Optimization of g-C ₃ N ₄ and fly ash additives in magnesium oxychloride cement via central composite design for enhanced mechanical and solar-activated self-cleaning performance

Magnesium oxychloride cement (MOC) offers high early strength and low carbon footprint, but its poor water stability limits its practical application. To overcome this limitation, this work proposes the incorporation of micro-(fly ash) and nano- g-C3N4) additives into the MOC, imparting self-cleaning activity under light irradiation using a central composite design with response surface methodology. The optimal formulation (3.5 wt% g-C3N4 and 2.5 wt% fly ash) increased the compressive strength from 57.67 to 82.04 MPa, enhanced nanomechanical properties (hardness >435 MPa, Young’s modulus >17 GPa), retaint 85% strength in water, and exhibited promising self-cleaning activity under solar-like irradiation (up to 80% of efficiency). These results are attributed to a synergistic reinforcing effect within the cement matrix, in which fly ash enhances matrix densification, while g-C3N4 acts as a nanoreinforcement. In addition, the aluminosilicate framework derived from the fly ash in the composite provides a positive surface charge that helps control pollutant adsorption, enhancing the self-cleaning efficiency.