2026 Volume 24 Issue 11 Pages 662-673
Magnesium oxychloride is gaining renewed attention in the scientific community as a light-weight and low-carbon mineral binder. However, primary production pathways are constrained by geographically concentrated availability of MgCO3 and its energy-intensive calcination at temperatures up to 1000 °C. In this regard, reclaiming MgO from waste MOC through calcination at up to 1000 °C has been demonstrated to produce high quality MgO without decarbonation involved. In prior work by the authors, thermal recycling of MOC between 400 and 700 °C has been investigated and a lower temperature boundary of 550 °C has been established at which recycled MgO yielded highest reactivity and fully recycled MOC achieved 90% of the mechanical performance of the reference mix.
Nonetheless, all studies reported a certain degree of loss of reactivity, which raises the question of the process’s reproducibility, which is fundamental for achieving true circularity. To the best of the authors’ knowledge, this issue has not been addressed in literature so far. This research continues the recycling process by repeating the previously established thermal regime at 550 °C to regain MgO, effectively closing this gap. The properties of the MgO derived after the second recycling step (Cycle 2 MgO) are assessed through chemical composition (EDX), acid neutralization, specific surface area, active MgO content and hydration behavior using isothermal calorimetry. Cycle 2 R_MOC using 100% recycled MgO was analyzed regarding its phase formation, microstructure and mechanical performance. Citric acid neutralization time increased with each recycling cycle. EDX analysis revealed that calcination at 550 °C for 1 h left approximately 8 and 12 wt% Cl in the recycled MgO after the first and second cycle. Under fixed-mass replacement, this residual chloride altered the effective MgO:MgCl2 ratio from 5.4 to 3.4 and 3.7, close to the stoichiometry of Phase 3. Residual chloride may bias the active MgO content and the reactivity indicators of recycled MgO and therefore has to be quantified and considered in the mix design. Open questions concern the predominance of Phase 5 in Cycle 2 R_MOC despite its high chloride content and the decrease in mechanical performance despite matrix densification.
