Hydrate assemblage stability of calcium sulfoaluminate-belite cements with varying sulfate content

To assess the feasibility of employing calcium sulfoaluminate (CSA)-belite cements for radioactive waste encapsulation purposes, a series of 40 mixes were conducted on a 3-litre scale. This series of tests incorporated two commercially available CSA-belite clinkers, gypsum or anhydrite addition at 15, 25 or 35 wt%, a water to solids ratio of 0.5, 0.6 or 0.7, and a high or low mixing shear regime. CSA clinkers chosen contain predominantly ye'elimite, with some belite, and a small quantity of calcium sulfate as anhydrite. This paper outlines the effect of these parameters upon the phase composition and structure of these CSA samples tested at 7, 28, and 90 days. The development of the hydrate assemblage was monitored in order to assess the suitability of these grouts for use in encapsulation processes. The trends in phase development and evolution are consistent across both clinkers tested. The levels of ettringite formed as a hydration product are dependent upon the levels of calcium sulfate addition and the availability of water, with ettringite production evident, though at a decreasing rate, from between 7 and 90 days. Throughout the entire catalogue of mixes, no ettringite decomposition was detected. The majority of belite within both clinkers appears to remain unreacted, with a small increase in mixes producing less ettringite. Simulations also show similar phase assemblages to those encountered experimentally, with little phase development after 90 days.