Early-Age Mechanical Evaluation of Alkali-Activated Ferro-Sulphoaluminate Cement Mortars Containing Nano-SiO2 or Nano-Al2O3

Ferro-sulphoaluminate cement is a potential precursor for inorganic protective coatings, yet the early-age mechanical response of alkali-activated formulations containing nano-oxide powders remains insufficiently defined. This exploratory study characterized the constituent powders and compared the 3 d flexural and compressive responses of five complete mortar formulations as follows: a non-activated reference, two nano-SiO2 formulations, and two nano-Al2O3 formulations. One independently prepared batch with three replica prisms was tested per formulation. Mean flexural strengths ranged from 3.79 to 4.99 MPa, with no statistically conclusive overall formulation difference (p = 0.089). N0 reached 45.80 MPa in compression, but with the lowest flexural strength, the activated nano-modified formulations had higher flexural strength and retained 46.5%–53.9% of the compressive strength of the N0 mean. Potential activator–binder incompatibility, excessively rapid early reactions, reduced workability, and impaired compaction are literature-supported hypotheses that may help explain this reduction. The high alkalinity of the activator may have contributed to rapid early reactions. Adding nano-additives (nano-SiO2 and nano-Al2O3) only could not modify this reverse effect. Future work should consider using a low-alkali activator and adding a setting retarder in ferro-sulphoaluminate cement. This study provides an early-stage dataset and further in situ validation and long-term performance testing must be performed in the future to evaluate adhesion, permeability, and durability properties.