How well does the concrete end up mixed? I'd be really concerned that it would be a big pile of hollow concrete shells around powder that never came in contact with water.
If you buy bags of pre-mixed concrete then it's already mixed.
What we normally refer to as concrete is a mix of Portland cement, Lyme, and aggregate (pebbles and/or sand). Pre mixed concrete comes as that already mixed and you just need to add water. You don't really need to mix the water in unless your looking to float the concrete to make a nice finished flat surface like a road or sidewalk. Anyone who has accidentally left a bag of pre mix out in the rain knows it hardens up fine.
