This study compares, for the first time, MgO-based (MB) cement and Portland-based (PB) cement for stabilizing earth mortars. While MB and PB earth mortars reach similar strength, MB cement stabilization demonstrates superior early-age performance. Thermogravimetric analysis, X-ray diffraction, ²⁹Si and ³¹P NMR spectroscopies show that the cement reacts in both systems and allow to establish the phase assemblages. The stabilized earth pastes contain less hydroxide phases, indicating a pozzolanic reaction in both cases.
MB-stabilized clay mortars retain about 1/3 of the compressive strength of pure MB mortar, while with PB this proportion is only 1/5. This difference demonstrates that MB is more compatible with clay minerals and more suitable for stabilizing earth mortars. If MB cement could be produced with renewable energy from CO₂-free sources (instead of from magnesite), stabilization of earth mortars with MB would be substantially more CO₂ efficient than with PB.