Changes on the nanostructure of cementitius calcium silicate hydrates (C–S–H) induced by aqueous carbonation

The nanostructure of the main binding phase of the hydrated cements, the calcium silicate hydrates (C–S–H), and their structural changes due to aqueous carbonation have been characterized using TEM, nitrogen physisorption, and SAXS. Synthetic C–S–H has been used for this purpose. Two different morphologies were identified, similar to the high density and low density C–S–H types. When submitting the sample to a CO₂ flux, the low density phase was completely carbonated. The carbonation by-products, calcium carbonate, and silica gel were also identified and characterized. The precipitation of the silica gel increased the specific surface area from 95 to 132 m²/g, and its structure, formed by particles of ~5 nm typical radius, was observed by small angle X-ray scattering. In addition, the resistance of the high density C–S–H to carbonation is reported, and the passivating effect of the precipitated calcium carbonate is also discussed. Finally, the results have been compared with carbonation features observed in Portland cement carbonated experimentally at downhole conditions.     

C–S–H/polyaniline nanocomposites prepared by in situ polymerization

Synthesis and characterization of a new cement-based polymer nanocomposite is reported. Calcium silicate hydrate (C–S–H) was prepared in the presence of aniline monomer followed by in situ polymerization to increase the degree of interaction between inorganic and organic phases. Two stoichiometrically different C–S–H systems were used. The properties of the C–S–H/polyaniline materials were studied using several analytical techniques including SEM, XRD, TGA, ²⁹Si MAS NMR and FTIR. It is suggested that the in situ polymerization method can effectively be employed for producing a C–S–H/Polymer nanocomposite. The extent of molecular interaction with the polymer depends on the chemical composition of the C–S–H. Production of a new range of polymer-modified cement-based systems having improved environmental stability and mechanical performance is promising.     

Antagonistic effect of superplasticizer and colloidal nano-silica in the hydration of Alite and Belite pastes

The dependence on the hydration rate for Alite and Belite clinker phases in the presence of a polycarboxylate superplasticizer PC SP upon addition of colloidal nano-silica CNS were monitored by means of Diffuse Reflectance Infrared spectroscopy DR-FTIR. Spectral signatures of clinker dissolution and product formation were acquired for both materials. The rates for the build-up of product vibrational band intensities were found to depend sensitively on addition of CNS. The hydration product was proposed to be calcium-silicate-hydrate C–S–H. Details in the spectral signatures were found to differ. Quantum chemical calculations were employed and found to be consistent with the interpretation that small clusters dominate the Alite C–S–H spectrum, whereas the Belite C–S–H spectrum results from extended polymers.     

Influence of granulated blast furnace slag on the reaction,structure and properties of fly ash based geopolymer

Ground granulated blast furnace slag (GBFS) has been used to alter the geopolymerisation behaviour of fly ash. The influence of varying amount of GBFS (5–50%) on the reaction kinetics has been studied using isothermal conduction calorimetry. It was observed that the reaction at 27 °C is dominated by the GBFS activation, whereas the reaction at 60 °C is due to combined interaction of fly ash and GBFS. The reaction product of geopolymerisation has been characterised using X-ray diffraction and scanning electron microscopy–X-ray microanalysis. Alumino–silicate–hydrate (A–S–H) and calcium–silicate–hydrate (C–S–H) gels with varying Si/Al and Ca/Si ratio are found to be the main reaction products. Coexistence of A–S–H and C–S–H gel further indicates the interaction of fly ash and GBFS during geopolymerisation. Attempt has been made to relate the microstructure with the properties of the geopolymers.     

A comparative study of the volume stability of C–S–H (I) and Portland cement paste in aqueous salt solutions

Ordinary Portland cement (OPC) paste specimens and compacted C–S–H (I) powders were immersed in distilled water and aqueous salt solutions of varying concentration to study their volume change behavior. Immersion resulted in ionic interaction leading to various degrees of expansion, leaching, softening and dissolution of the test samples. In all cases relatively rapid expansion occurred. The expansion of C–S–H (I) in aggressive media was found to be large and fast in MgCl₂, MgSO₄, LiOH, LiNO₃ and calcium (magnesium) acetate (CaMgAc) solutions. LiCl, CaCl₂ and NaCl were moderately aggressive towards C–S–H (I) depending on the solution concentration. Trends in the length change behavior of OPC paste are similar to that of synthesized C–S–H (I). Similarities were observed between the length change behavior of compacted C–S–H (I) and the swelling of smectite clays in contact with these osmotic media. The similarities are compatible with explanations of expansion provided by both the osmotic and the electrical double layer (EDL) theories. The relationship between the expansive behavior of C–S–H and both Na and Ca Montmorillonite in contact with aqueous salt solutions is discussed extensively in the context of its significance in cement science.     

Influence of Si:Al ratio on the microstructural and mechanical properties of a fine-limestone aggregate alkali-activated slag concrete

Three series of fine limestone aggregate, alkali-activated blast furnace slag (AAS) concretes were fabricated and tested; two through activation with waterglass/NaOH solution, of which one included NaCl as a retarding agent, and one activated by Na₂CO₃. Each of these series was made up of three formulae containing different amounts of Al₂O₃. The compressive strengths of the series activated by waterglass/NaOH after 28 days were ≈65 ± 5.3 MPa, a 22% increase compared to previously reported formulae containing no additional Al₂O₃. Increasing the amount of Al₂O₃ did not further increase strength, however. The Na₂CO₃-activated formulae had strengths of ≈35 ± 3 MPa after 28 days, representing no increase in strength over formulae not containing Al₂O₃ previously reported. X-ray diffraction showed the main binding phase to be calcium silicate hydrate (C–S–H) gel, as is commonly found in ordinary Portland cement (OPC). Fourier transform infrared spectroscopy showed little difference from the previously reported results for formulae not containing Al₂O₃ and strongly resemble the spectra reported elsewhere for C–S–H. Electron microscopy, coupled with energy dispersive spectroscopy, showed the cementing phase to be a single homogenous phase—not a mixed system of geopolymer and C–S–H gel—with a lower volume fraction of unreacted slag than formulae without Al₂O₃. The reason for the increase in strength of Al₂O₃-containing formulae is unclear, but is unlikely to be ascribed to the formation of large amounts of ‘geopolymers’ and may be related to a possible increase in reaction temperature of between 2 and 5°C, depending on amount of additive.     

The hydration of slag,part 1: reaction models for alkali-activated slag

Reaction models are proposed to quantify the hydration products and to determine the composition of C–S–H from alkali-activated slags (AAS). Products of the slag hydration are first summarized from observations in literature. The main hydration products include C–S–H, hydrotalcite, hydrogarnet, AFm phases (C₄AH₁₃ and C₂ASH₈) and ettringite. Then, three stoichiometric reaction models are established correlating the mineral composition of slag (the glass part) with the hydration products. Using the proposed models, quantities of hydration products and composition of C–S–H are determined. The models are validated with a number of experimental investigations reported in literature, yielding good agreement, i.e., these models can successfully predict the hydration reaction of AAS. The models are furthermore applied to calculate the retained water in the hydration products of AAS in different hydration states and a general hydration equation of AAS is derived. As an illustration to one of the model applications, chemical shrinkage of the AAS cement paste in different hydration states are predicted. The chemical shrinkage of AAS is shown to be remarkably higher than OPC. Furthermore, phase distribution in the hardened AAS paste and the porosity are calculated.     

Mechanical properties of calcium silicate hydrates

The dynamic mechanical properties of compacted samples of synthetic calcium silicate hydrate (C–S–H) were determined at variable stoichiometries (C/S ratio). The stiffness and damping properties of the C–S–H systems were monitored at various increments of mass loss from 11%RH following the removal of the adsorbed and interlayer water. The changes in the storage modulus (E′) and internal friction (tan δ) were discussed in terms of the state of water present in the nanostructure of C–S–H, the evolution of the silicate structure and the interaction of calcium ions in the interlayer region. Results were compared to those for the hydrated Portland cement paste and porous glass. It was shown that the C–S–H in the hydrated Portland cement has a complex yet analogous dynamic mechanical behavior to that of the synthetic C–S–H. The response of these systems upon the removal of water was explained by a layered model for the C–S–H. A mechanistic model was proposed to describe the changes occurring at various stages in the dynamic mechanical response of C–S–H.     

Contribution of 1H combined rotation and multipulse spectroscopy nuclear magnetic resonance to the study of tricalcium silicate hydration

Hydration products of tricalcium silicate (C₃S) are the calcium silicate hydrates (C–S–H) and portlandite. Silica fume, added to anhydrous cement in industrial formulations, reacts with portlandite and leads to C–S–H slightly different from the previous one (this reaction is called the pozzolanic reaction). C₃S hydration at 120 °C with and without silica fume has been studied by two ¹H nuclear magnetic resonance techniques: combined rotation and multi-pulse spectroscopy (CRAMPS) and magic angle spinning (MAS). The static spectra are broadened by the proton–proton dipolar interaction. The ¹H MAS technique does not allow us to remove the effects of this interaction completely and cannot be in this case a quantitative method. Therefore the CRAMPS technique, which can remove the broadening of the interaction because of the use of a multipulse sequence associated with the sample rotation, was used. It is shown that the CRAMPS NMR spectra allow us to describe the action of silica fume on the hydrates and to reveal the competition between portlandite formation around the C₃S grains and portlandite dissolution near the silica grains until the complete dissolution of portlandite. This revised version was published online in November 2006 with corrections to the Cover Date.     

Drying shrinkage of cement paste as measured in an environmental scanning electron microscope and comparison with microstructural models

A recently developed image-intensity-matching technique has been used to analyse images of cement paste which were dried in an environmental scanning electron microscope. Shrinkage that occurs during changes in relative humidity is reported, together with some of the influences of water-to-cement ratio, temperature and age. Results from microstructurally based models are compared with experimental results. The best fit of models to experiment is achieved if calcium silicate hydrate (C–S–H) is divided into two types: high density C–S–H, which does not shrink, and low density C–S–H, which does shrink. Approximate values of unrestrained shrinkage of the low density C–S–H are attained as a function of relative humidity. This revised version was published online in November 2006 with corrections to the Cover Date.     

On the formation of cementitious C–S–H nanoparticles

This work explores from a theoretical viewpoint the underlying growth mechanisms which govern the formation of the most important hydration product present in cementitious environments, the so called C–S–H (calcium–silicate–hydrate) gel. Aiming at identifying the basic building blocks which make up the cementitious C–S–H gel, we have performed ab-initio calculations at the Hartree Fock (HF) level. Two different growth mechanisms have been identified depending on the amount of Si and Ca ions, which naturally lead to the appearance of tobermorite-like and jennite-like nano-crystals.     

Grafting of gold nanoparticles and nanorods on plasma-treated polymers by thiols

Grafting of gold nanoparticles and nanorods on the surface of polymers, modified by plasma discharge, is studied with the aim to create structures with potential applications in electronics or tissue engineering. Surfaces of polyethyleneterephthalate and polytetrafluoroethylene were modified by plasma discharge and subsequently, grafted with 2-mercaptoethanol, 4,4′-biphenyldithiol, and cysteamine. The thiols are expected to be fixed via one of –OH, –SH or –NH₂ groups to reactive places on the polymer surface created by the plasma treatment. “Free” –SH groups are allowed to interact (graft) with gold nanoparticles and nanorods. Gold nano-objects were characterized before grafting by transmission electron microscopy and UV–Vis spectroscopy. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and electrokinetic analysis (zeta potential determination) were used for the characterization of polymer surface at different modification phases. It was proved by FTIR and XPS measurements that the thiols were chemically bonded on the surface of the plasma-treated polymers, and they mediate subsequent grafting of the gold nano-objects. On the surfaces, modified polymers were indicated some objects by AFM, size of which was dramatically larger in comparison with that of original nanoparticles and nanorods. This result and the other results of UV–Vis spectroscopy indicate an aggregation of deposited gold nano-objects.     

Analysis of carbonate and sulphate attack on concrete structures

The current study was designed to explain the progress and causes of damage to structural concrete exposed to atmospheric CO₂ and rainwater (23-year old slab) and interactions of atmospheric CO₂, rainwater and gypsum (50-year old columns). The testing programme included SEM with EDXA and XRD analysis, and tests determining compressive strength and water absorption. During the carbonation process the basic product identified in the concrete microstructure was calcite or “secondary” calcite, which appeared at the surface of concrete, in the cracks or in the form of dripstone. The formation of ettringite and calcite was observed in the concrete columns exposed to interactions of gypsum, carbon dioxide and rainwater for 50 years. Ultimately, the only product present on the exterior of the concrete was calcite. This is attributed to the conversion of ettringite to calcite due to a direct reaction with CO₂. The test results were compared to the theoretical hypotheses.     

Silicone-modified cellulose. Crosslinking of cellulose acetate with poly[dimethyl(methyl-H)siloxane] by Pt-catalyzed dehydrogenative coupling

Cellulose acetate was reacted in different ratios with poly[dimethyl(methyl-H)siloxane] containing 25 mol% Si–H side groups along the chain. A dehydrocoupling reaction between Si–H and C–OH groups occurred in presence of Karstedt’s catalyst, leading to the formation of Si–O–C bond, as proved by FTIR spectra, thus crosslinking the cellulose derivative. The networks were processed as films by casting before the end of the reaction and were investigated by different techniques to emphasize the morphology, thermal, dielectric and surface properties developed in correlation with the ratio between the two involved components (cellulose and siloxane derivatives). A decrease of the dielectric constant values of cellulose acetate was noticed throughout the studied frequency and temperature range as a result of crosslinking.     

Atmospheric effects on the thermally induced sintering of nanoparticulate gold films

Gold (Au) films were formed by sintering of Au nanoparticles (NPs) under gas flows of air, oxygen (O₂), nitrogen (N₂), or N₂ bubbled through formic acid (FA/N₂). The microstructure changes of the Au nanoparticulate films were studied when different atmospheres were applied. The Au film sintered under FA/N₂ showed the progressive agglomeration and grain growth with porosity in the film, while the film sintered under N₂ had NPs without participating grain growth. A necking between NPs was observed in the film, however, unnecked NPs were still found. The Au film sintered under O₂ atmosphere showed the NPs agglomeration with various sizes up to 50 nm. X-ray characteristic peaks of the (111)-preferred orientation were observed in all samples. All samples showed N–H stretching at 3200–3300 cm⁻¹ regardless of sintering atmosphere. Hydrocarbon chains (C–H) at 2850–3000 cm⁻¹ were detected in the film sintered under N₂. For the Au film sintered under O₂, C–H stretching at 2850–3000 cm⁻¹, C–H deformation at 1350–1470 cm⁻¹, and C–O stretching at 1200–1300 cm⁻¹ were observed. C–O stretching at 1600–1700 cm⁻¹ was observed for the film sintered under FA/N₂ atmosphere. The electrical resistance of the film was related with microstructures and organic residual materials left in the film. Even though either porosity or carbon residues were observed in the film, the sintering of NPs in FA/N₂ or N₂ showed the sheet resistance comparable to that of electroplated one.     

Preliminary investigations of the phase composition and fine pore structure of super-critically carbonated cement pastes

Super-critical carbonation of cement-based materials can lead to significant improvements in their properties. Preliminary investigations suggested that processing should be aimed at producing a matrix material with minimal amounts of Ca(OH)₂, anhydrous material and C—S—H gel along with a controlled pore structure. Using ²⁹Si MAS NMR and TGA as the principal investigative techniques it has been shown that moisture content during carbonation is a major factor in determining the phase composition and pore structure of the resulting matrix. Of the drying regimes studied, 60% DOD gave the greatest amount of conversion to calcium carbonate and silica gel.     

Use of analytical techniques for identification of inorganic polymer gel composition

In the present experimental study, a number of analytical techniques were used to identify the composition of gel and thus elucidate to a certain extent the mechanisms involved during synthesis of inorganic polymers. The raw materials used, low calcium slag from a ferronickel plant and commercial glass, were alkali activated by Na₂SiO₃ and KOH solutions. X-ray diffraction (XRD) is used to identify new formed phases; deconvolution of the amorphous peaks in X-ray powder diffraction patterns enables the quantitative estimation of the amorphous phases present. The morphology and composition of the inorganic polymer gel may be defined by optical reflection microscopy (ORM) and scanning electron microscopy (SEM). Fourier transform infrared spectroscopy (FTIR) is a useful tool for the identification of specific molecular structures including Si–O–Si and Al–O bonds, which define the degree of polysialation. Thermogravimetric analysis (TG) determines water evaporation rates in inorganic polymer structures by recording the weight loss under controlled heating. Finally, the effect of the presence in the starting mixture of heavy metals such as Pb or Cu and anions such as NO₃ ⁻ or SO₄ ²⁻ on the quality of the gel formed and subsequently on the compressive strength of inorganic polymers are studied and discussed.     

Mineral carbonation for carbon sequestration in cement kiln dust from waste piles

Alkaline earth metals, such as calcium and magnesium oxides, readily react with carbon dioxide (CO₂) to produce stable carbonate minerals. Carbon sequestration through the formation of carbonate minerals is a potential means to reduce CO₂ emissions. Calcium-rich, industrial solid wastes and residues provide a potential source of highly reactive oxides, without the need for pre-processing. This paper presents the first study examining the feasibility of carbon sequestration in cement kiln dust (CKD), a byproduct generated during the manufacturing of cement. A series of column experiments were conducted on segments of intact core taken from landfilled CKD. Based on stoichiometry and measured consumption of CO₂ during the experiments, degrees of carbonation greater than 70% of the material's potential theoretical extent were achieved under ambient temperature and pressure conditions. The overall extent of carbonation/sequestration was greater in columns with lower water contents. The major sequestration product appears to be calcite; however, more detailed material characterization is need on pre- and post-carbonated samples to better elucidate carbonation pathways and products.     

Influence of flue gas SO2 on the toxicity of heavy metals in municipal solid waste incinerator fly ash after accelerated carbonation stabilization

The influence of CO₂ content and SO₂ presence on the leaching toxicity of heavy metals in municipal solid waste incinerator (MSWI) fly ash was studied by examining the carbonation reaction of MSWI fly ash with different combinations of simulated incineration flue gases. Compared with raw ash, the leaching solution pH of carbonated ash decreased by almost 1 unit and the leaching concentrations of heavy metals were generally lower, with that of Pb decreasing from 19.45 mg/L (raw ash) to 4.08 mg/L (1# carbonated ash). The presence of SO₂ in the incineration flue gas increased the leaching concentrations of heavy metals from the fly ash to different extents after the carbonation stabilization reaction. The pH of the leaching solution was the main factor influencing the leaching concentrations of heavy metals. The increase in buffer capacity with the pH of carbonated ash caused an increase in heavy metal stability after the carbonation reaction. Accelerated carbonation stabilization of MSWI fly ash could reduce its long-term leaching concentrations (toxicity) of Cu, Pb, Se, and Zn. The leaching concentrations of heavy metals from carbonated ash also likely had better long-term stability than those from raw ash. The presence of SO₂ in the incineration flue gas increased the proportion of exchangeable state species of heavy metals; slightly increased the long-term leaching toxicity of Cu, Pb, Se, and Zn; and reduced the long-term stability of these metals in the fly ash after the carbonation reaction.     

Fabrication of porous low crystalline calcite block by carbonation of calcium hydroxide compact

Calcium carbonate (CaCO₃) has been widely used as a bone substitute material because of its excellent tissue response and good resorbability. In this experimental study, we propose a new method obtaining porous CaCO₃ monolith for an artificial bone substitute. In the method, calcium hydroxide compacts were exposed to carbon dioxide saturated with water vapor at room temperature. Carbonation completed within 3 days and calcite was the only product. The mechanical strength of CaCO₃ monolith increased with carbonation period and molding pressure. Development of mechanical strength proceeded through two steps; the first rapid increase by bonding with calcite layer formed at the surface of calcium hydroxide particles and the latter increase by the full conversion of calcium hydroxide to calcite. The latter process was thought to be controlled by the diffusion of CO₂ through micropores in the surface calcite layer. Porosity of calcite blocks thus prepared had 36.8–48.1% depending on molding pressure between 1 MPa and 5 MPa. We concluded that the present method may be useful for the preparation of bone substitutes or the preparation of source material for bone substitutes since this method succeeded in fabricating a low-crystalline, and thus a highly reactive, porous calcite block.