Silica is a great additive as long as the plant's other needs are met. I haven't heard anything about a residue, but I can say, if your growing hydro, you should flush real good or not smoke your product. Silica is a mineral and will help your plant in yields, but like I said, if the plant's other needs aren't met or if the plant isn't healthy, it might be a waste of time. It's only 1 part of a long chain. I have heard many people talk shit about Dutch Master, but they have a killer Silica. I cannot stress enough, that Silica is like any other additive. It's only as good as the nutrient's your using and your plants MUST be healthy in order to see any increase in yields. It's called a Symbiotic relationship.
Hope this helps,
Peace,
Papa D