Phase contrast tomography to study near-field effects of polypropylene fibres on hardened cement paste

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 mm ³. 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 10 ⁵ μm ³ 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.