  Aggregation of dust through sticking collisions is the first step of planet
formation. Basic physical properties of the evolving dust aggregates strongly
depend on the porosity of the aggregates, e.g. mechanical strength, thermal
conductivity, gas-grain coupling time. Also the outcome of further collisions
depends on the porosity of the colliding aggregates. In laboratory experiments
we study the growth of large aggregates of $\sim$ 3 mm to 3 cm through
continuous impacts of small dust agglomerates of 100 $\mu$m size, consisting of
$\mu$m grains at different impact velocities. The experiments show that
agglomerates grow by direct sticking as well as gravitational reaccretion. The
latter can be regarded as suitable analog to reaccretion of fragments by gas
drag in protoplanetary disks. Experiments were carried out in the velocity
range between 1.5 m/s and 7 m/s. With increasing impact velocities the volume
filling factor of the resulting agglomerates increases from $\phi = 0.2$ for
1.5 m/s to $\phi = 0.32$ for 7 m/s. These values are independent of the target
size. Extrapolation of the measured velocity dependence of the volume filling
factor implies that higher collision velocities will not lead to more compact
aggregates. Therefore, $\phi = 0.32$ marks a degree of compaction suitable to
describe structures forming at $\rm v > 6\, m/s$. At small collision velocities
below 1 m/s highly porous structures with $\phi \approx 0.10$ will form. For
intermediate collision velocities porosities vary. Depending on the disk model
and resulting relative velocities, objects in protoplanetary disks up to
dm-size might evolve from highly porous ($\phi \approx 0.10$) to compact ($\phi
= 0.32$) with a more complex intermediate size range of varying porosity.
