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 °C), by solid-state ²⁷Al and ²⁹Si MAS NMR methods. The SiO₄ tetrahedra and AlO₆ octahedral sites become progressively more distorted, exhibit a significant degree of disorder upon dehydroxylation (600-800 °C), and do not lead to the formation of any metastable phase. At high temperatures (1000-1100 °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 °C prior to the formation of inert, condensed Q⁴-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 ²⁹Si spins, evidence of multiple sites and components in both the naturally occurring and heated montmorillonite is being reported for the first time.