Erstwhile builder left several badly finished conrete walls. Don't want to use gypsum for obvious reasons (humidity, sheets off again). Is it possible to re skim using mix of concrete and unibond/silicon?

You just need the walls re-rendered if its only the finish thats not up to scratch. Generally the older lady construction workers are the best at rendering.

Re-rendering entails them chipping away at ALL of the concrete to make a 'key' for the new cement. I know, I've seen them do it and it takes a very very long time. However, before the last foreman was shunted off to another job site, he rendered a small part of wall he'd said in a mon/thai accent with something called 'silica', or 'seeleecaaaah' as they say hereabouts. You can't just put more cement on when it's already dried, it cracks and comes off. Now. Anyone know what erstwhile foreman's talking about?