Tobermorite Structure Research Articles

Molecular dynamics study on the structure and mechanical properties of tobermorite

The calcium silicate hydrate (C-S-H) structure has been indicated its atomic structure in the early stage of hydration resembles the geometry of tobermorite which is a natural calcium silicate hydrate. The mechanical properties of tobermorite crystals were investigated based on molecular dynamics (MD) method in this study. Uniaxial tension simulation in the z-direction was performed to the layered structure of tobermorite 9 Å, 11 Å and 14 Å at molecular level. The stress–strain relationships were obtained, and the structure analysis of tobermorite was carried out in terms of the relative concentration profile and mean square displacement. It has been found that tobermorite 9 Å has the greatest tensile strength compared with tobermorite 11 Å and 14 Å. It also shows that strong interaction between interlayer Ca ions and silicon-oxygen tetrahedra will increase the resistance of tensile loading. These findings may contribute to have a deep knowledge of the intrinsic property of C-S-H.

Hydrothermal solidification of underground construction wastes into building materials: Waste slurry recycling, industrial application and evaluation

A novel hydrothermal technology was developed to recycle underground construction wastes into tobermorite-based building materials and immobilize polyvalent Cr. The hardening mechanism involved in tobermorite synthesis and immobilization of Cr were studied by various techniques, including XRD, SEM, STA, XPS and ICP-OES. A simulated waste slurry was converted hydrothermally to well-crystalline tobermorite for both promoting strength and structurally accommodating Cr. A higher curing temperature and longer time facilitated the tobermorite formation, with a high proportion of tobermorite (∼93.67 wt%) being obtained at 200 °C for 24 h at a Ca/Si molar ratio of 0.6. Along with tobermorite formation, the in-situ immobilization of Cr in the tobermorite structure was also examined. Cr(Ⅲ) was found to replace the Ca2+ in the Ca–O octahedron to form Cr–O bonds or connect with H2O in the interlayer as Cr–OH bonds; whereas Cr(Ⅵ) groups linked with Ca2+ in the interlayer as stable CaCrO4, which adversely affected tobermorite formation. Both Cr(Ⅲ) and Cr(Ⅵ) had very low leaching rates from the solidified specimens, although the lower leaching rate of Cr(Ⅲ) (∼4.05% of 5000 ppm) compared to Cr(Ⅵ) (∼7.23% of 5000 ppm) confirmed that tobermorite had a stronger immobilization capacity for Cr (III). An actual waste slurry was used as the dominant raw material (∼70 wt%) for the hydrothermal synthesis of a tough tobermorite-based building material with a flexural strength (∼25.3 MPa), which was even higher than that of common concrete materials. The hydrothermal technology developed here has been successfully applied to the industrial production of standard bricks containing 50–70 wt% waste slurry that were fully compliant with Chinese standards. Life cycle assessments revealed that the hydrothermal slurry-recycled bricks effectively reduced environmental impacts by ∼13%–152% compared to similar building products, thereby demonstrating economic benefits and sustainability.

Composition, nanostructure and stability of Cu-modified C-A-S-H in antibacterial alkali-activated slag

Antimicrobial alkali-activated slag (AAS) has been envisioned as a biologically resistant binder for potential application in sewer structures against biogenic sulfuric acid attacks. This work investigates the composition, phase assemblage, micro- and nano-structural alterations of AAS binders incorporating Cu-based antimicrobial agents (i.e., copper oxide and copper nitrate), as well as their antimicrobial properties and degradation mechanism under biological corrosion caused by sulphur-oxidising bacteria Acidithiobacillus thiooxidans. The results show that the incorporation of copper oxide or copper nitrate increases the degree of hydration of slag but reduces the mean chain length (MCL) of C-A-S-H as Cu preferably affects the bridging AlO4-tetrahedra over SiO4-tetrahedra. The substitution of Cu for the CaO polyhedral at the edges and corners of the C-A-S-H main chain makes the Al linkage blocked. Cu also possesses a charge-balancing role, resulting in a lower protonation of C-A-S-H in the Cu-doped AAS than that in the neat AAS, where C-A-S-H is a Tobermorite structure with MCL between 5 and 11. The Cu-doped AAS pastes show excellent antimicrobial efficiency, in particular, the CuO-doped AAS demonstrates 100 % effectiveness in inhibiting biofilm formation, with the corrosion depth significantly reduced by a factor of 10. The Cu ion is released initially from the outer surface of C-A-S-H due to its low bond dissociation energy, followed by the weak position, namely the AlO4-tetrahedra at the bridging position, rendering the rupturing of the C-A-S-H structure.

Strength and Electrical Properties of Cementitious Composite with Integrated Carbon Nanotubes

The main objective of this work was to study the effects of carbon nanotubes (CNTs) on the strength and electrical properties of cement mortar. Molecular dynamic simulations (MDSs) were carried out to determine the mechanical and electrical properties of a cementitious composite and its associated mechanisms. To model the atomic structure of a calcium silicate hydrate (C-S-H) gel, tobermorite 11Å was chosen. Single-walled carbon nanotubes (SWCNTs) embedded in a tobermorite structure were tested numerically. In particular, it was concluded that a piezoelectric effect can be effectively simulated by varying the concentration levels of carbon nanotubes. The deformation characteristics were analyzed by subjecting a sample to an electrical field of 250 MV/m in the z-direction in a simulation box. The results indicated a progressively stronger converse piezoelectric response with an increasing proportion of carbon nanotubes. Additionally, it was observed that the piezoelectric constant in the z-direction, denoted by d33, also increased correspondingly, thereby validating the potential for generating an electrical current during sample deformation. An innovative experiment was developed for the electrical characterization of a cementitious composite of carbon nanotubes. The results showed that the electrostatic current measurements exhibited a higher electric sensitivity for samples with a higher concentration of CNTs.

Effects of Ca2+ →Mg2+ substitution on the properties of cementitious tobermorite

Using density-functional-theory calculations we assess the effects of the isovalent ${\mathrm{Ca}}^{2+}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}{\mathrm{Mg}}^{2+}$ substitutions on the mineral structure of tobermorite, a major analog of the main hydrated cement phase, calcium-silicate-hydrate (CSH). From the structural point of view, due to its smaller ionic radius, Mg substitution leads to an overall decrease of the lattice parameters for the 9 \AA{} (dry) and 11 \AA{} (hydrated) tobermorite structures used in the study. Furthermore, Mg doping at intralayer sites leads to a considerable distortion of the unit cell due to a changing oxygen coordination number. With the increasing amount of Mg doping, chemical bond analysis shows an overall increase in the bond strength with the crystal cohesion enhancement. In addition, Mg sites in the dry and hydrated tobermorite are characterized by an enhanced reactivity, which may be useful in capturing $\mathrm{C}{\mathrm{O}}_{2}$. With regard to the elastic properties, Mg doping leads to an overall stiffening of the elastic moduli. However, depending on the particular site, Mg doping may give rise to an increased elastic anisotropy. Overall, the present results indicate that Mg-based tobermorite structures may be useful prospects in the search for alternative components toward the development of high-performance, environmentally friendly cementitious materials.

Local structure and Ca/Si ratio in C-S-H gels from hydration of blends of tricalcium silicate and silica fume

At the microscale, C-S-H gels from alite, or neat Portland cements, has a Ca/Si ratio close to 1.80. At the nanoscale, C-S-H is described by a defective tobermorite structure which allows a maximum Ca/Si ratio close to 1.40. There is no agreement in the location of the extra 0.40 mol of Ca(OH)2 at the nanoscale. Atomistic modelling studies reported Ca(OH)2 species within the tobermorite interlayer space. Other works point toward a fine intermixing of defective tobermorite and nanoportlandite. Here, we have prepared a series of alite blended with silica fume and studied the pastes by several techniques including synchrotron pair distribution function (PDF). In the employed conditions, the C-S-H gel formed by the pozzolanic reaction has nearly the same local structure than the primary C-S-H gel. Furthermore, differential PDF points toward Ca(OH)2 excess having a local structure compatible with few-layer thick nanoparticles stretched along the c-axis.

Reaction mechanisms of alkali-activated materials

abstract: The Alkali-Activated Materials (AAM) are defined as materials obtained through the reaction between precursors and activators, and are separated into two classes depending on the products formed in the reaction, those rich in calcium, as the blast furnace slag, whose Ca/(Si+Al) ratio is higher than 1; and poor in calcium, which is the geopolymers subclass. In this review article, some bibliographical aspects were discussed regarding the discovery of these materials, through research conducted by Victor Glukhovsky and through the characterization of historical monuments by Davidovits, which began in the 50s and 60s and persist to the present day. The main products obtained in the alkaline activation reaction were also addressed, using the definition of polysialates and zeolites, in the case of geopolymers, and the tobermorite structure, in the case of materials rich in calcium. The main steps of the alkali-activated reaction, such as dissolution, condensation, polycondensation, crystallization, and hardening, were discussed. Some techniques for characterizing the AA reaction products were also examined, such as X-ray diffraction (XRD), nuclear magnetic resonance spectrometry (NMR), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Finally, the main factors that interfere in the kinetics of AA reactions were explored, in which the type of cure and the activating solution used in the alkali-activated materials production stands out.

Effect of Different Aluminum Substitution Rates on the Structure of Tobermorite.

Tobermorite and Al-substituted tobermorite with varying Al/(Si + Al) molar ratios were synthesized via the static hydrothermal method. The effect of different Al substitution ratios on the structure of tobermorite was investigated by XRD, SEM–EDS, TG–DSC, FT–IR, 29Si, and 27Al MAS–NMR. The interplanar crystal spacing of (002) increased with the increment of the Al substitution ratio. The maximum ratio of the Al substitution for Si in the structure of tobermorite was about 15 at %, which was limited by the compositions of tobermorite and the electrostatic repulsive forces caused by Al substitution. The DSC curve of the tobermorite with an Al/(Al + Si) molar ratio of 0.15 showed a larger exothermic peak area at 850 °C, indicating that the Al-O-Si bond was formed in the structure of tobermorite and the Al participated in the recrystallization. The 29Si and 27Al MAS–NMR results confirmed that tetrahedral Al was introduced into the structure of the tobermorite. New silicon sites, Q2(1Al) and Q3(1Al), formed by the Al substitution for Si in bridging and non-bridging sites, were detected. Compared to the tobermorite without Al substitution, the adsorption capacity of Al-substituted tobermorite with Al/(Al + Si) = 0.15 to Na+ was improved by 44%.

Nano-engineering of the mechanical properties of tobermorite 14 Å with graphene via molecular dynamics simulations

The tensile and shear properties of tobermorite 14 Å-based structures reinforced with a single graphene sheet (GS) interfacing with either the surface water, octahedral calcium, or tetrahedral silicate layer were investigated using molecular dynamics simulations. The GS contributed to a significant increase in the XY-plane tensile (180%–360%) and shear (90%–225%) strengths, stiffness, and toughness while degrading the out-of-plane properties of the tobermorite structures. The water interface promoted molecular friction (intermolecular forces) and resulted in greater in-plane fracture tensile strength and toughness but a lower shear toughness than when the GS interfaced with the dry solid surfaces.

Realization of Mechanical Properties Prediction from Nano- to Macro- Scale Structure: An Achievement of C-S-H Hydrated Phases

The performance prediction of C-S-H gel is critical to the theoretical research of cement-based materials. In the light of recent computational material technology, modeling from nano-scale to micro-scale to predict mechanical properties of structure has become research hotspots. This paper aims to find the inter-linkages between the monolithic "glouble" C-S-H at nano-scale and the low/high density C-S-H at the micro-scale by step to step method, and to find a reliable experimental verification method. Above all, the basic structure of tobermorite and the "glouble" C-S-H model at nano-scale are discussed. At this scale, a "glouble" C-S-H structure of about 5.5 nm3 was established based on the 11Å tobermorite crystal, and the elastic modulus ​​of the isotropic "glouble" is obtained by simulation. Besides, by considering the effect of porosity on the low/high density of the gel morphology, the C-S-H phase at micro-scale can be reversely characterized by the "glouble". By setting different porosities and using Self-Consistent and Mori-Tanaka schemes, elastic moduli of the low density and high density C-S-H ​​from that of "glouble" are predicted, which are used to compare with the experimental values of the outer and inner C-S-H. Moreover, the nanoindentation simulation is carried out, where the simulated P-h curve is in good agreement with the accurate experimental curve in nanoindentation experiment by the regional indentation technique(RET), thus the rationality of the "glouble" structure modeled is verified and the feasibility of Jennings model is proved. Finally, the studies from the obtained ideal "glouble" model to the C-S-H phase performance has realized the mechanical properties prediction of the C-S-H structure from nano-scale to micro-scale, which has great theoretical significance for the C-S-H structural strengthening research.

Advanced Rietveld refinement and SEM analysis of tobermorite in chemically diverse autoclaved aerated concrete

Changes of structural properties of tobermorite in autoclaved aerated concrete (AAC) for various compositions were characterized and the disadvantages of SEM analysis in this context are discussed. The influence of variations in the chemical composition of raw materials on lattice parameters, morphology and domain sizes of tobermorite was investigated by XRD and for comparison by SEM analysis. Particularly the effect of substitution by Al3+ and (SO4)2− in tobermorite structure was examined. The dimensions of coherently scattering domains were calculated based on the refinement of anisotropic peak broadening of tobermorite in XRD diffractograms using a Rietveld compatible approach. No effect of (SO4)2− on the domain sizes and lattice parameters of tobermorite could be observed. The amount of anhydrite detected by quantitative XRD analysis indicates that all of the available (SO4)2− is present as anhydrite. Lath-like shapes of domains and a larger c parameter are calculated whenever Al3+ is incorporated in a considerable amount. Formation of katoite can be observed very clearly in SEM micrographs whenever the amount of available Al3+ exceeds a distinct value in the dry mix. The effect of Al3+ and (SO4)2− on tobermorite morphology could not be observed clearly by SEM analysis in AAC samples.

Intermolecular dynamics of ultraconfined interlayer water in tobermorite: influence on mechanical performance.

Ultraconfined interlayer water within the tobermorite molecular structure is responsible for changes in the uniaxial tensile and compressive response of the family of tobermorites: 9, 11 and 14 Å. These confined interlayer water molecules are engaged in solvation of cations and anions within the tobermorite structure, which has been demonstrated through the intermolecular vibrational spectra and hydrogen bond lifetime of the water molecules. This study demonstrates that instead of ionization of water molecules (as proposed in an earlier study), breaking of hydrogen bonds of water is more plausible leading to solvation of ions within the molecular structure of tobermorite. A schematic of the coordinate covalent bonds between the water molecules and the cations and anions of the tobermorite structure has been proposed in this study.

Adsorption mechanisms of cesium at calcium-silicate-hydrate surfaces using molecular dynamics simulations

Understanding the mechanisms of interactions and immobilization of radionuclides in cement systems at the molecular level is key to developing safe and novel solutions for the long-term storage of nuclear wastes. The molecular level adsorption mechanisms of cesium (Cs+) ions onto tobermorite 9 Å, tobermorite 14 Å, and jennite – three crystalline structural analogues for calcium-silicate-hydrates, the main components in cement-based materials, were investigated using molecular dynamics (MD) simulations. Convergence monitoring of the simulations, using the root mean square displacement with average correlation analysis, indicated that MD trajectories of 10–15 ns were needed given the size of the selected computational cell to properly capture the dynamic process of adsorption. Cs+ ions adsorbed as inner-sphere complexes at the tobermorite surfaces, while they showed lower affinity for jennite. The charge and structure of the surface layer strongly influenced the adsorption mechanisms of Cs+ ions. Strong association with the hexagonal cavities of the silica sites were seen at the tetrahedral SiO4 surface of the tobermorite structures. Cs+ ion adsorption was attributed to co-ion adsorption at the octahedral CaO6 surface of tobermorite 9 Å, cation exchange at that of tobermorite 14 Å, and electrostatic interaction with the surface hydroxyl groups for jennite. Distribution coefficients of Cs+ ions onto cement pastes derived from molecular dynamics by taking the surface structure and dynamics of Cs+ ions directly into account showed a range of overlap with experimental results.

Nano-scale mechanical properties investigation of C-S-H from hydrated tri-calcium silicate by nano-indentation and molecular dynamics simulation

Calcium silicate hydrate (C-S-H) is the most important hydration product in cementitious material. Due to its complexity, the investigation on its mechanical properties is still in progress. In this study, synthesized C3S was utilized to produce relatively pure C-S-H gels with two different water/solid ratios. Then mechanical properties of the C-S-H were measured and characterized by nano-indentation and SEM/EDX/BSE. Two types of C-S-H with Ca/Si ratio around 1.8 were observed through statistical analysis of the indentation modulus (Er) and hardness (H). The looser one possessing lower Er around 25–30 GPa while the denser one possesses higher Er of 35.9 GPa. Both phases have higher stiffness than those of high density (HD) and low density (LD) phase obtained from cement pastes. Furthermore, molecular dynamics provides mechanical understanding of C-S-H gel at nano-scale. Based on the crystal structure of tobermorite 11 Å, 64 models with Ca/Si ratios from 1.3 to 2.0 were constructed for uniaxial tension test. The stiffness and cohesive strength of the C-S-H gel are greatly influenced by Ca/Si ratio, water content and nano-porosity. With increasing Ca/Si ratio from 1.3 to 2.0, long silicate chains were broken to shorter ones and more water molecules penetrated into defect regions. The destroyed silicate skeleton and the “hydrolytic weakening” effect resulted in the reduction of the mechanical properties of C-S-H gels. Concurrently considering the intrinsic porosity and chemical compositions, the appropriate micromechanical models are utilized to upscale the molecular properties to higher level.

Synthesis of Tobermorite Structure with Non-traditional Silica Components

The main aim of the article is research of kinetics of creation of calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH) phases in the autoclaved aerated concrete in dependence on type of raw materials. Regarding materials, this will be unique combinations of lime with fly ash, quartz sand or silica fume. The aim will be to prove the influence of ratio of individual types of materials on the CSH phase to determine optimal combinations of raw materials. The research focuses first on detailed analysis of the used materials. Next focus is kinetics of the creation of microstructures of silicate phases under ideal conditions of small-sized autoclave reactors (250 ml, 190 °C, 1.3 MPa). The produced samples are further analysed using of x-ray diffraction (XRD) and scanning electron microscopy (SEM) for qualitative valuation of created microcrystalline phases. The results show that the most suitable non-traditional silica component is ash from coal combustion. The optimal combination of amorphous and crystalline silicon in combination with Al 2 O 3 provided the highest proportion of synthesized tobermorite even in more economical autoclaving variants.

The influence of starting materials´ solubility on tobermorite structure formation under the hydrothermal conditions

Overall, the tobermorite mineral crystallises under the hydrothermal conditions from the CaO-SiO2-H2O system. The system of calcium hydrosilicates species is extremely complex and many factors play an important role. The solubility of starting materials belongs to one of the most important factors. Dissolution rates of SiO2 source are significantly affected by their particle size distribution. Also the starting cation´s concentration in the aqueous solution affects the yield of the hydrothermal reaction. The main aim of this contribution is to study the influence of water-to-solid ratio on tobermorite crystallization. The effect of particle size of various sources of SiO2 was also studied. The cost effective Ca and Si sources was primarily favoured. The C/S ratio of the experiments was set to 0.83. All syntheses were performed at 180 °C in laboratory steel autoclaves. The prepared powdered samples were characterized in order to determine the phase composition by using of X-Ray diffractometry. The thermal behaviour of prepared samples was studied by TG-DTA analyser. Morphology and particle size of synthetized powdered products was studied by scanning electron microscopy.

Formation of nanocrystalline tobermorite in calcium silicate binders with low C/S ratio

Pastes of calcium silicate binders with low CaO/SiO2 (C/S) ratio with an overall C/S molar ratio of 1.1 were produced by mixing these materials with water or with an aqueous solution of Na2SiO3. Both types of pastes (non-activated and Na2SiO3 activated) were analysed using X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). The observations show that the hydration process of this novel type of cements results in the formation of tobermorite nanocrystals embedded in the amorphous calcium-silicate-hydrate (C-S-H) matrix, both in the case of water and of water + Na2SiO3 hydration. Selected area electron diffraction (SAED) patterns obtained in the HR-TEM observations enabled a full characterization of the tobermorite crystalline structure (type, unit cell, space group, lattice angles and parameters). Molecular dynamics (MD) simulations of the identified tobermorite structure were performed at various temperatures, showing its viability and thereby supporting the experimental results. To the best of the authors' knowledge, this report is the first to describe the spontaneous formation of crystalline tobermorite in the form of nanodomains dispersed in the C-S-H phase upon the hydration of calcium-silicate binders.

Incorporation of Al and Na in Hydrothermally Synthesized Tobermorite

Calcium silicate hydrate and its Al‐substituted form synthesized by a hydrothermal process were investigated by X‐ray diffraction, compositional analysis, and magic‐angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, in order to determine the mechanism of Al and Na incorporation in the tobermorite structure with varying molar ratios of Ca/Si and Al/Si. At a high molar ratio of Ca/Si, the silicate chains of tobermorite are ruptured, the degree of polymerization of the silicate chains is lowered, and the high calcium concentration lowers the content of Na2O in the structure. Solid‐state 29Si and 27Al MAS NMR spectroscopy confirm that all Al atoms were incorporated in the silicate chains of tobermorite. The tetrahedrally coordinated Al (Al(IV)) could either act as the bridging tetrahedron () for the dreierketten chain of tobermorite, or be present in Q3 sites that link two dreierketten chains together. Therefore, the degree of polymerization of the silicate chains of tobermorite is increased at high molar ratio of Al/Si. Furthermore, the greater charge deficit due to the replacement of Si4+ by Al3+ ions is compensated by increased adsorption or binding of Na+.

Mechanical and hydration characteristics of autoclaved aerated concrete (AAC) containing iron-tailings: Effect of content and fineness

With the objective of reducing the negative impacts on environment and utilizing the secondary resource of tailings, the possibility of preparing AAC by using iron tailing was investigated. The bulk density and compressive strength were determined to indicating the feasibility of preparing AAC blocks from iron tailing. The morphology (FESEM-EDX), mineral constituent (XRD), thermal characteristics (TG-DSC) and crystal characteristics (29Si-NMR) of hydration products of AAC samples were analyzed to demonstrate the hydration characteristics of AAC products containing iron tailings effected by iron tailing content and fineness. The results indicated that the increasing content of iron tailing has negative effect on the mechanical property of AAC, and the finer of iron tailings can effectively enhance strength of AAC blocks. The main minerals in AAC products are C-S-H gel, tobermorite, anhydrite, hydrogarnet, and some residual minerals including quartz and calcite accompanied by ferric oxide and white mica in minor quantities. The increasing content of iron tailing obviously reduces the amount of calcium silicate hydrates; meanwhile, the finer of iron tailing accelerated the decomposition of white mica during the autoclaving process and has slight negative effect on crystallinity of tobermorite. It was also suggested that Al and Mg ions in iron tailings got into the structure of tobermorite during the hydrothermal reaction. This study provided the theoretical foundation for the utilizing of iron tailings in AAC production.

Molecular Dynamics Study of the Interaction of C-S-H and Chloride Ions

Molecular Dynamics was employed to investigate the interaction of calcium silicate hydrate (C-S-H), the primary hydration product of cement based materials, and chloride, causing severe durable problems of concrete. The 11Å tobermorite structure was chosen to describe the C-S-H structure and the CLAYFF force field was used. It is observed in the simulation that there are no bound chlorides at 303K, while a fraction of chlorides appear in the adsorption district of tobermorite/solution interface at 323K indicating the temperature increase can improve chloride sorption capacity of C-S-H. The formation of Ca-Cl cluster is found on the surface of tobermorite, which is assumed to promote the chloride sorption. The experimental results of sorption isotherms of C-S-H in CaCl2 and NaCl aqueous solutions with the same chloride concentration have proved this point. Other researchers have made the same conclusion by means of molecular dynamics modeling, NMR tests or zeta potential experiments.