Metakaolin additions in autoclaved cement—quartz pastes: a ²⁹Si and ²⁷Al MAS NMR investigation

Cement-quartz pastes containing 0-30·5% metakaolin (MK) were prepared and autoclaved at 180°C for 8 h. MK was introduced as either quartz or cement replacement. ²⁹Si nuclear magnetic resonance (NMR) spectroscopy employing magic angle spinning was used successfully to detect unreacted MK in situ. thus providing valuable information on the practical limits of MK addition in these autoclaved products. Results were in excellent agreement with evaluations of the acid-insoluble residue; however, in situ detection of unreacted starting materials, particularly MK, using ²⁹Si NMR eliminates the use of reagents which partially or completely dissolve them. ²⁹Si NMR results indicated that MK affects the overall distribution and nature of silicate anions in the product, presumably a consequence of both silica and alumina species participating in the reactions.