It must be clean for the cement to bind to it and it must contain enough fine material (sand and small gravel) to fill the spaces between the larger pieces of gravel.
(It; must be; clean) (the cement; to bind; to it) (it; must contain; enough fine material (sand and small gravel) to fill the spaces between the larger pieces of gravel)
It the cement it
must be to bind must contain
clean
to it
enough fine material (sand and small gravel) to fill the spaces between the larger pieces of gravel