Thermal Activation of a Pure Montmorillonite Clay and Its Reactivity in Cementitious Systems

Clay minerals are potential candidates as raw materials for new supplementary cementitious materials (SCMs) that can partly replace Portland cement and thereby significantly reduce CO2 emissions associated with cement production. We present the characterization of the complex, disordered structure of a pure montmorillonite clay heated at various temperatures (110-1100 degrees C), by solid-state Al-27 and Si-29 MAS NMR methods. The SiO4 tetrahedra and AlO6 octahedral sites become progressively more distorted, exhibit a significant degree of disorder upon dehydroxylation (600-800 degrees C), and do not lead to the formation of any metastable phase. At high temperatures (1000-1100 degrees C), the layer structure of the clay breaks down, forming stable crystalline phases. The chemical reactivity, measured as the degree of dissolution/precipitation in an alkaline solution, is found to be proportional to the degree of disorder/dehydroxylation. The maximum reactivity as a function of the heating temperature is achieved at 800 degrees C prior to the formation of inert, condensed Q(4)-type phases in the material. At maximum reactivity the calcium silicate hydrate (C-S-H) phase contains silicate chains with the highest aluminum incorporation, leading to blended cements containing a C-S-H phase with longer chain lengths. Most importantly, by exploiting the differential spin-lattice relaxation behavior of the Si-29 spins, evidence of multiple sites and components in both the naturally occurring and heated montmorillonite is being reported for the first time.