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At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Investigation of the changes in microstructure and transport properties of leached cement pastes accounting for mix composition

Ca-leaching of cement-based materials induces detrimental effects on properties related to long-term durability. A better understanding of leaching degradation in terms of alterations in mineralogy, microstructure, and transport properties is important for long-term assessments of concrete and reinforced concrete structures used in nuclear waste disposal systems or in hydro structures. However, the decalcification process is not easy to study because it is extremely slow. In this study, an ammonium nitrate (NH{sub 4}NO{sub 3}) solution of 6 mol/l was used to accelerate the leaching kinetics. The experiments were performed on cement paste samples with different water/powder (w/p) and limestone filler (LS) replacement ratios. Both the change of sample mass over time and the amount of calcium ion leached out were monitored during the test. Different post-analysis techniques including SEM/SEM-EDX, XRD/QXRD, MIP, ion chromatography, and N{sub 2}-adsorption were used to characterize the microstructural and mineralogical changes. The effect of accelerated leaching on transport properties was studied by measuring the changes in water permeability and diffusivity of dissolved gases. Results showed that the square-root-time law of degradation was applicable under accelerated conditions. Both higher w/p ratios and LS replacements increased the rate of leaching propagation; the former had a more significant effect. The accelerated leaching significantly altered the microstructure of the cement paste to a material with a higher specific surface area, increased total porosity and a shift to larger pore sizes. Those changes led to a significant increase in water permeability (one to two orders of magnitude) and diffusivity (less than one order) depending on degradation state.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Impact of a 70 °C temperature on an ordinary Portland cement paste/claystone interface: An in situ experiment

Radioactive wastes in future underground disposal sites will induce a temperature increase at the interface between the cementitious materials and the host rock. To understand the evolution of Portland cement in this environment, an in situ specific device was developed in the Underground Research Laboratory in Tournemire (France). OPC cement paste was put into contact with clayey rock under water-saturated conditions at 70 °C. The initial temperature increase led to ettringite dissolution and siliceous katoite precipitation, without monosulfoaluminate formation. After 1 year of interaction, partial decalcification and diffuse carbonation (calcite precipitation) was observed over 800 μm in the cement paste. At the interface, a layer constituted of phillipsite (zeolite), tobermorite (well-crystallised C-S-H) and C-(A)-S-H had formed. Globally, porosity decreased at both sides of the interface. Geochemical modelling supports the experimental results, especially the coexistence of tobermorite and phillipsite at 70 °C, minerals never observed before in concrete/clay interface experiments.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Thermodynamic modeling of hydrated white Portland cement–metakaolin–limestone blends utilizing hydration kinetics from {sup 29}Si MAS NMR spectroscopy

Hydration kinetics for the principal phases of Portland cement blends have been incorporated in thermodynamic modeling (GEMS package), utilizing degrees of hydration from {sup 29}Si MAS NMR. An empirical relationship for the reaction of these phases is established which includes three variable parameters that all can be estimated from the degrees of hydration. This approach is compared with thermodynamic equilibrium modeling (full hydration) for white Portland cement–metakaolin (0–30 wt.%) blends and for ternary blends of white Portland cement (65 wt.%)–metakaolin–limestone. The predicted phase assemblages have been compared with the phases identified by XRD, {sup 27}Al and {sup 29}Si MAS NMR which reveals that the incorporation of hydration kinetics improves the agreement between modeling and experiments. The results show also that the formation of strätlingite depends critically on the quantity of charge-balancing anions in the AFm phases, especially carbonate and sulfate anions, and on the degree of hydration for metakaolin.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Hydration products in sulfoaluminate cements: Evaluation of amorphous phases by XRD/solid-state NMR

The hydration of four sulfoaluminate cements have been studied: three sulfoaluminate systems, having different content of sulfate and silicate, and one blend Portland-CSA-calcium sulfate binder. Hydration was followed up to 90 days by means of a combination of X-ray diffraction and solid state MAS-NMR; Differential scanning calorimetry and Scanning electron microscopy were also performed in order to help the interpretation of experimental data. High amount of amorphous phases were found in all the four systems: in low-sulfate cements, amorphous part is mainly ascribed to monosulfate and aluminium hydroxide, while strätlingite is observed if belite is present in the cement; in the blend system, C-S-H contributes to the amorphous phase beyond monosulfate.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Electrical capacitance tomography to monitor unsaturated moisture ingress in cement-based materials

This paper reports the investigation of the feasibility of electrical capacitance tomography (ECT) for monitoring unsaturated moisture flow in cement-based materials. ECT is an imaging modality in which the electrical permittivity distribution within an object is reconstructed based on measured capacitances between electrodes attached on the object's surface. In a series of experiments, mortar specimens with different water-cement mass ratios (w/c) (0.25, 0.45 & 0.60) were monitored with ECT during the moisture ingress. The results demonstrate that ECT is able to image the moisture ingress within the specimens, and further, to distinguish between different moisture flow rates in mortars with different porosities resulting from differing w/c ratios. These findings suggest that ECT could provide a tool for monitoring, visualizing and quantifying the rate of moisture ingress in cement-based materials non-destructively.

36 MATERIALS SCIENCE↗
DOE OSTI2016

NMR relaxation of molecules confined inside the cement paste pores under partially saturated conditions

Nuclear Magnetic Resonance (NMR) relaxation studies of liquids confined inside cement samples mostly refer to water molecules and saturated pores. Here we extend these studies to the ethanol and cyclohexane molecules partially saturating a white cement paste. The two filling liquids were selected as representatives of polar and nonpolar molecules which should experience different interactions with the surface containing OH groups. The results on these two molecules are compared with those obtained on water filled sample. The transverse relaxation measurements, performed at different saturation degrees, clearly revealed three pore reservoirs in cement paste: intra-C-S-H sheet pores, inter-C-S-H gel pores and capillary pores. A two phase exchange model, describing the effective relaxation rate under partially saturated conditions, is also considered. The model allows finding of a relationship between the relaxation time distribution, the filling degree of the pores and the liquid morphology inside these pores.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Effect of electrical field on TSA failure of cement-based materials

The impact of electrical field on the sulfate attack of cement-based materials at low temperatures was studied to rapidly detect Thaumasite Sulfate Attack (TSA) in the laboratory and in future engineering. Sulfate attack of cement paste with 30 wt% replacement of CaCO{sub 3} powder soaked in Na{sub 2}SO{sub 4} solution, MgSO{sub 4} solution, or CaSO{sub 4} solution was measured under the condition of 5 ± 2 °C when an electrical field was applied. Appearance changes and compressive strength of the samples were tested to evaluate the corrosion degree. Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Raman Spectroscopic measurements were applied to study the microstructure of the corrosion product. The test results indicate that sample damage was obvious, compression strength declined rapidly, and ettringite and/or thaumasite were found in the corrosion product after applying the electrical field for 90 days. A muddy product with TSA characteristics was formed, and thaumasite composition was found in the corrosion product after applying the electrical field for 120 days when MgSO{sub 4} solution was used as the corrosion medium. It is confirmed that an electrical field can accelerate TSA failure in cement-based materials.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Can Electrical Resistance Tomography be used for imaging unsaturated moisture flow in cement-based materials with discrete cracks?

Previously, it has been shown that Electrical Resistance Tomography (ERT) can be used for monitoring moisture flow in undamaged cement-based materials. In this work, we investigate whether ERT could be used for imaging three-dimensional (3D) unsaturated moisture flow in cement-based materials that contain discrete cracks. Novel computational methods based on the so-called absolute imaging framework are developed and used in ERT image reconstructions, aiming at a better tolerance of the reconstructed images with respect to the complexity of the conductivity distribution in cracked material. ERT is first tested using specimens with physically simulated cracks of known geometries, and corroborated with numerical simulations of unsaturated moisture flow. Next, specimens with loading-induced cracks are imaged; here, ERT reconstructions are evaluated qualitatively based on visual observations and known properties of unsaturated moisture flow. Results indicate that ERT is a viable method of visualizing 3D unsaturated moisture flow in cement-based materials with discrete cracks. - Highlights: • 3D EIT is developed to visualize water ingress in cracked mortar. • Mortar with different size discrete cracks are used. • The EIT results are corroborated with numerical simulations. • EIT results accurately show the temporal and spatial variation of water content. • EIT is shown to be a viable method to monitor flow in cracks and matrix.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Coupling of chemical kinetics and thermodynamics for simulations of leaching of cement paste in ammonium nitrate solution

A novel numerical method is proposed in this paper to model the leaching of various types of cement pastes in the ammonium nitrate solution by coupling chemical kinetics with chemical thermodynamics. Arrhenius equation and Eyring equation are applied to predict the rate constant of fast dissolution of portlandite in the ammonium nitrate, and a modified rate law is developed by incorporating the saturation ratio of the reaction. The estimations of leached depths from the proposed method comply with the well-known square root-time relationship and they also agree well with the reported experimental results. Moreover, the evolutions of porosity and major hydrate compositions during leaching are obtained. It is found that a simple linear relation exists for the acceleration factor of leaching in 6 M of ammonium nitrate with respect to leaching in pure water regardless of the type of the cement clinker. - Highlights: • Chemical kinetics and thermodynamics are coupled to simulate the accelerated leaching. • The proposed method generates good estimations for leached depths and changes of porosity. • The proposed method can predict the detailed distributions of all the major cement hydrates. • A simple linear relation exists for predicting the widely scattered acceleration factors.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Impact of quantity of anhydrite, water to binder ratio, fineness on kinetics and phase assemblage of belite-ye'elimite-ferrite cement

One of the main challenge in belite, ye'elimite and ferrite cement is the control of the belite phase reactivity. We propose to study two mechanisms that impact belite hydration kinetic: the first mechanism is linked to available space for hydration products and can be seen as a decrease of water accessibility to cement surface and the second concerns the cement fineness. We used a thermodynamic model that can predict phase assemblage and we found good correlations between this model and the results obtained with XRD measurement and Rietveld refinement.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Automated coupling of NanoIndentation and Quantitative Energy-Dispersive Spectroscopy (NI-QEDS): A comprehensive method to disclose the micro-chemo-mechanical properties of cement pastes

Engineering cement-based composites requires a comprehensive understanding of the microstructure features governing macroscopic properties. This work aims to foster the latest chemo-mechanical technique to disclose the micro-mechanical properties of intimately intermixed phases. Considering a typical Portland cement paste as a study case, we compared the so-far developed nanoindentation analysis methods with a new automatic method coupling Nano-Indentation and Quantitative Energy-Dispersive Spectroscopy (NI-QEDS). Besides evincing advantages and disadvantages of previous methods, the NI-QEDS analyses enabled distinguishing chemical phases having strongly overlapping mechanical properties. Results suggested the presence at the sub-micrometre scale of extra calcium, aluminum and sulfur into (or in the vicinity of) outer C–S–H. The quantitative chemistry allowed identifying the rare occurrence of nanoindentation volumes located on ''pure'' phases (e.g., inner C–S–H). Finally, NI-QEDS provided new knowledge on cement pastes' micro-chemo-visco-mechanical properties and showed to be a powerful tool for investigating even more heterogeneous systems.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Hydration of MgO/SiO{sub 2} and Portland cement mixtures: A structural investigation of the hydrated phases by means of X-ray diffraction and solid state NMR spectroscopy

The phases formed during the hydration of several MgO/SiO{sub 2} and Portland cement mixtures were investigated by performing X-ray diffraction and {sup 29}Si and {sup 27}Al solid state NMR experiments on samples hydrated for 1, 7 and 28 days. The structural properties and relative amount of magnesium silicate hydrate (M-S-H) and calcium silicate hydrate (C-S-H) gels, which form as separate phases, were found to significantly depend on the initial composition of the mixtures. Faster hydration kinetics was observed for all mixtures with respect to the “pure” MgO/SiO{sub 2} and Portland cement samples, pH and excess of silica playing a major role. Moreover, the addition of the MgO/SiO{sub 2} mixture to Portland cement affects the formation of aluminum-containing hydrated phases.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Mechanical properties and durability performance of concretes with Limestone Calcined Clay Cement (LC3)

The adoption of any binder system for structural concrete depends on the performance characteristics desired for addressing the long-term deformation and durability concerns. The major properties influencing the performance includes the shrinkage characteristics governing the long-term deformation, and durability characteristics related to various transport mechanisms, governing the performance in different service conditions. This paper describes the potential of Limestone Calcined Clay Cement (LC{sup 3}) for use in structural concrete in comparison with Ordinary Portland Cement (OPC) and fly ash based blended cement (FA30). Three types of concrete mixtures were designed for the study, two based on achieving an equivalent strength grade (M30 and M50 concrete grade) with each binder, and the third with equal binder content and w/b ratio. Mechanical properties such as compressive strength and elastic modulus, and autogenous and drying shrinkage, along with various durability parameters of the different concretes were assessed. Oxygen permeability, rapid chloride penetration, chloride migration, resistivity development and water sorptivity were the various parameters considered for evaluation of durability performance. The results indicate the superiority of LC{sup 3} binder over other binders in producing durable concrete, especially in a chloride laden environment. The major reason for the better performance was attributed to the more compact and dense microstructure of the system with the LC{sup 3} binder against OPC and FA30. The drying shrinkage performance was seen to be similar for concrete with all three binders.

36 MATERIALS SCIENCE↗
DOE OSTI2018

A high-pressure X-ray diffraction study of the crystalline phases in calcium aluminate cement paste

Calcium aluminate cement (CAC) has wide application in civil engineering and castable refractory materials. The main binding phases of hardened CAC paste, CaO·Al{sub 2}O{sub 3}·10H{sub 2}O (CAH{sub 10}), inevitably converts to 2CaO·Al{sub 2}O{sub 3}·8H{sub 2}O (C{sub 2}AH{sub 8}), 3CaO·Al{sub 2}O{sub 3}·6H{sub 2}O (C{sub 3}AH{sub 6}) and Al(OH){sub 3} (AH{sub 3}), 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 CaAl{sub 2}O{sub 4} (CA), CAH{sub 10} and C{sub 3}AH{sub 6} 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 CAH{sub 10}. 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.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Comprehending the occupying preference of manganese substitution in crystalline cement clinker phases: A theoretical study

Highlights: • The element distributions of manganese in clinker are successfully predicted by atomic simulations. • Occupying preference of Mn in clinker follows the ''electronic properties matching'' principle. - Abstract: Understanding the dopant behavior in cement clinker has been an important issue due to the increasing use of alternative fuels and secondary raw materials for cement manufacturing following the rising demand for reduction of CO{sub 2} emissions and energy consumption. In this work, state-of-the-art ab initio calculations have been employed to systematically investigate manganese (Mn) doping mechanism in four dominant clinker phases. Corresponding experimental studies are incorporated to verify simulated results. A conspicuous preference of Mn for occupying the Fe site in ferrite is found based on formation energy analyses, which is in accord with our experiments and vast literatures. More in-depth analyses indicate the Mn doping mechanism follows the ''electronic structure matching'' principle, which influences the stability of doped structures by inducing localized coordination distortions. The work provides a fundamental perspective to investigate doped clinker, which facilitates the searching for other doping species, thereby designing steerable doping-enhanced cement.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of nano-silica and curing conditions on the reaction rate of class G well cement exposed to geological CO2-sequestration conditions

Chemical reactions of class G oil well cement submerged in CO{sub 2}-saturated brine were experimentally investigated to evaluate the effects of nano-silica and curing conditions. The progression of depths of reaction zones (depletion, carbonation, and degradation zone) with reaction time up to 62 days were quantitatively measured based on 3D X-ray CT in conjunction with the chemical analysis. The results show that there were no significant difference in the progress of carbonation zone regardless of the curing conditions and nano-silica. The measure of reaction depth for tested specimens concludes that the curing at the high pressure and temperature is more effective to prevent the depletion zone from progressing into the cement interior than the addition of the nano-silica, supported by XRD and NMR results. Image-based measurements, coupled with chemical analyses, quantified the evolution of the reactive zones in the oil well cement, providing insights into the underlying reaction mechanism.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Statistical variability of mechanical fields in thermo-poro-elasticity: Multiscale analytical estimations applied to cement-based materials at early-age

Highlights: • Standard deviation of stresses in C-S-H has the order of magnitude of the stresses applied at concrete scale. • Fluctuations increase with the stiffness and contrast of phases. • Fluctuations are dependent of mechanical load. • Transformation fields also cause mechanical fluctuations. - Abstract: Analytical homogenization provides information about the average mechanical fields but can also be used to obtain information about the statistical variability or fluctuations of these fields. Here, we present analytical estimations of the fluctuations of elastic stresses within cement-based materials (CBM). Mori-Tanaka, self-consistent and generalized self-consistent schemes are combined to represent the multiscale character of CBM. The second moment of mechanical fields is derived from the internal energy. The presence of transformation fields and resulting fluctuations are considered. We account for the evolution of the volume fractions of phases due to cement hydration. Scenarios considering or not the ITZ are studied. Our estimations are dependent on load and representation CBM microstructure. The obtained standard deviation of stresses in C-S-H are on the order of the stresses applied at concrete scale. We provide estimations of the compressive strength of a cement paste at early-age using our results of fluctuation of stresses.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Microstructural changes and mass transport in cement-based materials: A modeling approach

Highlights: • Concept to relate microstructural changes, mass transport, and chemical reactions in cement-based materials is presented. • Multi-physics modeling approach utilizing an extended version of the PNP equation and the geochemical code IPHREEQC. • Case studies indicate significant changes in mass transport properties due to alterations in the microstructure. • Microstructural changes depend on exposure and interactions between chemical reactions, pore structure, and mass transport. A generic modeling framework is presented to relate microstructural changes, i.e. changes in the pore volume distribution and phase assemblage, and mass transport in cement-based materials. The modeling framework accounts for mass transport and chemical equilibrium between ions in the pore solution and solid hydrates by means of an extended version of the Poisson-Nernst-Planck equation, in which the chemical equilibrium is solved by the external geochemical code IPHREEQC. Results of numerical studies concerning carbonation and leaching of a cement-based material indicate significant changes in mass transport properties due to chemical reactions leading to alterations in the microstructure. The resulting highly non-linear (both spatial and temporal) microstructural changes are thereby depending on the exposure and indicate that interactions between chemical reactions, pore structure changes, and mass transport have to be accounted for as a whole.

36 MATERIALS SCIENCE↗