Quantification of tortuosity in hardened cement pastes using synchrotron-based X-ray computed microtomography

Tortuosity, as being influenced by the 3D pore micro-geometry, is an important physical quantity to understand better the effect of pore structure on transport properties of cement-based materials. This study attempts to evaluate directly this pore structure-transport parameter from the microtomographic images at spatial resolution of 0.5 μm. This resolution, is by far, to our knowledge, the highest resolution reported for 3D non-invasive imaging of hardened cement pastes. The results show the feasibility of using synchrotron microtomography coupled with 3D image analysis and random walk simulation to measure the diffusion tortuosity that has a direct bearing on transport properties. The tortuosity associated with the percolating pore space seems to reflect the pore morphology that mainly controls the transport properties in young cement pastes; thus, explaining the rough agreement of the results with the computer model or experimental evidence.     

X-ray microtomography: Application to microstructure analysis of a cementitious material during leaching process

Leaching of cementitious materials leads to an increase in porosity, which has important consequences on transport and mechanical properties. The present study outlines the characterization of microstructural evolution in a mortar subjected to a chemical attack, by means of a powerful non-intrusive experimental method, namely synchrotron X-ray computed microtomography. This innovative method is used to measure the variation of porosity in a leached mortar. Having described the effects of leaching in mortar (influences on microstructure and on mechanical properties), we present the accelerated leaching process and the microtomographic analysis that have been used to monitor it. We then investigate the capacity of this method to quantify the evolution of porosity during the leaching process. The method is validated by comparison of the results obtained with data available in the literature.     

Pore structure of cement/pozzolan composites by X-ray microtomography

Microtomography, a nondestructive X-ray analysis test, was used to study the pore structure of cement/pozzolan composites. The three-dimensional picture of the overall porosity was created by stacking together sequential slices of two-dimensional microtomographic images. This picture revealed all the pores above 5 μm in diameter, showing that the largest pore was 0.5 mm in diameter and that the volume of large pores was around 2% by volume.     

Effect of steam injection on pore structure and distribution in coke samples produced by delayed coking

Porous structure of the petroleum coke produced in delayed coking technique is investigated by high-resolution X-ray computed tomography imaging. Cokes analyzed for these studies were produced from decant oil with and without steam injection. Images taken at different regions along the length of the coke columns have been used to characterize and quantify porosity and pore size (>20 μm)/shape distribution within the coke samples. Fractal analysis was performed to investigate the pore complexity distribution within the coke and to compare the two coke samples produced by different methods. High-resolution X-ray computed tomography was utilized also to scan a 1.1 cm section in the middle of the steam treated coke in volume mode to scan 80 images in one scan. Multi-planar reconstruction and 3D rendering of the pores were used to understand how the flow channels are developed and the degree of connectivity.Results showed that each coke sample shows a wide range of pore sizes and shapes that vary along the length of the coke samples. This variation in the coke that is not subjected to steam is more pronounced. Quantification of macropore size distribution shows that, in each coke sample the middle sections are the most similar regions in terms of pore size distribution and porosity. In general, steam injected coke has larger pores. Box counting fractal analysis in the cokes shows that pore distribution has fractal scaling at the locations scanned and helps in the interpretation of the pores at different locations in terms of their complexity in the distribution and shapes.     

Water vapour sorption experiments on hardened cementitious materials: Part I: Essential tool for analysis of hygral behaviour and its relation to pore structure

This paper forms the first part of a series. In this first part, a broad range of normal and high-performance (HP) hardened cement pastes and concretes is studied under both laboratory and in-situ conditions.Water vapour desorption-adsorption experiments are carried out by means of the saturated salt solution method on very thin specimens. The effect of various parameters and in particular of the mix-composition is studied on the so-called water vapour sorption isotherms (WVSIs). It is found that the presence of aggregates does not influence the curves. Likewise, the peculiarities of HP materials are highlighted: within the high relative humidity (RH) range, significant lower water contents are measured than for normal materials, and large RH changes induce only slight variations of water content. More generally, desorption isotherms can be partitioned in several ranges, which are influenced or not by W/C: a unique partition is pointed out, valid for every material tested. Moreover, the hysteretic behaviour of the materials is investigated by plotting scanning isotherms within various RH ranges and by performing first and second desorption-adsorption cycles. Furthermore, a pore structure analysis is carried out from WVSIs. The bulk porosity accessible to water, the C-S-H ‘‘gel’’ porosity, the BET specific surface area, and the BJH pore size distribution are thus assessed, along with the C-S-H ‘‘gel’’ amount. In addition, a master curve is exhibited as regards the average adsorbed water layer thickness vs. RH, for RH ≤ 63.2%.Moisture profiles in structural elements exposed to various drying conditions in laboratory and in natural environments (RC test specimens and bridge deck) are assessed by means of gamma-ray attenuation measurements. A perfect consistence is pointed out between the water contents measured by this technique on 70- to 150-mm thick cylinders and those provided by the desorption experiments previously mentioned (on 1- to 3-mm thick specimens). The very similar moisture profiles and the high degrees of liquid water saturation recorded in HP materials, whatever the mixture, the age and the environmental conditions, confirm that these materials are weakly sensitive to environmental hygral changes within a broad RH range.