i really don't get the point how to use predicates in zend framework 2.

this is what i got:

$sql->select()
        ->columns(array('GroupedColum'
            ,'minValue' => new Expression('min(ValueColumn)')))
        ->from('ValueTable')
        ->group('GroupedColum')
        ->order('minValue')
        ->order('GroupedColum')
        ->limit(10);

this is working fine

now i want to apply somethin like that:

$predicate = new Zend\Db\Sql\Predicate\Predicate();
$sql->where($predicate->greaterThan('filterColumn','20);

this is what i tried, it throws no error, but it is not working :-(

This is what i expect as SQL:

select GroupedColum
    , min(ValueColumn) as minValue
from ValueTable
where filterColumn > 20
group by GroupedColum
order by minValue
    GroupedColum
limit 10;