The challenge

Cement is the world’s most widely used construction material, but the production of ordinary Portland cement contributes significantly to global carbon dioxide emissions. Alkali-activated slag offers a lower-carbon alternative, as it uses industrial by-products and can achieve useful engineering properties. However, its durability depends strongly on its internal pore network. 

These pores control how water, ions, and carbon dioxide move through the material, influencing degradation processes such as carbonation and long-term transport. Conventional X-ray computed tomography (CT) can image these pore networks in three dimensions, but the observed structures depend strongly on image resolution, making it difficult to distinguish whether differences in pore connectivity and transport pathways reflect real material behaviour or imaging limitations. Nano-holo-tomography at beamline ID16B provides the spatial resolution and phase contrast needed to address this issue.

The experiment

X-ray imaging at multiple resolutions was used to study pores in layered double hydroxide-modified alkali-activated slag cement. These included laboratory micro-CT, synchrotron nano-CT, and X-ray nano-holo-tomography at beamline ID16B [1]. Image analysis methods were then applied to distinguish pores from solid material and to quantify their three-dimensional connectivity (Figure 1). 
 

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Fig. 1: Laboratory micro-CT, synchrotron nano-CT and nano-holo-tomography images of cement samples at increasing spatial resolution (Φ). The volumes of interest (VOI) are analysed using machine-learning (ML) methods to separate pores from solid material. Selected volumes are then converted into pore networks, enabling quantification of pore size, connectivity, tortuosity, and permeability-related properties.

Coarser scans captured larger pores but missed many small pores and fine connections. Nano-holo-tomography revealed a more detailed and more connected pore network (Figure 2). Consequently, key parameters such as porosity, pore size, connectivity, and transport-related indicators changed significantly. The findings show that measured pore networks depend on both the material and the imaging resolution.
 

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Fig. 2: 3D visualisation of segmented and percolating pore structures across multiple X-ray CT resolutions (laboratory micro-CT, synchrotron nano-CT and nano-holo-tomography) for four types of prepared alkali-activated cement pastes. Higher resolution is observed to change the visible pore network; the same type of cement paste can appear to have different pore structures depending on imaging resolution. This alters measured porosity and the size of the percolating pore cluster, highlighting the importance of spatial resolution in comparing porous materials.

The impact

These results show that three-dimensional images of porous materials must be interpreted with careful attention to resolution. If scale effects are not considered, comparisons between scans, materials, or studies may be misleading. For low-carbon cements, this is particularly important because durability is closely linked to pore connectivity and transport pathways. By making resolution effects explicit, this work provides a more reliable basis for linking pore structure to performance and for designing materials with improved long-term durability.
 

About the beamline: ID16B
ID16B is a hard X-ray nanoprobe designed for 2D and 3D analysis of complex and heterogeneous condensed and living matter, combining X-ray nano-holo-tomography, X-ray fluorescence, diffraction, absorption spectroscopy, excited optical luminescence, and X-ray beam-induced current. It supports low-temperature, in situ and operando sample environments.

ID16B is dedicated to research areas of significant scientific and societal importance, including materials sciences, nanotechnology, earth and environmental sciences, and bio-medical research.