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Author(s): Viseshchitra, P; Ercius, P; Monteiro, PJM; Scott, M; Ushizima, D; Li, J; Xu, K; Wenk, HR | Abstract: © 2020 American Ceramic Society (ACERS) Calcium silicate hydrate (C-S-H), is the principal hydration product of Portland cement that mainly contributes to the physical and mechanical properties of concrete. This paper aims to investigate the three-dimensional structure of C-S-H with Ca/Si ratios of 1.0 and 1.6 at the nanoscale using electron tomography. The 3D reconstructions and selected region of interest analysis confirm that the morphology of both C-S-H materials are foil-like structures. The difference between the two materials is the density of elongated structures. C-S-H with Ca/Si ratio 1.6 is clearly composed of denser particles compared to the other C-S-H material due to overlapping of the foil-like structure. Pore analysis shows that C-S-H 1.0 and C-S-H 1.6 have porosities 69.2% and 49.8% respectively. Pore size distribution also reveals that C-S-H 1.0 has pore size range between 0-250nnm and C-S-H 1.6 between 0-100nnm. The pore network's size of C-S-H 1.0 is significantly larger than 1.6. This study illustrates the capability of using electron tomography to determine the 3D nanoscale structure of cementitious products and to distinguish between C-S-H 1.0 and 1.6.

Nanocrystalline calcium silicate hydrate (C–S–H) is the binding phase of many low-CO2 cements. Understanding its structure–mechanical properties relationship is critical in designing sustainable co...

The mechanical and thermal properties of the gigatonnes of concrete produced annually are strongly affected by the anharmonicity of the chemical bonds in its main binding phase, nanocrystalline cal...

Calcium nitrate (CN) is used widely as an effectively inorganic setting accelerator and antifreeze admixture in concrete structures. In this paper, the multiscale investigation of CN on the hydration of C3S was studied by scanning transmission X-ray microscopy (STXM) combined with near-edge X-ray absorption fine structure (NEXAFS), 29Si MAS NMR, calorimetry, scanning electron microscope, and N2 absorption. It was concluded that the calcium silicate hydrate (C-S-H) surrounds the unhydrated C3S at 1-day hydrated C3S in the presence of calcium nitrate, while portlandite is partly in transformation and is formed partly. Based on Ca L3,2-edge NEXAFS spectra for 1-day hydrated C3S particle, calcium nitrate does not change the structure of the asymmetrically 7-fold coordination of calcium in the C-S-H. Calcium nitrate can accelerate the hydration of C3S to some extent and polymerization of the silicate chains within C-S-H considerably at early age, resulting in the increasing specific surface area.

Ancient Roman concrete presents exceptional durability, low-carbon footprint, and interlocking minerals that add cohesion to the final composition. Understanding of the structural characteristics of these materials using X-ray tomography (XRT) is of paramount importance in the process of designing future materials with similar complex heterogeneous structures. We introduce Materials Data Science algorithms centered on image analysis of XRT that support inspection and quantification of microstructure from ancient Roman concrete samples. By using XRT imaging, we access properties of two concrete samples in terms of three different material phases as well as estimation of materials fraction, visualization of the porous network and density gradients. These samples present remarkable durability in comparison with the concrete using Portland cement and nonreactive aggregates. Internal structures and respective organization might be the key to construction durability as these samples come from ocean-submersed archeological findings dated from about two thousand years ago. These are preliminary results that highlight the advantages of using non-destructive 3D XRT combined with computer vision and machine learning methods for systematic characterization of complex and irreproducible materials such as archeological samples. One significant impact of this work is the ability to reduce the amount of data for several computations to be held at minimalistic computational infrastructure, near real-time, and potentially during beamtime while materials scientists are still at the imaging facilities.

This research studied the hydration of C3S-C3A-calcium sulfate systems made of combinations of two C3S (pure triclinic and Al-doped monoclinic), two C3A (pure cubic C3A and Na-doped orthorhombic), and two calcium sulfates (gypsum and hemihydrate). For each system, the hydration of four different SO3 contents (0.25–2.0 wt%) was assessed by calorimetry. The optimum SO3 content was fixed from calorimetry results, and the mixtures were evaluated by in-situ XRD and TGA. The type of C3S was the factor that most affected the sulfate balance of the systems. The mixes with Al-C3S produced a higher amount of ettringite in the first hours, resulting in much earlier sulfate depletions when compared to the mixes with pure C3S. The mixes with ort-C3A also showed faster sulfate depletion due to its higher reactivity compared with cb-C3A. Finally, the replacement of gypsum by hemihydrate resulted in faster sulfate depletion caused by the faster hemihydrate dissolution P18-RT-720 cofinanciado con FEDER

Creep of the cement matrix affects the structural stability of concrete. In Portland cements, the creep is largely controlled by the binding phase calcium-(aluminum-)silicate-hydrate, or C-(A-)S-H. This phase has a lamellar structure and under deviatoric stress aligns its c-axis with the principal stress. However, the limiting resistance to this reorientation is unknown at the nanocrystalline level. Small-angle X-ray scattering shows that the lamellae thickness decreases under 100's MPa deviatoric stress. Deviatoric stress Raman spectroscopy shows that there are two ways that this break-up can occur. If the material's silicate chains are cross-linked, then strain in Si O bonds does not increase above certain stresses, indicating a relaxation adjacent to the Si O bond. If the chains are not cross-linked, then the silicate chains are broken up by rastering against each other, introducing defects. These results show that the plastic deformation of C-(A-)S-H is relevant for Portland cement creep.

Abstract Methane pyrolysis is an emerging technology to produce lower-carbon intensity hydrogen at scale, as long as the co-produced solid carbon is permanently captured. Partially replacing Portland cement with pyrolytic carbon would allow the sequestration at a scale that matches the needs of the H2 industry. Our results suggest that compressive strength, the most critical mechanical property, of blended cement could even be improved while the cement manufacture, which contributes to ~ 9% global anthropogenic CO2 emissions, can be decarbonized. A CO2 abatement up to 10% of cement production could be achieved with the inclusion of selected carbon morphologies, without the need of significant capital investment and radical modification of current production processes. The use of solid carbon could have a higher CO2 abatement potential than the incorporation of conventional industrial wastes used in concrete at the same replacement level. With this approach, the concrete industry could become an enabler for manufacturing a lower-carbon intensity hydrogen in a win–win solution. Impact Methane pyrolysis is an up-scalable technology that produces hydrogen as a lower carbon-intensity energy carrier and industrial feedstock. This technology can attract more investment for lower-carbon intensity hydrogen if co-produced solid carbon (potentially hundreds of million tons per year) has value-added applications. The solid carbon can be permanently stored in concrete, the second most used commodity worldwide. To understand the feasibility of this carbon storage strategy, up to 10 wt% of Portland cement is replaced with disk-like or fibrillar carbon in our study. The incorporation of 5% and 10% fibrillar carbons increase the compressive strength of the cement-based materials by at least 20% and 16%, respectively, while disk-like carbons have little beneficial effects on the compressive strength. Our life-cycle assessment in climate change category results suggest that the 10% cement replacement with the solid carbon can lower ~10% of greenhouse gas emissions of cement production, which is currently the second-largest industrial emitter in the world. The use of solid carbon in concrete can supplement the enormous demand for cement substitute for low-carbon concrete and lower the cost of the low-carbon hydrogen production. This massively available low-cost solid carbon would create numerous new opportunities in concrete research and the industrial applications.

The capability of neutrons to penetrate materials opaque to other non-destructive techniques combined with relatively high neutron attenuation cross section for hydrogen provide unique possibilities to reveal internal distribution of water and other hydrogen-containing substances in various bulky samples. In this study we compare the accuracy of water thickness reconstruction from two types of neutron imaging experiments: neutron radiography with a white beam spectrum and energy-resolved neutron imaging. Neutron transmission spectra available at the latter modality can be used for the extraction of certain material characteristics, such as microstructure and elemental composition, usually not accessible for the analysis of images integrated over the entire neutron spectrum. Distribution of water within the sample, however, can be reconstructed from both types of neutron imaging methods. The results of present study indicate that the accuracy of water mapping appeared to be comparable within the 2%–4% error in our experiments dominated by non-optimized calibration of background signal. The accuracy of water thickness reconstruction was verified with a calibration step wedge sample for both imaging modalities. We also demonstrate the applicability of this technique for the studies of water absorptivity and porosity of ancient Roman concrete samples extracted from Cosa and Portus Traiani sites. These reconstruction methods are not only limited to quantification of water or hydrogen containing substances, but can be implemented to any other material with relatively high neutron attenuation.

This work reports the properties of hybrid-fiber-reinforced concrete (HyFRC) made with expansive (Type K) cement. Combinations of metallic and non-metallic fibers at total fiber volume fraction of 1% were studied. The effectiveness of double hooked-end (DHE) steel fibers in concrete containing expansive cement is investigated for the first time in this study. The mechanical, physical, and microstructural properties of concretes have been assessed. Additionally, the fiber pull-out test was also performed to investigate the effectiveness of Type K cement in improving the fiber-matrix interfacial bond. The results indicate that Type K cement has small influence on the mechanical properties of concrete fabricated at the same water-cement ratio of 0.35 with a similar consistency. However, as expected, it enhances the volume stability of concrete subjected to drying condition. The pull-out resistance of steel fibers increased by 26% as a result of full replacement of ordinary Portland cement (OPC) with Type K cement. A deflection-hardening performance is achieved by introducing of DHE steel fibers in HyFRC. The partially replacement of DHE steel fibers with other type of fibers results in a reduction in the strengths of HyFRC. The results obtained in this study proves that the bond between fiber and cement matrix is enhanced by fully replacement of OPC with expansive cement, which subsequently improves the mechanical properties of HyFRC.

Efforts are currently underway to improve the performance of small electric antennas and resonator-based antennas embedded in the civil infrastructure. One of the main limitations of these small antennas is that they have narrow bandwidths which limit their use when embedded in concrete. The present work shows that by changing the amount of iron (III) oxide microparticles in a cementitious matrix, not only does the impedance matching of the embedded array improve by as much as 20 dB, it also produces a lesser shift in resonant frequency (RF) than the original cementitious matrix alone. Further analysis of antenna transmission coefficient S21 values shows that adding 4% iron (III) oxide into cement paste improves the RF power transmission into the material compared to the control by as much as 10 dB. Given that iron (III) oxide microparticles are easily available as admixture pigments for coloring mortar or concrete, it is possible to fast track implementation of this application in new construction. National Research Foundation (NRF) Submitted/Accepted version This research was funded by the Republic of Singapore’s National Research Foundation through a grant to the Berkeley Education Alliance for Research in Singapore (BEARS) for the Singapore Berkeley Building Efficiency and Sustainability in the Tropics (SinBerBEST) Program. BEARS was established by the University of California, Berkeley as a center for intellectual excellence in research and education in Singapore.

A complete understanding of the effect of gypsum on the C3S hydration and the role of aluminum has not been achieved. The effects of gypsum (at 2.5 wt% and 5.0 wt%) on the hydration of tricalcium silicate (C3S) and aluminum-doped C3S (Al-C3S) hydration are assessed. Isothermal calorimetry, thermodynamic modeling, XRD, TGA, and 27Al and 29Si MAS-NMR were performed to analyze gypsum's influence on the hydration of C3S and Al- C3S. The inclusion of gypsum retarded the initial hydration (first 3 h) for both C3S and Al-C3S due to the interaction between the sulfate ions and C3S. In contrast, gypsum enhanced the hydration of both C3S and Al-C3S afterward. The removal of Al3+ ions of the pore solution by the formation of ettringite is not the mechanism behind the enhancement of C3S hydration by gypsum, which mainly results from changes in C-S-H morphology and increases in the ionic strength. CAPES (Coordination for the Improvement of Higher Education Personnel) [88882.439908/2019-01]. Spanish Junta de Andalucía [UMA18-FEDERJA-095 and P18-RT-720] Graduate Program in Civil Engineering: Construction and Infrastructure (PPGCI) of the Federal University of Rio Grande do Sul (UFRGS). CNPq (Brazilian National Council for Scientific and Technological Development) through the research fellowship PQ CNPq.

Calcium (alumino)silicate hydrate (C-(A-)S-H) is the main binding phase in blended cement concrete. Understanding the chemistry and structure of C-(A-)S-H is essential to optimizing concrete properties such as compressive strength and durability; yet questions remain around the coordination environments of Ca and Al in its structure with various chemical compositions and equilibration temperatures. C-(A-)S-H with Ca/Si = 0.6–1.6, Al/Si = 0–0.1, and equilibrated at 7–80 °C is studied by nanoscale soft X-ray spectroscopy at the Ca L2,3- and Si K-edges. Highly distorted CaO7 complexes occur in the intralayer of C-(A-)S-H irrespective of Ca/Si, Al/Si, and temperature. Zeolitic Ca in the interlayer of C-(A-)S-H is highly distorted from an ideal octahedral coordination. Third aluminate hydrate is either not Ca-bearing or its Ca is structurally similar to C-(A-)S-H and does not resemble the Ca in AFm-phases. Increasing aluminosilicate chain polymerization in C-(A-)S-H shifts the Si K-edge to higher energies, implying Al uptake in the bridging and/or cross-linked sites, as well as a contraction of Si O bond lengths. C-(A-)S-H exhibits a foil-like morphology, with individual foils comprised of nano-sized platelets with comparable thickness regardless of Ca/Si or Al/Si at 7–50 °C. Coarser C-(A-)S-H foils occur at 80 °C and higher Al/Si ratios relative to lower temperatures and Al content.

With the aim of creating more sustainable building materials, calcium silicate hydrate (C-S-H), the primary binding phase in modern concrete, was integrated with poly(ethylene glycol) (PEG). At the crystal lattice scale, synchrotron radiation-based high-pressure X-ray diffraction (HP-XRD) reveals that the incorporation of PEG increases the ab-planar stiffness of C-S-H leading to a higher bulk modulus, while molecular simulation results indicate polymers are not likely intercalated into the interlayer region. At a higher length scale, nanoindentation measurements show the introduction of PEG lowers the packing density of C-S-H particles and, thus, decreases the indentation modulus. However, a high creep resistance of the C-S-H/PEG sample is still maintained, which suggests a meso-composite may form to restrict the rapid translation between neighboring calcium silicate sheets. Furthermore, 29Si nuclear magnetic resonance (NMR) shows that the presence of PEG shortens the mean chain length of C-S-H, making dimer the predominant structure.

The magnesium and sodium sulfate attacks on Portland cement paste in the presence of applied electric fields were studied, and the mineralogical alterations were investigated by both experiments and thermodynamic modeling. When an electric current flows out of the cement paste, the electric migration of ions induced sulfate ingress and decalcification. Compared with the specimen exposed to Na2SO4, that exposed to MgSO4 for 28 d proceeded to a later degradation stage, which is characterized by the decomposition of ettringite, portlandite, and AFm phases, and the formation of CaSO4. Thermodynamic modeling indicates a neutralization process induced by the electric migration of OH−, which is potentially responsible for the decomposition of ettringite. When an electric current flows into the cement paste, the Mg2+ and Na+ showed different migration behavior. Mg2+ was incorporated to form brucite and M-S-H–like products in a shallow area (~100 μm) on the surface of the specimen, whilst a part of the Na+ could be bonded to form Na-rich silica gel with the other part penetrating through the specimen. By coupling the pore solution chemistry obtained from thermodynamic modeling with the Nernst-Planck equation, the migration behaviors of the ionic species (SO42-, Mg2+, and Na+) were analyzed.

International audience; X-ray computed tomography (CT) is a non-destructive technique that offers a 3D insight into the microstructure of thick (opaque) samples with virtually no preliminary sample preparation. Since its first medical application in 1973, the technique has improved dramatically in terms of acquisition times and resolution. The best resolution achievable for both parallel-and cone-beam setups is presently sub-micron. The macroscopic properties (stiffness, resistance, permeability, durability,. . .) of cementitious materials are known to be significantly affected by their multiscale microstructure. This calls for x-ray CT investigations of cementitious materials down to the smallest length-scales. The present review first provides background information on the technique (including image-processing). It then covers various applications of x-ray CT to cementitious materials: imaging of the porous network, durability experiments, damage experiments,… , cutting-edge nanotomography experiments. Current challenges such as time-and chemically-resolved experiments are also discussed.

To promote diatom-based biofuel productions in clean energy, this paper explores the potential of using biosilica as a renewable and regenerative byproduct from diatom-based biofuel production in cementitious materials. High-purity diatomite (DE), a biosilica model compound, is selected to investigate the microstructure and durability performance of diatom-based cementitious composites. The replacement of portland cement with 30 wt.% DE significantly enhances the resistance to leaching, reduces the rapid chloride permeability, but increases the drying shrinkage at early ages. The microstructure of DE-containing matrix is refined due to the lime-silica reaction, and the interfacial transition zone of DE-containing concrete is densified. Overall, the DE-induced microstructural changes enhance the durability performance of DE-containing cementitious composites, which in turn demonstrates the feasibility of using biosilica as a sustainable cement substitute. The use of biosilica from diatom-based biofuel production can potentially reduce the CO2 emissions of carbon-intensive concrete production and promote the development of clean energy.

Dicalcium silicate (C 2 S) is an important clinker mineral in Portland and belite-calcium sulfoaluminate cement. However, there is still a lack of information on the local degree of silicate polymerization and calcium coordination in C 2 S hydration products. This study aimed to fill this gap by characterizing the hydration of two C 2 S polymorphs, the β- and α′ H - types, using scanning transmission X-ray microscopy coupled with ptychographic imaging. The results showed that the coordination of the Ca species in β- and α′ H -C 2 S had a distorted, cubic-like symmetry, whereas the Ca coordination of calcium silicate hydrate (C-S-H), the main hydration product, was structurally similar to that of tobermorite. The outer hydration product (Op) of both polymorphs exhibited an increasing degree of silicate polymerization over time. The inner hydration product (Ip) of β-C 2 S polymerized slower than Op; however, the degree of silicate polymerization of both Ip and Op in the α′ H -C 2 S hydration system was comparable. The polymerization degree of Op was relatively heterogeneous, whereas, in α′ H -C 2 S, the polymerization degree was more homogeneous. Ptychographic imaging shows that the Op of α′ H -C 2 S exhibits coarser fibrils than the Op of β-C 2 S, and a clear Op-Ip interface of hydrated β-C 2 S is observed.

The existing macroscale models of the calcium (alumino)silicate hydrate (C-(A-)S-H), the main binder of concrete, assume that the nanocrystallites maintain random orientation under any loading conditions. However, using synchrotron-radiation-based XRD, we report the development of preferred orientation of nanocrystalline C-A-S-H, from random at ambient pressure to strongly oriented under uniaxial compression with lateral confinement. The c-axes of the nanocrystals tend to align with the primary load. This preferred orientation is preserved after removing of external loading. The texture, quantified using a standard Gaussian fiber orientation distribution function (ODF), was used to calculate the averaged bulk elastic tensor of oriented C-(A-)S-H. It changes from isotropic (without texture) to transversely isotropic (with texture). Our results provide direct evidence of the reorientation of nanocrystalline C-(A-)S-H as a mesoscale mechanism to the irreversible deformation of cement-based material. The implications of these results for modeling the mechanical property of C-(A-)S-H at the macroscale are discussed.

Calcium aluminate cement (CAC) has wide application in civil engineering and castable refractory materials. The main binding phases of hardened CAC paste, CaO·Al2O3·10H2O (CAH10), inevitably converts to 2CaO·Al2O3·8H2O (C2AH8), 3CaO·Al2O3·6H2O (C3AH6) and Al(OH)3 (AH3), leading to a significant change in the mechanical properties of the CAC matrix. This work investigates the mechanical properties of the main crystalline components in hydrated and/or converted CAC systems, using synchrotron-radiation-based high-pressure X-ray diffraction. The anisotropic deformations of CaAl2O4 (CA), CAH10 and C3AH6 along each crystallographic direction are investigated, along with their bulk moduli. The density-driven stiffening hypothesis is validated for the studied phases and other cement-based minerals. An atom-scale topological analysis is proposed to explain the unusually high stiffness of CAH10. The results provide fundamental information to understand the mechanical properties of single cement-based phases at molecular level, and enable predicting the changing mechanical properties of converted CAC matrix using homogenization models.

The higher reactivity of orthorhombic C3A (ort-C3A) in sulfate-containing solutions, compared with cubic C3A (cb-C3A), was previously related to the differences in crystal structure or the sodium in the ort-C3A pore solution. We analyzed the hydration of cb-C3A (in water and NaOH solution) and Na-doped ort-C3A in the presence of gypsum and hemihydrate. Calorimetry, in-situ XRD, TGA, and rheological tests were conducted. NaOH accelerated the hydration of cb-C3A, but ort-C3A still presented higher ettringite formation rate and earlier sulfate depletion. Ort-C3A pastes showed 10-20 times higher viscosities and yield stresses. Replacing gypsum by hemihydrate increased the ettringite precipitation rate and anticipated the sulfate depletion of ort-C3A but did not significantly influence cb-C3A hydration. The crystallization of hemihydrate into gypsum resulted in early (

Global warming is the gradual increase in global temperatures owing to trapping of heat in the atmosphere by greenhouse gases. As of January 2020, 121 countries and 1 region, including Japan, pledged to achieve carbon neutrality by 2050. One of the measures for achieving this goal is the promotion of low-carbon urban development, which includes urban greening. Oysters are produced in all districts of Japan but Hiroshima Prefecture is particularly famous for its high production levels. The production of oysters generates oyster shells as a by-product, which are used commercially as livestock feed and a soil conditioner. However, the reuse of oyster shells remains limited. Pervious concrete is a special type of concrete in which single-sized aggregates are bound together with cement paste or mortar and is considered an environmentally friendly material because of its high water permeability. Therefore, we investigated whether oyster shell pervious concrete (OyPC) containing crushed oyster shell aggregate would be suitable for growing plants for potential application as a planting foundation for urban greening. Zoysia tenuifolia sod or Kentucky bluegrass (Poa pratensis) seeds were directly placed or sown in soil on the surfaces of OyPC and crushed stone pervious concrete specimens, which were then placed in plastic containers for either 72 days (short-term planting tests) or 802 days (long-term planting test). OyPC had a higher plant-growing performance than crushed stone pervious concrete in both the short- and long-term planting tests and also pumped the water from the container to moisten the sod and the soil covering the seeds, assisting the growth of plants. The grass on OyPC that has a small particle size could grow without water retention material which is indispensable to crushed stone pervious concrete for greening. These findings indicate that OyPC would be feasible for application as a planting foundation for urban greening.

Calciuam-silicate-hydrate (C-S-H) is the principal binding phase in modern concrete. Molecular simulations imply that its nanoscale stiffness is ‘defect-driven’, i.e., dominated by crystallographic defects such as bridging site vacancies in its silicate chains. However, experimental validation of this result is difficult due to the hierarchically porous nature of C-S-H down to nanometers. Here, we integrate high pressure X-ray diffraction and atomistic simulations to correlate the anisotropic deformation of nanocrystalline C-S-H to its atomic-scale structure, which is changed by varying the Ca-to-Si molar ratio. Contrary to the ‘defect-driven’ hypothesis, we clearly observe stiffening of C-S-H with increasing Ca/Si in the range 0.8 ≤ Ca/Si ≤ 1.3, despite increasing numbers of vacancies in its silicate chains. The deformation of these chains along the b-axis occurs mainly through tilting of the Si-O-Si dihedral angle rather than shortening of the Si-O bond, and consequently there is no correlation between the incompressibilities of the a- and b-axes and the Ca/Si. On the contrary, the intrinsic stiffness of C-S-H solid is inversely correlated with the thickness of its interlayer space. This work provides direct experimental evidence to conduct more realistic modelling of C-S-H-based cementitious material.

This paper analyzed the air-void system in air-entrained concrete in the presence of calcium salts and developed a new approach method, random “parking” method, for studying it. The foam index test studied indicated that the introduction of calcium salts did result in a drop in foaming power at 0.5 min, but the foam ability after 60 min was enhanced in surfactant-water solution. The Powers spacing factor in the air-entrained hardened concrete decreased gradually in the presence of CaCl2 or Ca(NO3)2. The probability density function of air-void distribution showed that the air-void size in air-entraining concrete was reduced with the addition of CaCl2 and Ca(NO3)2. The random parking method developed herein was compared with the experimental Powers spacing factor, and the concept of the alternative spacing factor, defined as the average distance between the surfaces of the two nearest air voids in concrete, was used. It was indicated that the alternative spacing factor is in close proximity to the experimental Powers spacing factor.

Cenosphere particles are hollow, but due to their hard shells, they can be used in cementitious composites to produce ultra-lightweight cement composites (ULCC) with high strength and low thermal conductivity. This study integrates thermal conductivity with mechanical experimental research, microscopic investigation, and numerical simulations to provide new insights into the behavior of these advanced composites. The microstructure of ULCC samples was characterized using synchrotron high-resolution microtomography, transmission electron microscopy and scanning electron microscopy. Composite models were used to predict the thermal conductivity of the cenospheres based on the experimental results of ULCC thermal conductivity and the porosity of the samples.

The main binding phases of calcium aluminate cement (CAC) concrete, CaO·Al2O3·10H2O (CAH10) and 2CaO·Al2O3·8H2O (C2AH8), slowly convert to 3CaO·Al2O3·6H2O (C3AH6) and Al(OH)3 (AH3). This reaction significantly speeds up at a temperature higher than ∼30 °C, and over time leads to significant strength loss in CAC concrete. Because of the lack of direct evidence that simultaneously probes morphological and chemical/crystallographic information, intense debate remains whether the conversion is generated by a solid-state or through-solution reaction. The conversion of CAH10 at an elevated temperature is studied herein using synchrotron-radiation-based X-ray spectromicroscopy capable of acquiring near edge X-ray absorption fine structure data and ptychographic images with a resolution of ∼15 nm. We show that, when stored at 60 °C, CAH10 first converts to C2AH8 by solid-state decomposition, followed by the through-solution formation of C3AH6. The C3AH6 crystallizes from both the relics of dissolved C2AH8 and fro...

In many planar antenna array designs, impedance transformers are required to interconnect the elements to ensure that their impedances are matched. However, impedance transformers take up space and reduce area utilisation. If each element is electrically small and able to function individually as an electrically small antenna (ESA), they can be combined into an array without using impedance transformers. In this study, a stubbed hexagonal shaped folded dipole ESA of one tenth of the wavelength is proposed and developed. This metamaterial inspired design of loading the folded dipole with split ring resonator overcomes the problem of fabricating ESA of one tenth of the wavelength using typical printed circuit board fabrication technologies for the 2.45 GHz band. To show the potential of using this ESA as a unit element for antenna array without using impedance transformers, a seven-element array is designed and fabricated. By optimising the element separation distance, and stub lengths, the ESA array shows good S(11) of less than −25 dB, and gain improvement of up to 12 dB compared with a single unit ESA.

We carry out a multi-scale investigation of the fracture properties of the gel produced by the alkali-silica reaction in concrete. Milimeter-sized alkali gels are tested via scratch tests at the micrometer level. Scanning electron microscopy reveals curved fracture surfaces produced during the scratch test, warranting a fracture mechanics analysis. By application of a nonlinear fracture mechanics model to the recorded load and depth data, we estimate the fracture toughness of ASR gel to be K c = 0.62 MPa m . In turn, by combining micro-indentation and scratch tests, we can estimate the fracture energy G f = 11.2 J/m2. These experimental results are important for the multi-scale modeling of the alkali-aggregate reaction.

Author(s): Geng, G; Myers, RJ; Kilcoyne, ALD; Ha, J; Monteiro, PJM | Abstract: Tricalcium aluminate (cement clinker phase), gypsum, katoite, ettringite, and calcium monosulfoaluminate hydrate (abbreviated as kuzelite) are the major minerals in the hydration reaction of tricalcium aluminate in the presence of gypsum and have critical impacts on the kinetics and thermodynamics of early-age cement hydration mechanisms. Here, spectroscopic analysis of these minerals is conducted using scanning transmission X-ray microscopy (STXM). Their Ca L2,3-edge near edge X-ray absorption fine structure (NEXAFS) spectra are measured and correlated to the known Ca coordination environments. The results indicate that these minerals have unique Ca environments that can be differentiated from one another based on the intensities and positions of the absorption peaks at 346.5-348.5 and 350.5-351.5 eV. It is concluded that Ca in tricalcium aluminate (cubic and orthorhombic polymorphs) and katoite is in cubic-like coordination with negative 10Dq, whereas Ca is in an octahedral-like coordination with positive 10Dq in ettringite, gypsum, and kuzelite. For tricalcium aluminate, the Ca atoms in both polymorphs are in similar chemical environments with slightly more distortion in the orthorhombic polymorph. As a common issue in STXM experiments, absorption saturation of NEXAFS spectra is also investigated. It is demonstrated that the optical density difference between pre- and post-edge absorption levels provides a reliable indication of the sample thickness in the systems studied. The present work provides a reference for the STXM study of the calcium (sulfo)aluminate reactions in cement hydration and natural aqueous environments, and in the former case, provides a more complete understanding of a system that may serve as a low-C alternative to Portland cement.

This paper presents a trans-scale finite element model based on hydraulic-thermal-mechanical coupling equations of porous medium to simulate the behavior of concrete during freezing and thawing. Unlike previous models that regard concrete as homogeneous material, this paper aims to establish a macro-size concrete model with detailed micro-structure, the aggregates (mm) and air voids (μm) are randomly generated by the Monte Carlo method, and the interfacial transition zones (μm) with random thickness are built around the aggregates. The capillary pores (nm) of concrete is expressed by the relationship between the percentage of frozen ice and the radius of capillary pore. The simulation results reveal the mechanism of air voids protecting the concrete from freezing and thawing damage, and the mechanical field shows the freezing and thawing damage occurs preferentially at the interfacial transition zone, because the stress concentration due to the irregular distribution of aggregates and water resistance of aggregates. The temperature boundary conditions were also varied to study the influence of the temperature change rate on hydrostatic pressure and volume stress. The effect of the cement paste's pore structure, including the spacing of air voids, on the ability of concrete to resist freezing and thawing is presented.

Nano/micro-indentation has been extensively used to examine elastic properties and creep of cement-based materials. However, there have been inconsistencies in the measured elastic moduli and debate on the creep mechanism of calcium (alumino) silicate hydrates (C-(A-)S-H). To bring new insights, we examined the deviatoric stress-induced preferred orientation of calcium aluminosilicate hydrate (C-A-S-H), portlandite, and ettringite compacts, using synchrotron-based X-ray diffraction. Intense preferred orientations were observed in C-A-S-H compacts, and the c-axis of unit cells is highly aligned with the compression direction; weaker c-axis related fiber-type texture formed in portlandite and ettringite. Higher humidity, greater compacting pressure, and/or increased duration of the pressure intensified the C-A-S-H preferred orientation, which is facilitated by gel-pore water lubrication. This pressure-induced time-dependent preferential re-orientation of crystallites can contribute to the creep of cement-based materials, and it may cause overrepresentation of the soft c-axis of C-(A-)S-H unit-cell in nanoindentation experiments and underestimation of Young's modulus.

Polycarboxylate ether (PCE), a commonly used superplasticizer, is known to influence the morphology of ettringite during the early hydration of C3A- and ye'elimite-containing cements. According to existing theories, such morphological changes may be crucial to the expansive behavior of these cements. This paper studied the expansion of ye'elimite-anhydrite pastes and found the use of PCE to reduce expansion after 4 days of curing. Hydration studies were conducted by calorimetry, X-ray diffractometry, scanning electron microscopy, mercury intrusion porosimetry, inductively coupled plasma–optical emission spectrometry, and X-ray microtomography. The results show that the influences of PCE on the morphology of ettringite and the hydration of ye'elimite were quite small after 2 days. Based on the crystal growth theory, the range of pores in which ettringite can grow was calculated to explain the observed expansive behaviors. The presence of ettringite nano-crystals in aluminum hydroxide was also revealed and considered as a possible expansion mechanism.

Roman-era concrete is the iconic embodiment of long-term physicochemical resilience. We investigated the basis of this behavior across scales of observations by coupling time-lapse (4-D) tomographic imaging of macroscopic mechanical stressing with structural microscopy and chemical spectroscopy on Roman marine concrete (RMC) from ancient harbors in Italy and Israel. Stress–strain measurements revealed that RMC creeps and exhibits a ductile deformation mode. The low permeability of concrete samples was linked to mortar-dominated microstructures showing no debonding with the aggregates. Structural and chemical imaging shows the presence of well-developed sulfur-rich, fibrous minerals that are intertwined and embedded in a crossbred matrix having the chemical traits of both a calcium-aluminum–silicate-hydrate and a polymerized alkali-alumino-silicate. This latter likely reflects the ultra-alkaline volcanic nature of the primary source materials. We hypothesize that the fine interweave of sulfur-rich fibers within this crossbred matrix enhances aggregate bonding, which altogether contributes to the durability of RMC.

Calcium (alumino)silicate hydrate (C-(A-)S-H) is the critical binding phase in modern Portland cementbased concrete, yet the relationship between its structure and stoichiometry is not completely understood. In this study, Raman spectroscopy is used to analyze the effects of varying Ca/Si molar ratio (0.6-1.6), Al/Si molar ratio (0.0-0.1), and synthesis temperature (7-80 degrees C) on the chemical composition and atomic configuration of C-(A-)S-H. The experimental results indicate that increasing Ca/Si molar ratio produces less cross-linked C-(A-)S-H structures, while the addition of Al into the system increases the long-range order of its chain-like structure. Furthermore, increasing the synthesis temperature leads to the formation of more polymerized structures, especially in the Al-containing samples. The Raman spectra also suggest the formation of vaterite in C-S-H samples synthesized at low temperatures. Finally, this study reveals that uptake of atmospheric CO2 in C-S-H and C-A-S-H favors the formation of long-range ordered chain-like structures.

In this research, formulae for predicting compressive strength and global warming potential (GWP) per cubic meter of concrete are proposed. These equations allow for prediction of properties based on the water-to-binder ratio for concrete mixtures containing cement as the only binder, as well as for concretes containing Class F fly ash (FA) and ground granulated blast furnace slag (GGBS) as partial cement replacement. To meet multi-criteria demands of mechanical properties and lower GWP, a method for obtaining the optimal water-to-binder ratio, for given replacement level, is presented. In this research, optimization is based on a direct ratio of GWP to compressive strength, but the methodology has the potential to be extended to other environmental impact categories and material property relationships. For the classes of concrete examined, the analyses showed that high levels of cement replacement with GGBS may provide the best, hence resulting in the lowest, ratio of GWP to compressive strength for the mixtures examined. Additionally for the classes of concrete examined, the results showed that optimal water-to-binder ratios for the best ratio of GWP to compressive strength occurred at relatively high strengths (∼50–70 MPa). Yet, the majority of concrete used in countries such as the U.S. is lower strength concrete (

Two-dimensional (2D) image analysis method is developed for evaluating coarse aggregate characteristic and distribution in concrete, based on concrete cross-section image. Coarse aggregate characteristic includes particle size, particle grading, particle roundness and particle orientation. Coarse aggregate distribution includes particle distributing homogeneity, particle distance and mortar to coarse aggregate area ratio (MAAR). The 2D image analysis method is performed on one conventional concrete and four self-consolidating concrete. Except exhibiting the ability of the 2D image analysis method, the analysis results also show the consistency of 2D information and 3D information, by comparing particle grading with sieving curve, and MAAR with mortar to coarse aggregate volume ratio (MAVR). Meanwhile, the 2D image analysis method can reveal the influence stochastic error, such as vibration and non-uniform sampling, on the homogeneity of concrete.

Many reinforced concrete structures susceptible to corrosion damage are subjected to externally applied loads, causing cracking. These cracks increase the permeability of the material, accelerating the ingress of corrosion-inducing deleterious agents. In this paper, the effect of multiple microcracking and macrocrack formation on corrosion initiation was investigated. A hybrid fiber-reinforced concrete (HyFRC), which forms ductile, distributed microcracking prior to dominant crack localization due to multiple tiers of fiber reinforcement, is being studied for its performance against corrosion damage. The effect of matrix cracking on corrosion initiation was studied with beam specimens preloaded in flexure prior to long-term corrosion exposure. Reinforced HyFRC composites were found to have a delayed corrosion initiation response due to reductions in crack widths and suppression of splitting cracks, compared to conventional reinforced concrete. The influence of microcracks on corrosion is studied using X-ray micro-computed tomography (μCT) on reinforced fiber-reinforced cementitious composites and reinforced mortar preloaded in tension.

The effects of three commonly used superplasticisers (SPs) – sulfonated naphthalene formaldehyde (SNF), sulfonated acetone formaldehyde (SAF) and polycarboxylates (PCB) – on the hydration process, products and microstructure of tricalcium aluminate (C3A) pastes in the presence of gypsum were systematically investigated using isothermal calorimetry, x-ray diffraction, environmental scanning electron microscopy, infrared spectroscopy and magic angle spinning–nuclear magnetic resonance spectroscopy. The results show that the hydration reaction rate of C3A paste at the initial stage (0–2 h) was promoted by the incorporation of SPs, especially in the C3A–SNF paste. Hydration was rapidly reduced at the early stage (2–12 h), but an acceleration in C3A–SNF/PCB pastes occurred again at the late stage. The morphology of the C3A–SNF/SAF paste changed from thin needle-shaped crystals at the initial stages to thick and coarse rod-like crystals at late stages. In the C3A–PCB paste, very few crystals occurred at an early age, while a large amount of prismatic crystals were observed at the later age. Chemical environment studies revealed that the SPs did not visibly change the position or shape of the characteristic bands of aluminates, but only influenced their relative intensities. A predominant resonance, assigned to six-coordinated aluminium as [AlO6] type, was observed for all the pastes with and without SP. The interaction mechanism between SP and C3A was also explored.

As awareness and concern for greenhouse gas emissions grow, methods for reducing a significant industrial emission source, the production of cement for use in concrete, have been appearing. Assessments of these methods typically do not take into account the influence of material properties and environmental impacts concurrently. When both are considered, rarely are properties considered other than concrete strength. In this research, methods for assessing environmental impacts with mechanical properties concurrently for design of non-reinforced concrete are presented. Tools for assessing environmental impacts based on the volume of material needed as defined by several material properties and design parameters, such as required moment of inertia and cross-sectional area, are investigated. The indices for comparison developed in this research are applied to a database containing 190 concrete mixtures with varying constituents both with and without Supplementary Cementitious Materials (SCMs). For the mixtures and comparison indices examined, concrete mixtures containing SCMs consistently offered the best combination of associated global warming potential (GWP) and material properties. However, for several cases, it was shown that the use of Portland cement as the sole binder could provide comparable indices of GWP and material properties to concrete mixtures containing replacement binder.

Alkali-Silica reaction (ASR) is a physicochemical process that can deteriorate concrete and is a recurring engineering problem. In this study three different cylindrical samples affected by ASR were prepared: a plain mortar and two composite mortars containing fibers (polypropylene and a polymer hybrid), which were analyzed at the microtomography (μCT) beamline 8.3.2 at the Advanced Light Source (ALS). In general, three different features were observed during the 136 day observation period: (1) aggregate dissolution, (2) crack propagation from inside the aggregate, through the cement matrix, and at the ITZ, and (3) the alkali-silica gel filling cracks and voids. In addition, accelerated mortar bar tests were utilized to observe ASR's expansive effect in the plain and composite mortars, and the fibers' ability to restrain expansion due to ASR.

This paper reports an investigation of the recycled concrete (RC) microstructure using synchrotron microtomography (μCT) at the Advanced Light Source combined with Scanning Electron Microscopy analysis. The study evaluated the influence of 50% of recycled concrete aggregate (RCA) and its water absorption compensation on the RC microstructure. The following variables were studied: a) the compressive strength of the original concrete used to obtain the RCA (40 and 80 MPa) and b) the initial moisture condition of the RCA (Saturated Surface Dry and Oven Dry). The microtomographic images showed the mixtures cast with RCA in the dry condition developed an evident macropore network surrounding the RCA particle that was not observed in the mixtures using RCA in the SSD condition. SEM images confirmed the initial findings from μCT and showed that the thickness of the interfacial transition zone in RC is in the same order of magnitude as the reference concrete.

There is renewed interest in using advanced techniques to characterize ancient Roman concrete due to its exceptional durability and low-carbon footprint. In the present work, samples were drilled from the “Hospitium” in Pompeii and were analyzed by synchrotron microtomography (μCT) and neutron radiography to study how the microstructure, including the presence of induced cracks, affects their water adsorption. The water distribution and absorptivity were quantified by neutron radiography. The 3D crack propagation, pore size distribution and orientation, tortuosity, and connectivity were analyzed from μCT results using advanced imaging methods. Porosity was also measured by mercury intrusion porosimetry (MIP) as a reference. Ductile fracture patterns were observed once cracks were introduced. Compared to Portland cement mortar/concrete, the Pompeii samples had relatively high porosity, low connectivity, and a similar coefficient of capillary penetration. In addition, permeability was predicted from models based on percolation theory and pore structure data to evaluate the fluid transport properties. Understanding the microstructure of ancient Pompeii concrete is important because it could inspire the development of modern concrete with high durability.

Debates remain on the influencing factor and long-term performance of C-S-H-seeded paste. In this study, phase-pure seeds were prepared from diluted Ca3SiO5 hydration. The seed structures were determined using synchrotron-based X-ray diffraction, X-ray absorption spectroscopy, and transmission electron microscopy. Nano-foils and similar Ca and O environments were present in seeds of different Ca/Si; nanofibers were present in high-Ca seeds only which showed lower silicate polymerization. Calorimetry and setting results showed a greater hydration acceleration with high-Ca seeds; seed shape dominated the acceleration. 0.5 wt% high-Ca seeds increased 1- and 28-days paste strength by 300% and 20%, respectively. Life-cycle assessment showed negligible (