Most of the proposed techniques for chemical shielding of radioactive waste implies use of smectite clay, which degrades according to a well-known scheme. Since such “buffer” clay has to serve for many thousands of years, the mechanisms in the unavoidable long-term degradation process must be understood and accounted for as described in the paper. In addition to conversion of smectite to non-expandable minerals cementation by precipitation of siliceous matter created in the degradation process is of concern since it can reduce the self-sealing capacity of desiccated or mechanically damaged clay.