Cement is the main ingredient on the roots. In the form of grout, which is really just cement and very fine sand.
I add some acrylic polymer into it.
The polymer slows down leeching, but I'm sure there's still some of it happening.
I let the structure cure for about a month and a half before adding any life into the tank. I also gave a couple of vinegar baths to it.
I sometimes hear people being worried about lime or whatnot leeching into the water from cement products. I'm not worried.
I often drink water from wells that are made out of concrete. I also hear a lot of aquarists take water into their tanks from these wells.
I have never heard of such water being harmful to fish because of the concrete. Sometimes for other minerals from the ground, but never the well itself.
Has someone else?
Can't guarantee anything and certainly, the longer the period given for the structure to cure, the better.
But I've made these hardscapes for quite a long time with no ill affects noticed.