Direct three-dimensional observation of the microstructure and chemistry of C3S hydration
Introduction
Tricalcium silicate (C3S)1 is a major component of portland cement. It reacts with water to form calcium silicate hydrate (CSH) and calcium hydroxide (CH). This hydration process determines most of the early-age properties of concrete and has been studied for decades. If one could understand and predict the kinetics and microstructure development during this process, strategies could be designed to control it and thereby improve the quality and economics of concrete. However, the mechanisms of hydration are not fully understood [1], [2], and values of many of the thermodynamic and kinetic properties needed to make accurate predictions have not been measured [1], [2], [3], [4], [5], [6]. In fact, agreement has still not been reached even about the basic mechanisms of early-age hydration due to a lack of sufficiently detailed experimental observations, especially of direct, in-situ evolution of the microstructure that can be used to guide numerical simulations [1], [2], [3], [4], [5], [6], [7], [8], [9].
Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), transmission electron microscopy (TEM), atomic force microscopy (AFM), white light interferometry, and nanoindentation have been used to study cement hydration and to characterize the hydrated microstructure, especially the calcium silicate hydrate product (CSH) [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. Generally, CSH is classified as “inner” or “outer” products, according to their location relative to the original boundary of the C3S grains [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. However, this classification is limiting because of the difficulty of imaging techniques to locate the original boundary of the C3S grain in a hydrated microstructure. The products have also been classified based on their apparent density and chemistry [13], [14], [15], [16], [17], [20]. The molar ratio of calcium to silicon (Ca/Si) in CSH is commonly accepted to be between 1 and 2 depending, on the hydration environment when the products are formed [22], [28], [29], [30], [31], [32]. Recent work with SEM-EDS and nanoindentation has suggested that there is likely an inter mixing of CH with CSH in hardened paste samples [20]. Different degrees of mixing could lead to different Ca/Si of hydration product that are possibly outside typical values [12], [33].
Electron microscopy has also suggested that the dissolution of C3S is controlled by the formation and coalescence of etch pits on the grain surfaces [18], [19], [25], [26], [35]. These etch pits seem to form preferentially where mechanical damage from grinding or crystalline imperfections are present in the material [18], [19], [35]. Despite these observations being of great insight, this work has not been able to make direct observations of the evolution of the in-situ structure over time. In addition, these imaging techniques require careful sample preparation that may introduce artifacts. Furthermore, since the surfaces are examined at such high magnification, the observations are highly localized and may not statistically reflect the structure or kinetic behavior at greater length scales. These challenges make it difficult to obtain quantitative measurements that are representative and can aid in the development and validation of predictive models based on chemistry and physics.
Soft X-ray microscopy has been used to take in-situ time-lapse nanoscale transmission images while C3S is reacting at an average water-to-solid (w/s) ratio of five [35], [36]. The technique provides information about the structure of the hydration products and the process of formation during in-situ reactions. However, analyzing the images is challenging because the 3D microstructure is projected onto a 2D transmission image. This is similar to the radiographs shown in this paper. Results from this work have suggested that the hydration process occurs not only on the surface, but also inside of the C3S particle, although the technique cannot resolve the specific location and is only roughly quantitative [35].
Recent advances with synchrotron hard X-ray nanoprobes have allowed X-ray nano-computed tomography (nCT) and nano X-ray Fluorescence (nXRF) to become a reality. These techniques are capable of non-destructively imaging at a nanoscale resolution [37], [38], [39], [40], [41]. The sample preparations for these techniques are also minimal, so this technique lends itself to evaluating processes that change over time. These techniques can be combined with other experimental methods and the results can be used as a starting point or as a comparison to computer simulations [37], [38], [39], [40], [41], [42], [43].
X-ray computed tomography (CT) is commonly used in the medical sciences to non-destructively image the internal structure of organisms. This technique combines a series of X-ray radiographs at small angles of rotation to produce a 3D tomograph [44], [45]. In addition to the structural information, the materials investigated have different contrast depending on their X-ray absorption. The X-ray absorption is a function of the density and mass attenuation coefficient. For a given X-ray energy level, the mass attenuation coefficient is a function of the average atomic number with some discontinuities caused by X-ray absorption edges [46], [47], [48]. No X-ray absorption edges were encountered for the instrument settings and materials investigated. If there is significant contrast in X-ray absorption among the constituent materials, the data can be used to separate them in the images. The material interfaces are sometimes highlighted by edge refraction [49], [50], which can be used to find edges in low-contrast images. The use of nCT for construction materials has been quite limited. However work has been done to investigate the 3D structure of aluminosilicate geopolymer gel and the early stages of reaction of fly ash [37], [38].
nXRF is a powerful tool when combined with nCT because it can provide elemental maps with detection limits better than a part per million (ppm). In this technique, a primary X-ray beam illuminates a sample and an energy dispersive detector is used to measure the fluorescence X-rays emitted by the sample. Each chemical element fluoresces at characteristic energies from the small region, so rastering the primary X-ray beam over the sample enables the creation of 2D chemical maps for the image [38], [40], [51].
In this paper, nCT is primarily used to investigate individual C3S particles before and then after different time periods of hydration. The full 3D tomography from nCT gives direct observations of the behaviors of C3S hydration at both the surface and inner structure of the samples. The quantitative results from this technique provide accurate measurements of different hydration behaviors at different time periods. Thanks to the non-destructive nature of nCT, the same sample can then be scanned by nXRF. The resulting elemental maps give useful information about the chemical composition of the hydration product.
The goal of this work is to establish the utility of nCT and nXRF to study C3S hydration during its first several hours. Additional work is ongoing to further refine the techniques to improve fundamental understanding of the mechanisms and measuring the relevant structural and kinetic properties that are needed by numerical models of chemical and microstructural evolution; this will be the subject of future publications.
Section snippets
Materials
The triclinic C3S powder used in this study was produced by Mineral Research Processing2 (Meyzieu,
Sample preparation and hydration
With the low X-ray energy used in these techniques, it was not possible to examine hydration in situ at industrially relevant w/s; a much more dilute suspension was required to permit X-ray transmission. However, dilute suspensions tend to promote extremely rapid initial dissolution rates of C3S, potentially causing them to dissolve completely before measurements could be made. Past research using calorimetry measurements on continually stirred suspensions has shown that mixtures with solutions
Results and discussion
A summary of the samples and changes in their dimension and volume are shown in Table 2. The y dimension is the measurement of the particle in the same direction as the X-ray beam used to interrogate the sample. The x dimension is perpendicular to the y dimension but in the same horizontal plane. Finally, the z dimension is perpendicular to the y dimension in the vertical plane.
Samples were investigated at three different time periods of hydration and at two different temperatures. For the C3S
Summary
This paper highlights the utility of using nCT and nXRF to measure the 3D structure and chemistry of the early hydration of C3S at 15 nm and 150 nm length scales. The hydration of triclinic C3S particles in 15 mmol/L lime solution at w/s = 5 was examined as a function of time and temperature. The volumetric growth and variation of X-ray absorption and chemistry has been measured and will serve as a useful bench mark for future experiments and simulations.
The following observations were made for
Acknowledgements
This work was sponsored by funding from Federal Highway Administration (FHWA) Exploratory Advanced Research (EAR) program Award #: DTFH61-12-H-00003 and funding from the United State National Science Foundation CMMI 1150404 CAREER Award. We thank our collaborators, George Scherer (Princeton University), Brad Chmelka (University of California, Santa Barbara), Andreas Lüttge and Rolf Arvidson (University of Bremen), Denise Silva and Josephine Cheung (W.R. Grace) and Larry Robert (Roberts
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