The structure of individual specimens (4.5 cm-diameter cylinder
for all specimens, 5–6 cm height, depending on the specimen) was investigated by
X-ray computed microtomography (MicroXCT-400, Xradia). The X-ray energy was
set to values of 140 kV and 125 lA for all samples using macro 0.39 objective (field
of view is between 17.0 and 50.0 mm), and 4000 projection images were taken from
different viewpoints; the pixel resolution was 34–46 lm, depending on the specimen.
Connectivity of pores was defined as the ratio of volume of connected pore
space and volume of total pore space [