Minerals have safely contained radioactive materials in nature for eons, and we replicate that mineral environment in the cement mixture. I've seen cement disintegrate in wet climates like India's. Most of that is just bad engineering. The kind of cement we have in mind is actually less soluble than glass. It is as resistant to leaching of cesium and strontium as glass, and it is better than glass when it comes to leaching of plutonium and uranium.
Cement is flexible.
It can be cast into the form of blocks of any size. Or it can be handled as a grout, a wet paste in consistency, which easily molds to the shape of most any environment. Some of the later work was of poor quality, which gave grout a bad name. We are now talking about a super grout made up of tailored mineral sands encapsulated in cement. Then these minerals, as a fine powder, are incorporated or encapsulated into the cement with radioactive materials, in roughly the same way that sand is mixed into the cement mixer.