Search results for "calcium silicate hydrate"
showing 10 items of 33 documents
Onset of cohesion in cement paste
2004
It is generally agreed that the cohesion of cement paste occurs through the formation of a network of nanoparticles of a calcium-silicate-hydrate ("C-S-H"). However, the mechanism by which these particles develop this cohesion has not been established. Here we propose a dielectric continuum model which includes all ionic interactions within a dispersion of C-S-H particles. It takes into account all co-ions and counterions explicitly (with pure Coulomb interactions between ions and between ions and the surfaces) and makes no further assumptions concerning their hydration or their interactions with the surface sites. At high surface charge densities, the model shows that the surface charge of…
Aqueous solubility diagrams for cementitious waste stabilization systems. 3. Mechanism of zinc immobilizaton by calcium silicate hydrate.
2002
Zinc oxide was added during hydration of alite (C3S) as an analogue for solidification/stabilization by cement of metal-bearing hazardous waste. Curing of samples was stopped at various intervals between 8 h and 100 d, and the reaction products were analyzed by both X-ray diffraction (XRD) and X-ray absorption spectroscopy (EXAFS at Zn, Ca, and Si K-edges). Calcium zincate hydrate (CaZn2(OH)6 x 2H2O) initially formed together with calcium silicate hydrate (CSH) vanishes from X-ray diffractograms after 14 d, and no other crystalline Zn-bearing phase could be detected thereafter. EXAFS Zn K-edge data analysis reveals that Zn(O,OH)4 tetrahedra continue to determine the first shell coordination…
From C–S–H to C–A–S–H: Experimental study and thermodynamic modelling
2015
Abstract It has long been known that the stoichiometry of C–S–H varies with the calcium hydroxide concentration in solution. However, this issue is still far from understood. We revisit it in both experimental and modelling aspects. A careful analysis of the solubility confirms the existence of three different C–S–H phases, defined as Ca 4 H 4 Si 5 O 16 , Ca 2 H 2 Si 2 O 7 and Ca 6 (HSi 2 O 7 ) 2 (OH) 2 , respectively. The variation of the Ca/Si ratio of the three phases has been described by surface reactions: the increase of the Si content is accounted for by silicate bridging, the increase of calcium content and the surface charge are accounted for by reactions involving silanol groups v…
Solubility of Zn(II) in Association with Calcium Silicate Hydrates in Alkaline Solutions
1999
The binding of Zn(II) to the cement mineral calcium silicate hydrate (CSH) was investigated in a well-defined laboratory system. CSH (Ca:Si = 1:1) was synthesized by coprecipitation with varying contents of Zn(II). Zn(II) was added in the proportions 0, 0.1, 1.0, 5.0, and 10% in exchange for Ca. The resulting CSH was characterized by X-ray diffraction. The solid phases were then equilibrated in aqueous suspensions, and the solubilities of Ca, Si, and of Zn(II) were determined as a function of pH and Zn(II) content in the solid phase. The solubility of Ca and Si in equilibrium with the CSH phases was in agreement with that predicted by thermodynamic calculations. Dissolved Zn(II) concentrati…
Experimental study of Si–Al substitution in calcium-silicate-hydrate (C-S-H) prepared under equilibrium conditions.
2009
International audience; C-A-S-H of varying Al/Si and Ca/(Al+Si) ratios have been prepared introducing C-S-H (Ca/Si=0.66 and 0.95) at different weight concentrations in a solution coming from the hydration of tricalcium aluminate (Ca3Al2O6) in water. XRD and EDX (TEM) analyses show that using this typical synthesise procedure, pure C-A-S-H is obtained only for calcium hydroxide concentrations below 4.5 mmol L−1. Otherwise, calcium carboaluminate or strätlingite is also present beside C-A-S-H. The tobermorite-like structure is maintained for C-A-S-H. A kinetic study has shown that the formation of C-A-S-H is a fast reaction, typically less than a few hours. The Ca/(Al+Si) ratio of C-A-S-H mat…
The growth of charged platelets.
2014
Growth models of charged nanoplatelets are investigated with Monte Carlo simulations and simple theory. In a first model, 2-dimensional simulations in the canonical ensemble are used to demonstrate that the growth of a single weakly charged platelet could be limited by its own internal repulsion. The short range attractive interaction in the crystal is modeled with a square well potential while the electrostatic interactions are described with a screened Coulomb potential. The qualitative behavior of this case can also be described by simply balancing the attractive crystal energy with the screened Coulomb repulsion between the crystal sites. This repulsion is a free energy term dominated b…
Electrokinetic Properties which Control the Coagulation of Silicate Cement Suspensions during Early Age Hydration
1998
The coagulation of cement particles during early age hydration has been previously identified as the first step of the setting and hardening of cement pastes. By hydrating Ca3SiO5and a silicate-rich clinker under controlled conditions, a correlation between the coagulation of the suspensions and the electrokinetic properties of particles is established. The zeta potential, and hence the surface charge, of particles in suspension depends on the calcium content of the medium. At low concentrations of Ca2+, the zeta potential of Ca3SiO5particles, calcium silicate hydrate (C–S–H), and clinker is negative (<−30 mV) and the suspensions are well dispersed. A strong coagulation occurs at intermedia…
Prediction of Long-Term Chemical Evolution of a Low-pH Cement Designed for Underground Radioactive Waste Repositories
2012
Low-pH cements, also referred as low-alkalinity cements, are binders with a pore solution pH ≤ 11. They can be designed by replacing significant amounts of Portland cement (OPC) (≥40 %) by silica fume, which can be associated in some cases to low-CaO fly ash and/or ground granulated blast furnace slag to decrease the heat output during hydration by dilution of OPC and improve the mechanical strength of the final material. With the prospect of using these materials in a geological repository, it is of main importance to estimate their long-term properties and the influence of external and internal factors (chemical composition of the binder, storage temperature) on their characteristics. For…
Hydration of alite containing aluminium
2011
Abstract The most important phase in Portland cement is tricalcium silicate, which leads during its hydration to the nucleation and growth of calcium silicate hydrate [referred to as C–S–H, (CaO)x–SiO2–(H2O)y]. The development of this hydrate around the cement grains is responsible for the setting and hardening of cement pastes. The general term for designating the tricalcium silicate in cements is alite. This name relates to all polymorphs containing various foreign ions inserted in their structure. These ions may influence the intrinsic reactivity, and once released during the dissolution, they may interact also with C–S–H. One of the most likely species to be inserted in the alite struct…
Two-Step Nucleation Process of Calcium Silicate Hydrate, the Nanobrick of Cement
2018
Despite a millennial history and the ubiquitous presence of cement in everyday life, the molecular processes underlying its hydration behavior, like the formation of calcium–silicate–hydrate (C–S–H), the binding phase of concrete, are mostly unexplored. Using time-resolved potentiometry and turbidimetry combined with dynamic light scattering, small-angle X-ray scattering, and cryo-TEM, we demonstrate C–S–H formation to proceed via a complex two-step pathway. In the first step, amorphous and dispersed spheroids are formed, whose composition is depleted in calcium compared to C–S–H and charge compensated with sodium. In the second step, these amorphous spheroids crystallize to tobermorite-typ…