Evaluation of Tortuosity in Cemented Sand Using X-Ray Computed Microtomography

Cemented sand is a common way for soil stabilization or ground improvement. Tortuosity is an important parameter that has a significant impact on flow and transport characteristics of porous media and related to permeability and diffusion coefficient. This study aims to quantitatively investigate the tortuosity of the cemented sand with different cement content using X-ray computed tomography (CT) based random walk simulation. The cemented samples were scanned using microCT and converted to 3D pore space through image processing. Random walk simulation was applied to the reconstructed 3D pore space to calculate the tortuosity. Higher cement content gave an increase in tortuosity and a decrease in porosity. This indicates that the addition of cement makes pore space more tortuous and decreases both permeability and diffusion coefficient which are being treated as important parameters for durability of construction materials

Understanding the Reactivity of Dicalcium Silicate by Density Functional Theory

Ordinary Portland Cement (OPC) is a mixture consisting of various phases. Because of mineralogical complexity, the relationship between chemical reactivity of the mixture and each individual phase is still unanswered question. In this study, computational method based on Density Functional Theory (DFT) was applied to investigate the chemical reactivity of different polymorphs of dicalcium silicate crystals. At first, computationally generated dicalcium silicate crystals were geometrically optimized to achieve targeted convergence criteria for computing the total internal energy, lattice parameters, and atomic arrangement at 0K. The simulations performed explain well the thermodynamic stability as well as the synthesis temperatures of the different polymorphs of dicalcium silicate

Normal and Anomalous Self-Healing Mechanism of Crystalline Calcium Silicate Hydrates

The origin of different stability of crystalline calcium silicate hydrates was investigated. The tobermorite crystal has been used as an analog of cement hydrate that is being mostly manufactured material on earth. Normal tobermorite is thermally unstable and transforms to amorphous at low pressure. Meanwhile, anomalous tobermorite with high Al content does not significantly transform under high pressure or high temperature. Conducted X-ray absorption spectroscopy explains the weak stability of normal tobermorite which was originally hypothesized by the role of zeolitic Ca ions in the cavities of silicate chains. Atomic simulations reproduced the experimentally observed trend of pressure behavior once the ideal structures were modified to account for the Al content as well as the chain defects. The simulations also suggested that the stability of tobermorite under stress could be rationalized as a self-healing mechanism in which the structural instabilities were accommodated by a global sliding of the CaO layer.J.M. acknowledges support by a grant (20SCIP-C159063-01) from Construction Technology Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government. H.M. acknowledges the financial support from the Gobierno Vasco (project IT912-16). The work in San Sebasti ' an (R.D., J.S.D, A.A) was supported by the Spanish Ministry of Science and Innovation with RTI2018-098554-B-I00, PID2019-105488GB-I00 and PCI2019-103657 research grants, the Gobierno Vasco UPV/EHU (Project No. IT-1246-19), and the European Commission from the NRG-STORAGE project (GA 870114). The Institute of Engineering Research in Seoul National University provided research facilities for this work. The Ca-XAS and HPXRD experiments were performed at XAFCA beamline in Singapore Synchrotron Light Source (SSLS) and 12.2.2 beamline in Advanced Light Source (ALS), respectively. The ALS supported by a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231 and the Consortium for Materials Properties Research in Earth Sciences under NSF Cooperative Agreement EAR 1606856. The authors thank Prof. Simon M. Clark, Dr. Yonghua Du, and Dr. Shibo Xi for helpful discussions and beamline experimental supports

Unlocking the Secrets of Al-tobermorite in Roman Seawater Concrete

Ancient Roman syntheses of Al-tobermorite in a 2000-year-old concrete block submerged in the Bay of Pozzuoli (Baianus Sinus), near Naples, have unique aluminum-rich and silica-poor compositions relative to hydrothermal geological occurrences. In relict lime clasts, the crystals have calcium contents that are similar to ideal tobermorite, 33 to 35 wt%, but the low-silica contents, 39 to 40 wt%, reflect Al3+ substitution for Si4+ in Q2 (1Al), Q3 (1Al), and Q3 (2 Al) tetrahedral chain and branching sites. The Al-tobermorite has a double silicate chain structure with long chain lengths in the b [020] crystallographic direction, and wide interlayer spacing, 11.49 Å. Na+ and K+ partially balance Al3+ substitution for Si4+. Poorly crystalline calcium-aluminum-silicate-hydrate (C-A-S-H) cementitious binder in the dissolved perimeter of relict lime clasts has Ca/(Si+Al) = 0.79, nearly identical to the Al-tobermorite, but nanoscale heterogeneities with aluminum in both tetrahedral and octahedral coordination. The concrete is about 45 vol% glassy zeolitic tuff and 55 vol% hydrated lime-volcanic ash mortar; lime formed wt% of the mix. Trace element studies confirm that the pyroclastic rock comes from Flegrean Fields volcanic district, as described in ancient Roman texts. An adiabatic thermal model of the 10 m2 by 5.7 m thick Baianus Sinus breakwater from heat evolved through hydration of lime and formation of C-A-S-H suggests maximum temperatures of 85 to 97 °C. Cooling to seawater temperatures occurred in two years. These elevated temperatures and the mineralizing effects of seawater and alkali- and alumina-rich volcanic ash appear to be critical to Al-tobermorite crystallization. The long-term stability of the Al-tobermorite provides a valuable context to improve future syntheses in innovative concretes with advanced properties using volcanic pozzolans

Testing superabsorbent polymer (SAP) sorption properties prior to implementation in concrete: results of a RILEM Round-Robin Test

This article presents the results of a round-robin test performed by 13 international research groups in the framework of the activities of the RILEM Technical Committee 260 RSC "Recommendations for use of superabsorbent polymers in concrete construction''. Two commercially available superabsorbent polymers (SAP) with different chemical compositions and gradings were tested in terms of their kinetics of absorption in different media; demineralized water, cement filtrate solution with a particular cement distributed to every participant and a local cement chosen by the participant. Two absorption test methods were considered; the tea-bag method and the filtration method. The absorption capacity was evaluated as a function of time. The results showed correspondence in behaviour of the SAPs among all participants, but also between the two test methods, even though high scatter was observed at early minutes of testing after immersion. The tea-bag method proved to be more practical in terms of time dependent study, whereby the filtration method showed less variation in the absorption capacity after 24 h. However, absorption followed by intrinsic, ionmediated desorption of a specific SAP sample in the course of time was not detected by the filtration method. This SAP-specific characteristic was only displayed by the tea-bag method. This demonstrates the practical applicability of both test methods, each one having their own strengths and weaknesses at distinct testing times

Experimental and Theoretical Studies on Mechanical Properties of Complex Oxides in Concrete

Despite the enormous amount of concrete consumed, fundamental understanding on the structural properties of hydrated oxides in concrete is still an open question. Due to the structural hierarchies and heterogeneous characteristics of concrete, accurate experimental and theoretical studies have not been well-developed for measuring mechanical properties of crystalline and amorphous materials in concrete. Lack of the information makes the development of constitutive relation of them to the macroscopic concrete properties difficult. The objective of this thesis is to compute mechanical properties of various phases in concrete. Unconventional methods of high pressure x-ray diffraction, absorption, and first-principles calculation are applied to calcium silicate hydrates (tobermorite 14 Å, 11 Å, 9 Å, and jennite), calcium aluminate hydrates (hemi-carboalumiante, monocarboaluminate, strätlingite, and hydrogarnet), tricalcium aluminate, and alkali-silica reaction gel. From the systematic comparison of both experiment and simulation, a comprehensive understanding of structural mechanism is achieved. For tobermorites, interlayer thickness which is related to the number of interlayer water molecules determines its compressibility. Hemicarboalumiante and strätlingite shows a pressure-induced dehydration while monocarboaluminate with the full occupancy of carbon oxide group behaves stable and incompressible under pressure. In the cases of tobermorite 14 Å and monocarboaluminate, linear density approximation in the first-principles calculation predicts the experimentally measured bulk modulus with high accuracy. Based on the excellent agreement of bulk moduli between experiment and simulation, it can be concluded that computed elastic properties of shear and Young's modulus, Poisson's ratio, and elastic tensor coefficients are highly reliable. In addition, combining x-ray diffraction and absorption methods allows accurate measurement of density variation under pressure which is used to compute the bulk modulus of alkali-silica reaction gel.The fundamental structural properties obtained in both experiments and simulations given in this thesis will pave a path toward not just to increase our knowledge of the structural mechanism of concrete but also to optimize concrete design for better structural performance

Chloride Adsorption by Calcined Layered Double Hydroxides in Hardened Portland Cement Paste

This study investigated the feasibility of using calcined layered double hydroxides (CLDHs) to prevent chloride-induced deterioration in reinforced concrete. CLDHs not only adsorbed chloride ions in aqueous solution with a memory effect but also had a much higher binding capacity than the original layered double hydroxides (LDHs) in the cement matrix. We investigated this adsorption in hardened cement paste in batch cultures to determine adsorption isotherms. The measured and theoretical binding capacities (153 mg g−1 and 257 mg g−1, respectively) of the CLDHs were comparable to the theoretical capacity of Friedel's salt (2 mol mol−1 or 121 mg g−1), which belongs to the LDH family among cementitious phases. We simulated chloride adsorption by CLDHs through the cement matrix using the Fickian model and compared the simulation result to the X-ray fluorescence (XRF) chlorine map. Based on our results, it is proposed that the adsorption process is governed by the chloride transport through the cement matrix; this process differs from that in an aqueous solution. X-ray diffraction (XRD) analysis showed that the CLDH rebuilds the layered structure in a cementitious environment, thereby demonstrating the feasibility of applying CLDHs to the cement and concrete industries.This publication was based on work supported in part by Award No. KUS-l1-004021 and No. KUS-C1-018-02, made by King Abdullah University of Science and Technology (KAUST). We acknowledge Sasol Company for a sample of Pural MG 63 HT

Self-heating capacity of electrically conductive cement composites: Effects of curing conditions

The objective of this study was to examine the effects of curing conditions on the self-heating performance of electrically conductive cement composites (ECCCs), which are being developed for use in the accelerated curing of concrete. ECCC samples composed of the same mix proportion were cured under three different conditions (i.e., moisture, air, oven) with different temperature and humidity values to have varying thermal properties. The three curing conditions caused a variation in the thermal expansion of ECCCs, which seems to be attributed to different degrees of desiccation and change in pore structure; a drier curing condition induced a decrease in the amount of bound water as well as an increase in the porosity and average pore size. This phenomenon from a drier curing was deemed responsible for the reduction in thermal expansion, and it was favorable for the self-heating capability of ECCCs to be maintained for a longer period (at least 24 h). The improved self-heating performance of ECCCs due to a drier curing was confirmed by a higher maximum surface temperature of ECCCs, a longer duration at around the maximum surface temperature, and a smaller increase of electrical resistivity during the voltage application. Thus, curing in oven is considered best suitable for the fabrication of ECCC blocks that are designed to accelerate the curing of concrete

Effect of casting geometry on distributions of segmented steel fibers in ultra-high performance fiber-reinforced concrete

© 2022 Elsevier LtdIn this study, influence of flow distance and casting direction to properties of steel fibers in ultra-high-performance fiber-reinforced concrete (UHPFRC) was investigated using 3D data on the fibers that accurately obtained through multiple microcomputed tomography (micro-CT) scans and image processing. 3D images of bundled fibers were separated to individual strands by means of a 3D watershed algorithm. The variation in fiber properties according to distance from the casting point and the casting direction was quantitatively analyzed. The results indicated that many fibers were distributed at the casting position for the horizontally casted specimen. The fibers tended to move along the major upward flow of the UHPFRC mixture, which in turn, gradually inclined upward and oriented horizontally in concentric circles. However, different major flow patterns and fiber orientations appeared depending on the casting direction due to the wall effects induced by formwork boundaries. It was also found that fiber content is the dominant factor affecting the 3D dispersion of fibers.N

Effect of Tartaric Acid on Hydration of a Sodium-Metasilicate-Activated Blend of Calcium Aluminate Cement and Fly Ash F

An alkali-activated blend of aluminum cement and class F fly ash is an attractive solution for geothermal wells where cement is exposed to significant thermal shocks and aggressive environments. Set-control additives enable the safe cement placement in a well but may compromise its mechanical properties. This work evaluates the effect of a tartaric-acid set retarder on phase composition, microstructure, and strength development of a sodium-metasilicate-activated calcium aluminate/fly ash class F blend after curing at 85 °C, 200 °C or 300 °C. The hardened materials were characterized with X-ray diffraction, thermogravimetric analysis, X-ray computed tomography, and combined scanning electron microscopy/energy-dispersive X-ray spectroscopy and tested for mechanical strength. With increasing temperature, a higher number of phase transitions in non-retarded specimens was found as a result of fast cement hydration. The differences in the phase compositions were also attributed to tartaric acid interactions with metal ions released by the blend in retarded samples. The retarded samples showed higher total porosity but reduced percentage of large pores (above 500 µm) and greater compressive strength after 300 °C curing. Mechanical properties of the set cements were not compromised by the retarder