…”

Section: Introductionmentioning

confidence: 99%

Confined Water Dissociation in Disordered Silicate Nanometer-Channels at Elevated Temperatures: Mechanism, Dynamics and Impact on Substrates

Zhao

et al. 2016

Langmuir

The effects of elevated temperature on the physical and chemical properties of water molecules filled in the nanometer-channels of calcium silicate hydrate have been investigated by performing reactive molecular dynamics simulation on C-S-H gel subjected to high temperature from 500 to 1500 K. The mobility of interlayer water molecules is temperature-dependent: with the elevation of temperature, the self-diffusivity of water molecules increases, and the glassy dynamic nature of interlayer water at low temperature transforms to bulk water characteristic at high temperature. In addition, the high temperature contributes to the water dissociation and hydroxyl group formation, and proton exchange between neighboring water molecules and calcium silicate substrate frequently happens. The hydrolytic reaction of water molecules results in breakage of the silicate chains and weakens the connectivity of the ionic-covalent bonds in the C-S-H skeleton. However, the broken silicate chains can repolymerize together to form branch structures to resist thermal attacking.

“…The properties of the water confined in gel pores or in the vicinity of C-S-H surfaces have been studied by various experimental techniques. By using 1 H nuclear magnetic resonance (NMR), [1][2][3] the water in C-S-H gels was distinguished into three types: chemically bound water that is incorporated into the structure and forms a strong chemical bond with the calcium silicate structure, physically bound water that is deeply adsorbed near the surface and capillary water that is not bound and diffuses freely into the capillary pores. Furthermore, the quasi-elastic neutron scattering (QENS) technique 4 characterizes the different water types by quantitatively using the diffusion coefficient.…”

Section: Introductionmentioning

confidence: 99%

Water transport in the nano-pore of the calcium silicate phase: reactivity, structure and dynamics

Phys. Chem. Chem. Phys.

Zhao

et al. 2015

Reactive force field molecular dynamics was utilized to simulate the reactivity, structure and dynamics of water molecules confined in calcium-silicate-hydrate (C-S-H) nano-pores of 4.5 nm width. Due to the highly reactive C-S-H surface, hydrolytic reactions occur in the solid-liquid interfacial zone, and partially surface adsorbed water molecules transforming into the Si-OH and Ca-OH groups are strongly embedded in the C-S-H structure. Due to the electronic charge difference, the silicate and calcium hydroxyl groups have binomial distributions of the dipolar moment and water orientation. While Ca-OH contributes to the Ow-downward orientation, the ONB atoms in the silicate chains prefer to accept H-bonds from the surface water molecules. Furthermore, the defective silicate chains and solvated Caw atoms near the surface contribute to the glassy nature of the surface water molecules, with large packing density, pronounced orientation preference, and distorted organization. The stable H-bonds connected with the Ca-OH and Si-OH groups also restrict the mobility of the surface water molecules. The significant reduction of the diffusion coefficient matches well with the experimental results obtained by NMR, QENS and PCFR techniques. Upon increasing the distance from the channel, the structural and dynamic behavior of the water molecules varies and gradually translates into bulk water properties at distances of 10-15 Å from the liquid-solid interface.