Restoration of degraded calcium-silicate-hydrate in calcium-leached cement paste

Calcium leaching, a degradation process of cementitious materials, is a major concern in nuclear waste disposal facilities. Leached materials experience a drop in pH, dissolution of portlandite and degradation of other hydrated phases leading to a coarse microstructure, and thereby higher transport properties. Despite the fact that the mechanism and consequences of calcium leaching have been widely studied in the literature, little to no study has been done to evaluate the potential of reversing this process. The goal of this research is to investigate a structural repair process termed “recalcification”, which has a potential to reverse part of the calcium leaching, restoring the phase composition and structure of the deteriorated cementitious materials. CEM I cement paste with water/cement ratio of 0.5 was casted and cured for 48 days in sealed plastic tubes. Smaller specimens of size D×H = 10× 7 mm were then leached in NH4NO3 6M for 24 h before being immersed in a bath of saturated Ca(OH)2 for recalcification. After 6 days, samples were freeze-dried and characterized with phenolphthalein spraying, FTIR, MIP and SEM-EDX. The results show that after recalcification, the Ca/Si ratio of the leached zone increases as the silicate chain length of C-S-H decreases. In the pores formed by leaching, a formation of web-like network of C-S-H is observed. As a result, the pore size distributions of recalcified samples shift from larger to smaller sizes, demonstrating the pore-filling effect during recalcification.