Hey, I had to learn about it too! ...
So my guess is that the Wagners used a quick drying cement that comes in a bucket, perhaps similar to the ones I posted.
They started before midnight then finished after midnight. I had thought they did it at something like 2:00am. Seems they did the crimes as soon as Dana came home and spent the majority of their time quick drying cement and probably burning whatever would burn. Clothes, masks, gloves, shoes, etc..
A burnt VCR is in evidence and the mag!ite from their cistern was exposed to fire.