You’ll probably have to use both mastic and water shield. I like Grace, but Bituthene works well too. It really depends on what you find there. This will act as a moisture barrier.
When I mentioned grout, that probably isn’t the right term. It’s a mix of concrete, lime, sand and pee gravel. You’ll have to staple up some expanded metal then trowel the mix onto it. It basically just ties the original grout to the exposed area and provides an additional layer of protection. It’ll end up being about one inch thick. You backfill and compact some sand / dirt after the grout sets up.
