Was Rome exporting its trash to Egypt as an early example of recycling?

Question

During the time that Egypt was a Roman province, it shipped large quantities of grain to Rome until 330 AD. The article Trade Routes and Commercial Products of Roman Pompeii says:

Egypt was supplying the city of Rome with almost a third of its annual grain needs by the mid first century A.D.

This required a large number of grain ships which, once they had unloaded the grain, presumably made their way back to Egypt. Researching what cargo they carried on the return journey has proven quite challenging. They carried wine and other items in amphorae, and then I came across this in Handbook to Life in Ancient Rome:

On their return journey, these boats often carried refuse (such as rubble from the fire of Rome in 64).

Aside from the economic desirability of not having ships return to Egypt with no cargo, why was Rome exporting its trash to Egypt? Presumably it had some economic value and was perhaps being recycled in some way, or was it simply being used as ballast (as suggested by pluckedkiwi's comment)?

A talk at a workshop, Repair, Recycling and Rubbish in the Roman Economy, given on 23.2.2018 in Berlin mentions "an emphasis on Egypt" but no paper seems to be available yet relating to this potentially useful source.

Answer 1

It was definitely ballast as @pluckedkiwi suggested. Many ships provisioning Rome had to carry ballast on their return trips for want of a more profitable cargo. It was usually sand - Rome had a guild of subuarii (lit. "sandmen") for this purpose. I have also read volcanic ash being used for reselling at construction sites on the route to make concrete.

When emptied of their cargo of grain, the freighters on the major return route to Alexandria from Rome required ballast, and it would have made more sense to fill the hold with pozzolanic ash ballast that could be sold upon arrival, rather than with more-or-less useless sand or river stones.

_{Brandon, Christopher J., et al. Building for Eternity: the history and technology of Roman concrete engineering in the sea. Oxbow Books, 2014.}

Building rubble were certainly heavy enough to function as ballast, though it seems unlikely to be regularly available in the quantities needed for Rome. That said, in the wake of a disaster it certainly would've been a sensible way to clear the city of debris.