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This paper follows the hydration of both portland cement and tricalcium silicate pastes between 30 min and 16 h of hydration. In-situ fast X-ray computed tomography (fCT) was used to make direct observations of the air-filled void formation in w/s of 0.40 to 0.70 with a micron resolution.The results show that over the first hour of the acceleration period the volume of air-filled voids reaches a maximum value and then decreases during the acceleration period and stays constant. The void distribution changes from a few coarse voids to a large number of smaller and more uniformly distributed voids. This behavior is suggested to be controlled by changes in the ionic strength that cause exsolution of dissolved air from the pore solution. Keywords: X-ray computed tomography, Cement hydration, Void evolution, Calorimetry, Induction period, Exsolution

The external penetration of ions into cementitious materials plays an important role in long-term durability of concrete structures. Current test methods to study bulk diffusion of ions in cement-based materials are typically destructive, time consuming and labor intensive. This research work uses laboratory transmission X-ray microscopy (TXM) and X-ray computed microtomography (μCT) to image iodide diffusion in cement paste. These techniques can non-destructively image the distribution of the dissolved ions at 8.8 μm resolution in minutes with a 2D analysis and a few hours for 3D. This method does not aim to measure the size of the pores, instead it aims to image the movement of the fluid through the pores by imaging an electron dense tracer with similar behavior as chloride ions. The tests were completed on cement paste mixtures with water-to-cement ratios (w/c) of 0.35, 0.40, and 0.45. The findings were validated with micro X-ray fluorescence (μXRF) imaging. Iodide and chloride diffusivity is also compared for paste samples with w/c of 0.40 and shown to provide comparable results. Results are also presented of 3D ion diffusion and void filling at high degrees of saturation.

Corrosion of steel reinforcement in concrete is not easy to detect until cracking has initiated. When there is a question about Cl intrusion into concrete, cores are taken, the sample is ground in controlled thicknesses, and then the powder is analyzed by titration. This paper uses micro X-ray fluorescence (μXRF), a non-destructive chemical imaging technique, to obtain equivalent or better information than profile grinding methods with less human involvement. Fifteen different comparisons are made between profile grinding and μXRF from the same concrete and the results are comparable. Examples are also presented where the spatial measurements made by μXRF provide insights that are not possible with the conventional profile grinding method. This means the Cl profile can be obtained with μXRF with less human effort while providing additional insights not possible with the typical profile grinding analysis.

Silane is a commonly used surface treatment to reduce fluid entry into the concrete matrix. This work uses micro X-ray fluorescence (μXRF) to image the presence of silane coatings in field samples and the changes made to the paste chemistry. There are many advantages that μXRF has over other imaging techniques due to the large spot size and the high energy levels. Because of this, μXRF can rapidly investigate large areas and requires minimal sample preparation. Quantitative measurements are made to show that there is a reduction in the amount of sulfur and an increase in potassium in the hydrophobic regions formed by silane coatings. These measurements provide important insights into modifications made to the concrete matrix when silane sealers are used. The mechanisms for the observed chemistry changes are discussed and a simple field or laboratory test method is presented that uses a dye to detect silane coatings.

The reasons for the start and end of the induction period of cement hydration remain topic of controversy. One long-standing hypothesis is that a thin metastable hydrate forming on the surface of cement grains significantly reduces the particle dissolution rate; the eventual disappearance of this layer re-establishes higher dissolution rates at the beginning of the acceleration period. However, the importance, or even the existence, of this metastable layer has been questioned because it cannot be directly detected in most experiments. In this work, a combined analysis using nano-tomography and nano-X-ray fluorescence makes the direct imaging of early hydration products possible. These novel X-ray imaging techniques provide quantitative measurements of 3D structure, chemical composition, and mass density of the hydration products during the induction period. This work does not observe a low density product on the surface of the particle, but does provide insights into the formation of etch pits and the subsequent hydration products that fill them.

Advances in nano-computed X-ray tomography (nCT), nano X-ray fluorescence spectrometry (nXRF), and high-performance computing have enabled the first direct comparison between observations of three-dimensional nanoscale microstructure evolution during cement hydration and computer simulations of the same microstructure using HydratiCA. nCT observations of a collection of triclinic tricalcium silicate (Ca(3)SiO(5)) particles reacting in a calcium hydroxide solution are reported and compared to simulations that duplicate, as nearly as possible, the thermal and chemical conditions of those experiments. Particular points of comparison are the time dependence of the solid phase volume fractions, spatial distributions, and morphologies. Comparisons made at 7 h of reaction indicate that the simulated and observed volumes of Ca(3)SiO(5) consumed by hydration agree to within the measurement uncertainty. The location of simulated hydration product is qualitatively consistent with the observations, but the outer envelope of hydration product observed by nCT encloses more than twice the volume of hydration product in the simulations at the same time. Simultaneous nXRF measurements of the same observation volume imply calcium and silicon concentrations within the observed hydration product envelope that are consistent with Ca(OH)(2) embedded in a sparse network of calcium silicate hydrate (C−S−H) that contains about 70 % occluded porosity in addition to the amount usually accounted as gel porosity. An anomalously large volume of Ca(OH)(2) near the particles is observed both in the experiments and in the simulations, and can be explained as originating from the hydration of additional particles outside the field of view. Possible origins of the unusually large amount of observed occluded porosity are discussed.

Disagreements about the mechanisms of cement hydration remain despite the fact that portland cement has been studied extensively for over 100 years. One reason for this is that direct observation of the change in microstructure and chemistry are challenging for many experimental techniques. This paper presents results from synchrotron nano X-ray tomography and fluorescence imaging. The data show unprecedented direct observations of small collections of C3S particles before and after different periods of hydration in 15 mmol/L lime solution. X-ray absorption contrast is used to make three dimensional maps of the changes of these materials with time. The chemical compositions of hydration products are then identified with X-ray fluorescence mapping and scanning electron microscopy. These experiments are used to provide insight into the rate and morphology of the microstructure formation.

While there is great interest in characterizing and modifying materials at the nanoscale, progress has been slow because few techniques allow for critical observations at this length scale. This work presents a data fusion technique that combines synchrotron-based X-ray nano computed tomography and nano X-ray fluorescence to non-destructively investigate complex nanoscale materials and provide combined three-dimensional (3-D) renderings of microstructure and chemistry. The technique has been named nano tomography-assisted chemical correlation (nTACCo) and is demonstrated on fly ash particles with nanoscale chemical inhomogeneities. Our findings show that nTACCo is capable of providing the concentration and location of seven different nano-inclusions within a particle. This work also provides direct observations of reactivity and chemical distribution of fly ash. This ability to combine 3-D structure and chemistry at the nanoscale will provide unprecedented tools for nanoscience in material science, biology, chemistry and medical science.

A novel data fusion technique is presented that combines X-ray computed tomography (CT) and electron probe microanalysis to investigate fly ash particles. The technique is called Tomography Assisted Chemical Correlation (TACCo). This technique fuses 2D compositional data with 3D X-ray CT data. The method produces 3D constituent and microstructure maps. Results and observations are presented that show the power of the technique for data visualization as well as quantitative analysis.

This paper uses fast X-ray Computed Tomography (fCT) to study the change in air-filled voids during hydration of portland cement pastes of different water to cement ratios and with fly ash and limestone additives between 30 min and 12 h of hydration with deionized and de-aired water. For the deionized samples, the total volume of the air-filled voids increased during the induction period and decreased as the acceleration period began and finally stayed constant. The samples with 20% replacement of fly ash and 20% replacement of limestone showed the same trends as the OPC samples but with a reduction in the observed air-filled void formation. The air-filled void distribution in these samples changed from a limited number of large voids at the beginning of the measurements to a system with a significantly greater number of uniformly distributed smaller voids. The changes in the air-filled voids were not significant in pastes made with 50% fly ash or with de-aired water in the same testing condition. This might be explained by differences in pore solution chemistry and a reduced amount of dissolved air in the mixing water. A mechanism is proposed for a change in ionic concentration in solution to cause release of the dissolved air within the mixing water.

The pore characteristics, mineral compositions, physical and mechanical properties of the subarkose sandstones were acquired by means of CT scan, X-ray diffraction and physical tests. A few physical models possessing the same pore characteristics and matrix properties but different porosities compared to the natural sandstones were developed. The 3D finite element models of the rock media with varied porosities were established based on the CT image processing of the physical models and the MIMICS software platform. The failure processes of the porous rock media loaded by the split Hopkinson pressure bar (SHPB) were simulated by satisfying the elastic wave propagation theory. The dynamic responses, stress transition, deformation and failure mechanisms of the porous rock media subjected to the wave stresses were analyzed. It is shown that an explicit and quantitative analysis of the stress, strain and deformation and failure mechanisms of porous rocks under the wave stresses can be achieved by using the developed 3D finite element models. With applied wave stresses of certain amplitude and velocity, no evident pore deformation was observed for the rock media with a porosity less than 15%. The deformation is dominantly the combination of microplasticity (shear strain), cracking (tensile strain) of matrix and coalescence of the cracked regions around pores. Shear stresses lead to microplasticity, while tensile stresses result in cracking of the matrix. Cracking and coalescence of the matrix elements in the neighborhood of pores resulted from the high transverse tensile stress or tensile strain which exceeded the threshold values. The simulation results of stress wave propagation, deformation and failure mechanisms and energy dissipation in porous rock media were in good agreement with the physical tests. The present study provides a reference for analyzing the intrinsic mechanisms of the complex dynamic response, stress transit mode, deformation and failure mechanisms and the disaster mechanisms of rock media.

Strut-and-tie modeling is a design tool that can be used to address complicated problems where common assumptions made in sectional (beam) analysis do not apply. One early step in strutand-tie modeling requires the user to envision a series of load paths within a structure. The literature contains little guidance on how to choose these load paths. This paper demonstrates how strain energy calculations can be used to quantify differences between different strut-and-tie models (STMs). The results provide insight and guidance for choosing an STM with minimum strain energy for simply supported beams. In addition, methodologies are provided that give the engineer the ability to judge the STM chosen and to affect positive changes if desired. Important conclusions include: 1) a three-panel STM shows improved efficiency over a single-panel STM; 2) the most efficient three-panel STM shows a characteristic geometry with a perpendicular compression strut and tension diag onal; 3) an analytical approach is discovered that supports using distributed longitudinal steel in D-regions; and 4) a simple method is presented to find the STM with the minimum strain energy . The work is directly applicable to deep beams and members in bending near reactions.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.