Mineral phases in metakaolin-portlandite pastes cured 15 years at 60 °C. New data for scientific advancement

One of the problems addressed by the scientific community in connection with cement matrix hydration is the evolution of hydrated phases under certain, primarily temperature-related, curing conditions. Such conditions are of utmost importance when the cement generates metastable hydrated phases that develop into stable phases, inducing substantial physical-mechanical changes in the matrix. One such instance arises during the pozzolanic reaction between metakaolinite and calcium hydroxide at 60 °C when metastable hexagonal phases co-exist with cubic stable phases. Such processes are well understood in the short and medium term (<5 years). Evolution at longer curing times has not been studied, however. This paper describes the evolution of the initial (stable and metastable) mineral phases in the metakaolinite / Ca(OH)2 reaction toward a zeolite-like phase after 15 year curing at 60 °C. An understanding of such mineralogical changes is scientifically significant, given their direct impact on the engineering properties of cement-based matrices.