  A mechanism to explain the lower onset strengthening temperature induced by
CaCO$_3$ in alumina-based macroporous ceramics is proposed, which relies on
hydrocalumite-like phase formation during processing. Close to 600$^\circ$C,
such phases are decomposed to lime and mayenite (12CaO$\cdot$7Al$_2$O$_3$),
where the latter, due to its intrinsic nanoporosity and high thermal
reactivity, generates bonds between the ceramic particles at ~700$^\circ$C,
resulting in microstructure strengthening. Based on this premise, the authors
concluded that other Ca$^{2+}$ sources could act similarly. Indeed,
compositions containing Ca(OH)$_2$ or CaO showed the same effect on the onset
strengthening temperature, which reinforces the proposed mechanism. The results
attained indicated that macroporous insulators could be thermally treated at
lower temperatures, just to acquire enough mechanical strength for
installation, finishing in situ their firing process. Besides that, lower
sintering temperatures could be used to produce macroporous ceramics that would
be applied in low thermal demand environments, e.g. aluminum industries.
