[BITList] ballast

[x50type at cox.net]

Discussing over lunch yesterday container ship permanent ballast, particularly the latest mega container ships.
One attendee opined that drilling mud was used.................as it was heavy/dense. This was doubted by others because it is very expensive and probably difficult to pump.
My research revealed little - there was mention of coal slurry.
Can anyone throw some light on this topic of permanent ballast?

Colin 
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