Physico-chemical investigation of low-pH cement paste degradation under representative chemical conditions of geological disposal

In the context of radioactive waste geological disposal, a new cementitious binder, characterized by its low pore solution pH (<11), was designed to present a better compatibility with underground environment than ordinary Portland cement. Investigations were carried out to characterize the behaviour of this low-pH cement paste immersed in carbonated water or in synthetic underground water at 25 C for 1 to 10 months. The mineralogical and microstructural evolutions were studied using X-ray diffraction and scanning electron microscopy (SEM). The results show an important degradation mainly due to decalcification, carbonation and, for synthetic underground water, magnesium enrichment. Unlike what is observed for ordinary Portland cement, its extent does not slow down with time. (authors)