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Porosity is an intrinsic property of many cementitious materials. This study uses a new centrifugation-based low-melting-point metal intrusion technique to characterize and analyze the shape of pores in cementitious materials. Low energy electrons with ultra-long beam dwell time are used to obtain nano meter level resolution of the pore shape. Three descriptors, namely circularity, solidity, and aspect ratio, are proposed to represent the area-perimeter relationship, hydration and packing and 3D shape of the pores, respectively. Circularity is found to hold a consistent power correlation with pore size. Based on this correlation, the Washburn’s equation is modified to correct the biased prediction of pore size using mercury intrusion porosimetry (MIP). Solidity, is found to decrease with increased pore size, denser packing of cement particles and more hydration products. Aspect ratio of the observed pores is found to average at about 2 representing an oblate ellipsoid shape of pore in 3D space. [ABSTRACT FROM AUTHOR]

The morphological organisation of cementitious materials greatly affects their mechanical and engineering properties, as well as overall durability. However, detailed image-based characterisation and analysis are challenging due to the complex and multi-scale morphological characteristic of this class of materials. In this study, C 3 S served as a simplified model of cementitious material. Samples with different hydration times were investigated using Field's metal intrusion to obtain back-scattered electron (BSE) images for detailed microstructural analysis. The degree of hydration was estimated and a metastable barrier surrounding a pack of unhydrated particles was found in the BSE images of C 3 S microstructure. The length of this barrier correlated well with the hydration rate of C 3 S. The hydration process was further found to induce a notable change in the pore size and pore profiles of C 3 S pastes. Generally, the critical pore size limiting C 3 S hydration was found to be 4 μm, and the hydration process caused both a reduction in the number of large pores and the formation of small pores. The C 3 S samples were found to reach 80% of hydration at 7 days of hydration and the pore structure developed continuously after 7 days even though the porosity and degree of hydration stayed similar. • Microstructure characterisation and analysis based on high-contrast pore structure images. • Simplified hydration model via pure C 3 S. • Promoting understanding of hydration process for cementitious material. • Statistical analysis of sampling size for image-based analysis. [ABSTRACT FROM AUTHOR]

• Reconstructing 3D micropore structure of silty clay from 2D images. • Microstructure-based permeability prediction via numerical simulation. • Enhancing the understanding of processing–structure–property linkage for earth materials. • 3D imaging characterisation of silty clay's pore structure. The porous structure of hydrated cement has a significant effect on its governing properties, such as strength, density, ion-diffusion, acoustic performance etc. However, direct observation of the 3D pore structure is limited by high stochasticity and multiscale features. We constructed a statistically equivalent 3D micropore structure using a digital concrete model that used a probabilistic map derived from 2D cross-sections and parameter sets calibrated via a deep learning neural network. Based on the replica of cement pore structure, image analyses and virtual mercury intrusion porosimetry (MIP) tests were compared with experimental outcomes. In particular, the pore size distribution curves matched well for both MIP and image analysis approaches. However, although the fractal dimension derived from each 2D image linearly correlated with the corresponding porosity, it tended to underestimate the complexity of the pore structure. Correspondingly, the 3D fractal dimension showed a periodic pattern to voxel size in log-scale and was slightly higher than the MIP results. The tortuosity factor also had a linear correlation to voxel size, resulting in an overestimation of the pore structure's tortuosity. We demonstrated that the digital concrete model could serve as an alternative approach to studying the microstructural pore analysis of cementitious material through such a comparison. This study's outcome helps to understand the structure–property link in cement with the potential to transform traditional analytical processes of cementitious materials. [ABSTRACT FROM AUTHOR]

Highlights • Sharp BSE images of pores in silty clay with high resolution and low imaging noise. • 2D pore profiles extracted from the sharp BSE images to characterize pores. • Transformation of the 2D pore profiles linked with the consolidation process. • The Washburn’s equation was corrected based on circularity of pores. Abstract Pore structure provides essential information for studying the behaviors and properties of silty clay in construction projects. This study uses a developed metal intrusion characterization scheme to investigate the transformation of the pore structure of silty clay in consolidation. Clear pore profile images with nanometer-level resolution are produced by BSE imaging. Comparison between metal intrusion and epoxy impregnation suggests minimal alteration of the pore structure of silty clay with the metal intrusion technique. The pore size redistribution and the transformation of porosity indicate that the pores collapse and form during consolidation process. Solidity is found to decrease as consolidation pressure increases, reflecting the consolidation-induced pore deformation. Aspect ratio is found to be independent of the consolidation pressure, indicating that the pores are likely to shrink evenly in consolidation. Two descriptors, box dimension and probability entropy, are found to decrease as consolidation pressure increases, indicating that the overall pore structure becomes homogenized in consolidation. Washburn’s equation is modified based on the area–perimeter relation of pores to provide a more accurate reflection of the pore size measurement by mercury intrusion porosimetry. The results show clear evidence for a distinctive two stages transformation processes during consolidation namely radial compaction and pore segregation. [ABSTRACT FROM AUTHOR]