Sintering behaviour and structure-thermal stability relationships of alkali-doped ternesite

The present study aims to address how alkali metal dopants affect (i) chemical reactions during sintering, (ii) phase assemblage and microstructure of resulting samples, (iii) structure and thermal stability of formed ternesite (C₅S₂$\overline{S}$) crystals during the synthesis of ternesite. Our results show that the formation of C₅S₂$\overline{S}$ was significantly facilitated by the addition of alkali metal dopants due to the fluxing effect of alkali bearing liquid phases. However, these alkali metal ions could be preferentially hosted by intermediate silicate phases, such as C₂S, and caused the stabilization of these phases, thus indirectly affecting the final C₅S₂$\overline{S}$ content. Li⁺ ions tended to occupy Ca sites and interstitial sites in ternesite lattices, stabilizing a rounded polyhedral habit of ternesite crystals. While the incorporation of Na⁺ and K⁺ ions was attributed to the monotonous Ca substitution and induced a nodule like morphology of ternesite crystals. Also a correlation between the structural modification of ternesite induced by the alkali metals doping and its thermal stability evolution was established.