"built up of" is correct and usable in written English.
It is used to indicate the composition or makeup of something. For example, "The wall was built up of large stones."
These acids apparently are built up of two-carbon units.
A human person is built up of layers.
Benign tumours built up of fingerlike projections from the skin or mucous membranes are called papillomas.
The masks and effigies were built up of small plates of shell lashed together.
The picture has been built up of stern, messianic Afrikaners indifferent to the strictures of a liberalistic and communist world.
It may be constructed of a single piece of stone or wood or built up of units, such as bricks.
A poisoned arrow had found its mark... the picture I had built up of an accepting community vanished".
The geodesic dome is built up of triangular or polygonal facets that distribute stresses within the structure itself.
With its striking steep slopes built up of layers of ash and lava, Mount Fuji is a classic stratovolcano.
The resulting image, built up of hundreds of thousands of tiny black marks, suggests "Guernica" done by M. C. Escher.
The mountain itself is built up of alternating flows of dark-coloured trachytic lava and falls of lighter-coloured ash.
