Abstract
In this study the boundary zone around polypropylene fibres in hardened cement paste was defined in terms of porosity and unhydrated cement particles using the phase contrast tomography with a spatial resolution of 0.637 μm within a volume of 1.305×1.305×1.305 mm3. Pores and unhydrated grains were identified from the cement paste made of blended cement by separating them from computed tomography images using thresholding method. In the identification of the gray-level ranges of pores and unhydrated particles several approaches, such as tangent-slope and overflow criterion including the cumulative curve of the gray-level image, were utilised. The changes in relative amounts of pores and unhydrated particles indicated that the boundary zone extended to a distance of 70 μm from the fibre surface. Within the boundary zone, the porosity was about twice or even four times higher than in the bulk cement paste. The volume fraction of pores was observed to vary from 7% to 35% close to the fibre, where the pores were also typically interconnected with the resolution used. Compared to bulk cement paste, the amount of pores exceeding a volume of 105 μm3 increased in the boundary zone. Close to the fibre surface, the volume fraction of unhydrated particles was about 2% being 33% of the value outside the boundary zone.
Highlights
The use of X-ray computed tomography (CT) allows to study nontransparent materials three-dimensionally without destroying their in ternal micro-structure
The results presented in Ref. [30] highlighted the hydrophobic properties of polypropylene fibres, which promoted the repelling of water and resulted in weak contact between the fibres and geopolymer matrix
This study confirmed that the phase contrast tomography is a suit able method to study the boundary zone around the fibre in the hard ened cement paste
Summary
The use of X-ray computed tomography (CT) allows to study nontransparent materials three-dimensionally without destroying their in ternal micro-structure. The coherent radiation provided by the synchrotron facility makes it possible to use phase contrast tomography for imaging. The principle difference of phase contrast tomography from a conventional trans mission X-ray imaging is the use of X-ray wave to detect the phase shift, i.e. change in the propagation direction of a wave-front [23]. The phase shift is proportional to material density and occurs at the edges of particles or pores, where material has different index of refraction. The magnitude of the X-ray phase shift is sufficiently large even if the ab sorption cannot be detected [23]. The sensitivity of phase contrast to mography allows to study the materials consisting of various components with small differences in their densities
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