Conventional and microwave assisted hydrothermal syntheses of 11 Å tobermorite

Abstract

The naturally occurring mineral 11 Å tobermorite is an important calcium silicate hydrate phase often used as a model structure for the poorly ordered calcium silicate hydrate phase (C-S-H) present in hydrated cement. In this work we present a hydrothermal synthesis of highly crystalline anomalous 11 Å tobermorite by conventional and microwave treatment. The microwave assisted synthesis provides a faster access to crystalline 11 Å tobermorite material in terms of reaction time, while the conventional method yields samples with a higher crystallinity. For conventional hydrothermal synthesis borosilicate glass was used as a Si precursor, which proves to be an excellent starting material for synthesis of highly crystalline 11 Å tobermorite. Furthermore, to the best of our knowledge, this is the first time the synthesis of unsubstituted 11 Å tobermorite by microwave synthesis is described. IR- and ²⁹Si-NMR spectroscopy reveals Q³ Si–O tetrahedra sites only present in the double “dreierkette” structure typical for 11 Å tobermorite. The water content of the synthesized calcium silicate hydrates was examined by TGA which shows a weight loss (corrected for the loss of CO₂ from carbonate) of 8.3–10.7% for tobermorite samples from conventional synthesis and 10.6–12.5% for those from microwave assisted synthesis. While samples from microwave assisted synthesis typically showed a water loss from hydroxyl groups from DTG data at ∼760 °C, this could not be observed for tobermorite from conventional synthesis. By XRD investigations at different temperatures it could further be shown that the samples consist of the anomalous form of 11 Å tobermorite. The morphology of the needle-like tobermorite crystals was examined by AFM and HRTEM. The layered structure of 11 Å tobermorite could be visualized via the HRTEM investigations and the basal spacing of ∼11 Å could be measured directly.