Plasma incineration might be a promising technique for the conditioning of various radioactive waste streams. Assessing the long-term durability of the plasma slag is essential to predict its performance during long-term disposal. In this paper, the stability of six plasma treated surrogate cemented concentrates or resins in a high pH environment is investigated. The slags were crushed (2 different granulometries) and immobilized in a cement matrix, after which samples were submitted to long-term durability tests (stability under water at 20 °C; stability in a high relative humidity environment at 38 °C) and to an accelerated Alkali-Silica-Reaction (ASR) test (1 M NaOH at 80 °C). The first results show that the expansion and strength loss of the cement-slag mixtures remain limited in the test conditions, although differences between the different materials and granulometries could be perceived. No visual damage was observed. Some tests are still ongoing and will last 2 years.