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.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films