Structure of supercritically dried calcium silicate hydrates (C–S–H) and structural changes induced by weathering

The nanostructure of supercritically dried calcium silicate hydrates was researched. This particular drying procedure was used to avoid nanostructure modifications due to conventional drying processes. Thus, in this study, the as-precipitated cementitious C–S–H structure was obtained for the first time. A specific surface area 20 % larger than conventionally dried C–S–H was measured. Given the importance of this nanostructured phase for the properties of hydrated cements, especially when in contact with CO₂-rich environments, the supercritically dried C–S–H was weathered for 2 weeks. The structural effects of this weathering process on the C–S–H were researched and calcium carbonate microcrystal precipitation or the presence of silica by-product are reported. Calcite and aragonite polymorphs were observed, as well as nanoporous silica forming globular arrangements. In addition, 2 weeks of weathering was not enough to carbonate the entire C–S–H sample.     

Microindentation creep of monophasic calcium–silicate–hydrates

Microindentation creep results for monophasic synthetic C–S–H (C/S = 0.6–1.5), 1.4 nm tobermorite, jennite and calcium hydroxide at 11%RH are reported. Creep results for well hydrated cement paste and C₃S ‘composite’ systems are also described. The significance of the co-linear behavior of creep modulus functions of indentation modulus and indentation hardness for C–S–H obtained by microindentation and nanoindentation methods is discussed. The porosity dependence of creep modulus and the general equivalence of density values determined by helium pycnometry and by calculations employing unit cell dimensions (obtained using X-ray crystallography techniques) are also discussed in terms of postulates for the existence of two types of C–S–H. Comment on the compatibility of the creep modulus data for 1.4 nm tobermorite and jennite with models of C–S–H present in cement paste is provided.     

Molecular dynamics simulation of the nonlinear behavior of the CNT-reinforced calcium silicate hydrate (C–S–H) composite

Calcium–Silicate–Hydrate (C–S–H), which is the major constituent of the cement at the nanoscale, is responsible for the strength and fracture properties of concrete. This research is dedicated to the numerical study of enhanced mechanical properties of C–S–H reinforced by embedding carbon nanotube (CNT) in its molecular structure. Series of molecular dynamics (MD) simulations indicate that the tensile strength of CNT-reinforced C–S–H is substantially enhanced along the direction of CNT as compared to the pure C–S–H. The results of tensile loading reveal that CNT can efficiently bridge the two sides of cracked C–S–H. In addition, CNTs can severely intensify the “transversely isotropic” response of the CNT-reinforced C–S–H. Furthermore, the pull-out behavior of CNT reveals that the force-displacement response can be estimated by a bilinear model, which can later be used for simulation of cohesive crack propagation and multiscale simulation of crack bridging at macro scale specimen of CNT-reinforced cement.     

Mechanical properties of calcium silicate hydrate (C–S–H) at nano-scale: A molecular dynamics study

Calcium silicate hydrate (C–S–H) is the most important binding phase of cement paste and can determine the mechanical properties of cementitious material. The multi-scale nature of C–S–H gel brings about challenges in interpreting the mechanical properties of C–S–H. C–S–H models at the nano scales are commonly formulated by the aggregation of colloids that consist of layered molecular structures. In this study, the mechanical properties of C–S–H were investigated at the molecular level (∼5 nm). Uniaxial tension testing in the x, y, z directions was applied to the layered structure of C–S–H. The structural difference in the layer leads to heterogeneous mechanical properties in three directions. Particularly in the z direction, the C–S–H layer connected by H-bonds and Ca²⁺, has the weakest tensile strength. Resulting from the investigation, it can be concluded that the simulation can provide molecular insights into the linkage between molecular and larger scale structures.     

Effect of temperature on C–S–H gel nanostructure in white cement

Different ages of white cement pastes hydrated at 100 % RH and 25 or 65 °C were characterised with ²⁹Si MAS NMR spectroscopy. The findings showed that raising the curing temperature from 25 to 65 °C accelerated hydration of the belite phase considerably, inducing a sixfold rise in its one day degree of hydration, while alite phase hydration grew by a factor of only 1.5 in the first day. Moreover, the C–S–H gel formed at the higher temperature had a longer mean chain length and a higher initial uptake of Al³⁺. Lastly, curing at a higher temperature stabilised only one crystalline aluminate phases, calcium hemicarboaluminate.     

Effect of calcium–silicon ratio on microstructure and nanostructure of calcium silicate hydrate synthesized by reaction of fumed silica and calcium oxide at room temperature

A C–S–H series with calcium–silicon ratio 0.6–3.0 was synthesized by pozzolanic reaction. Phase composition, nanostructural and morphological characteristics were determined using XRD, XRF, SEM and ²⁹Si NMR. Most of the samples were phase-pure, poorly crystalline C–S–H. Significant changes in the nanostructure of the C–S–H samples were observed when the calcium–silicon ratio reached values of 0.8, 1.0 and 1.5. At calcium–silicon ratio 0.8 the basal XRD peak began to develop, crosslinking between layers was seen below this ratio but not above, and there was a substantial decrease in mean silica chain length at this ratio. At calcium–silicon ratio 1.0 there was a pronounced microstructural change from granular to reticular and another substantial decrease in mean chain length (indicated by an abrupt increase in the Q¹ peak intensity and decrease in the Q² peak intensity). At calcium–silicon ratio 1.5 the basal XRD peak began to diminish again, the mean silica chain length decreased further, and isolated tetrahedra (Q⁰) were observed.     

Nano-mechanical characterization of synthetic calcium–silicate–hydrate (C–S–H) with varying CaO/SiO2 mixture ratios

Synthetic calcium silicate hydrate (C–S–H) made with calcium to silicate (C/S) mixture ratios of 0.9, 1.2 and 1.5 respectively is characterized. C–S–H was produced by extracting calcium oxide (CaO) from calcium carbonate (CaCO₃) and then mixing it with micro-silica (SiO₂) and deionized water to make slurry. The slurry was continuously mixed for 7 days, then the excess water was removed and thermo gravimetric analysis (TGA) was conducted. The drying method was equilibrated to 11% relative humidity (RH). The stoichiometric formula of the synthetic C–S–H were approximated as C_0.7SH_0.6, C_1.0SH_0.8 and C_1.2SH_2.4 for C/S mixture ratios of 0.9, 1.2 and 1.5 respectively. The dried powders were characterized using X-ray diffraction analysis (XRDA), and ²⁹Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The powders were also compacted with 95 MPa pressure and nanoindentation of the compacted specimens were then undergone to mechanically characterize the synthetic C–S–H. The experiments provide insight on the nanoscale mechanical characteristics of C–S–H.     

Insights into the mechanisms of nucleation and growth of C–S–H on fillers

A complete understanding of the mechanisms upon which a filler acts in a cement-based material, e.g. as a C–S–H nucleation and/or growth-inducing factor, is of high importance. Although various studies report on accelerated cement hydration in the presence of fillers, the reason behind these observations is not completely understood yet. This work contributes to this subject, by providing an experimental evidence on the (electro) chemical aspects of the filler surface modification in the model solution, simulating the pore solution of cement paste. The nature of the various interactions with regard to the affinity of a filler surface towards C–S–H nucleation and growth was discussed in detail in this work with regard to zeta potential measurements of micronized sand and limestone particles in the model solutions. These results are further supported by microscopic observations of morphology and distribution of hydration products on the filler surfaces, together with considerations on thermodynamic principles in view of hydration products formation and distribution. The C–S–H nucleation and growth appeared to be due to the interactions between a filler surface and calcium ions in the pore solution. These interactions were determined by the chemical nature of the filler surface. The interaction mechanisms were found to be governed by relatively weak electrostatic forces in the case of micronized sand. This was reflected by a non-significant adsorption of calcium ions on the filler surface, resulting in non-uniformly distributed and less stable C–S–H nuclei. In contrast, the nucleation and growth of C–S–H on limestone particles were predominantly determined by donor–acceptor mechanisms, following moderate acid–base interactions. Consequently, a strong chemical bonding of calcium ions to a limestone surface resulted in a large amount of uniformly distributed C–S–H nuclei.     

Circular patterns of calcium oxalate monohydrate induced by defective Langmuir–Blodgett film on quartz substrates

The defective Langmuir–Blodgett (LB) film of dipalmitoylphosphatidylcholine (DPPC) on quartz injured by potassium oxalate (K₂C₂O₄) was used as a model system to induce growth of calcium oxalate crystals. Atomic force microscopy (AFM) indicated that circular defective domains with a diameter of 1–200 μm existed in the LB film. Scanning electron microscopy (SEM) showed circular patterns of aggregated calcium oxalate monohydrate (COM) crystallites were induced by these defective domains. It was ascribed to that the interaction between the negatively-charged oxalate ions and the phosphatidyl groups in DPPC headgroups makes the phospholipid molecules rearranged and exist in an out-of-order state in the LB film, especially at the boundaries of liquid-condensed (LC)/liquid-expanded (LE) phases, which provide much more nucleating sites for COM crystals.     

Evaluation of the combined deterioration by freeze–thaw and carbonation of mortar incorporating BFS,limestone powder and calcium sulfate

The durability performance of cementitious material is traditionally based on assessing the effect of a single degradation process. However, this study investigates the coupled deterioration properties of mortar incorporating industrial solid waste—ground granulated blast furnace slag (BFS) and different mineral admixtures, such as calcium sulfate (CS) and limestone powder (LSP). The combined deterioration properties caused by carbonation and frost damage in the mortar sample were experimentally investigated with respect to accelerated carbonation and freeze–thaw tests. Different degrees of deterioration, i.e. after subjected to 12, 30 and 60 freeze–thaw cycles, were induced in the freeze–thaw tests. The experimental investigation of single degradation revealed that the compressive strength, frost resistance and carbonation resistance decrease as the BFS replacement ratio increases by weight from 0 to 45%. The less amount of CH in the BFS cement leads to the carbonation progress more easily. Moreover, to achieve the same strength as ordinary Portland cement, 2 wt% CS and 4 wt% LSP in the BFS mortar are required. However, the data shows that incorporating LSP into the BFS mortar produces a lower frost resistance. The combined damage tests revealed that different deterioration degrees resulting from 12, 30 and 60 freeze–thaw cycles slightly decreased the carbonation resistance, which is related to the decrease in the inkbottle pore volume due to its water retention characteristics. Simultaneously, the pre-carbonation deterioration could effectively decrease the surface mass scaling of the freeze–thaw and the pore structure undergoes densification due to pre-carbonation.     

Structure and micro-nanomechanical characterization of synthetic calcium–silicate–hydrate with Poly(Vinyl Alcohol)

The principal phase of hardened Portland cement pastes is calcium silicate hydrate (C–S–H), which influences the physical and mechanical properties of construction materials. In this work calcium silicate hydrate (C–S–H) was synthesized, with the addition of Poly(Vinyl Alcohol) (PVA), for the development of C–S–H/polymer nanocomposites. Among the polymers available, PVA is indicated by the literature as one of the most viable for producing C–S–H/polymer complexes. However, no consensus exists regarding the kind of interaction. The resulting compounds were characterized by XRD, FT-IR, TGA, carbon content (CHN), TEM, SEM and elastic modulus and hardness were measured by instrumented indentation. The set of results presented do not confirm the intercalation of PVA in the interlayer space of C–S–H, but presented evidence of the potential for intercalation, since changes in the structure clearly occurred. A significant change in the micro-nanomechanical properties of C–S–H occurred in the presence of PVA.     

Preliminary investigation of novel bone graft substitutes based on strontium–calcium–zinc–silicate glasses

Bone graft procedures typically require surgeons to harvest bone from a second site on a given patient (Autograft) before repairing a bone defect. However, this results in increased surgical time, excessive blood loss and a significant increase in pain. In this context a synthetic bone graft with excellent histocompatibility, built in antibacterial efficacy and the ability to regenerate healthy tissue in place of diseased tissue would be a significant step forward relative to current state of the art philosophies. We developed a range of calcium-strontium-zinc-silicate glass based bone grafts and characterised their structure and physical properties, then evaluated their in vitro cytotoxicity and in vivo biocompatibility using standardised models from the literature. A graft (designated BT109) of composition 0.28SrO/0.32ZnO/0.40 SiO(2) (mol fraction) was the best performing formulation in vitro shown to induce extremely mild cytopathic effects (cell viability up to 95%) in comparison with the commercially available bone graft Novabone (cell viability of up to 72%). Supplementary to this, the grafts were examined using the standard rat femur healing model on healthy Wister rats. All grafts were shown to be equally well tolerated in bone tissue and new bone was seen in close apposition to implanted particles with no evidence of an inflammatory response within bone. Complimentary to this BT109 was implanted into the femurs of ovariectomized rats to monitor the response of osteoporotic tissue to the bone grafts. The results from this experiment indicate that the novel grafts perform equally well in osteoporotic tissue as in healthy tissue, which is encouraging given that bone response to implants is usually diminished in ovariectomized rats. In conclusion these materials exhibit significant potential as synthetic bone grafts to warrant further investigation and optimisation.     

Effect of pyrophosphate ions on the conversion of calcium–lithium–borate glass to hydroxyapatite in aqueous phosphate solution

The conversion of glass to a hydroxyapatite (HA) material in an aqueous phosphate solution is used as an indication of the bioactive potential of the glass, as well as a low temperature route for preparing biologically useful materials. In this work, the effect of varying concentrations of pyrophosphate ions in the phosphate solution on the conversion of a calcium–lithium–borate glass to HA was investigated. Particles of the glass (150–355 μm) were immersed for up to 28 days in 0.25 M K₂HPO₄ solution containing 0–0.1 M K₄P₂O₇. The kinetics of degradation of the glass particles and their conversion to HA were monitored by measuring the weight loss of the particles and the ionic concentration of the solution. The structure and composition of the conversion products were analyzed using X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy. For K₄P₂O₇ concentrations of up to 0.01 M, the glass particles converted to HA, but the time for complete conversion increased from 2 days (no K₄P₂O₇) to 10 days (0.01 M K₄P₂O₇). When the K₄P₂O₇ concentration was increased to 0.1 M, the product consisted of an amorphous calcium phosphate material, which eventually crystallized to a pyrophosphate product (predominantly K₂CaP₂O₇ and Ca₂P₂O₇). The consequences of the results for the formation of HA materials and devices by the glass conversion route are discussed.     

Nanomechanical investigation of the effects of nanoSiO2 on C–S–H gel/cement grain interfaces

In this paper, nanoindentation and viscoelastic modulus mapping were employed to study the influence of nanoSiO₂ on the properties of the interface between C–S–H gel and cement grains. The interface width measured by modulus mapping was around 200 nm as compared to a rough estimation of less than 5 μm by nanoindentation, due to the fact that 2 orders of magnitude increase in spatial resolution can be achieved with modulus mapping. Although the influence of nanoSiO₂ on the interface width was not significant, its impact on nanomechanical properties of the interface was marked. The data suggest an improvement of modulus and hardness of the interface by nanoSiO₂ in early age. This interface, which could be regarded as a layer surrounding cement grains, become denser by the addition of nanoSiO₂.     

In situ chemical modification of C–S–H induced by CO<Subscript>2</Subscript> laser irradiation

Fire-induced compositional changes lead to strength loss and even failure in cement and concrete. Calcium silicate hydrate (C–S–H) gel, the main product of cement hydration, dehydrates at 25–200 °C, while temperatures of 850–900 °C alter its structure. A Raman spectroscopic study of the amorphous and crystalline phases forming after CO₂ laser radiation of cement mortar showed that C–S–H dehydration yielded tricalcium silicate at higher, and dicalcium silicate at lower, temperatures. Post-radiation variations were identified in the position of the band generated by Si–O bond stretching vibrations.     

Effect of the POSS–Polyimide nanostructure on its mechanical and electrical properties

Nanocomposite films consisted of Polyhedral Oligomeric Silsesquioxane (POSS) filler in a Polyimide (PI) matrix were prepared. The effect of the nanocomposites’ structure on its mechanical and electrical properties was evaluated with respect to survival in the low Earth orbit (LEO) environment. The POSS–PI structure consists of POSS nano-aggregates formed in the bulk and on the surface. The aggregates’ size and distribution are POSS content-dependant. The fracture mechanism during hypervelocity impact at extreme temperature conditions was studied. The hypervelocity impacts of the POSS–PI films result in a brittle fracture, compared to ductile fracture in the case of PI, and in formation of radial cracks. A model based on formation and coalescence of voids around the aggregates, when load is applied, is suggested to explain the effect of the POSS content on the POSS–PI fracture mechanism. The size and density of the POSS aggregates also affect the nanocomposite’s volume electrical resistivity. An inverse dependence exists between the POSS aggregates’ surface density and the nanocomposites’ volume electrical resistivity.     

Finite element modeling of nanoindentation on C–S–H: Effect of pile-up and contact friction

In this paper we computationally study the indentation response of a rigid axisymmetric indenter on a semi-infinite elasto-plastic material of the Mohr–Coulomb type. The finite element method is used to quantify the effect of material properties (E, c, φ) and contact friction (μ) on the indentation response of C–S–H phases. The high E/c-ratio for both C–S–H phases, together with their cohesive-frictional behavior, leads to pile-up phenomena around the penetrated probe. The influence of all these parameters on the actual area of contact and its subsequent effect on commonly extracted quantities of the indentation test, namely indentation modulus (M) and indentation hardness (H), is investigated. It is shown that contact friction affects the contact area between the indenter and indented material and as a consequence interferes, to a certain extent, with the procedure for estimating elastic and plastic material properties. The effect is more pronounced for the hardness measurements.