Properties and hydration mechanism of foamed magnesium oxysulfate cement under acid modification

Foamed magnesium oxysulfate cement (FMOSC) as a new type of porous building material has many advantages, but deficiencies such as low strength and poor water resistance limit its application to an extent. To mitigate them, this study aims to optimize the performance of the FMOSC system using the modifiers 1-Hydroxyethylidene-1,1-diphosphonic acid (HEDP) and potassium dihydrogen phosphate (KDP). The individual and combined effects of the two modifiers on the compressive strength, water resistance, and pore structure of FMOSC are compared. The microstructure and phase composition of FMOSC are further analyzed using X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), laser microscopic Raman spectroscopy (Raman), thermogravimetric-differential scanning calorimetry (TG-DSC), and field-emission scanning electron microscopy-energy dispersive spectroscopy (FESEM-EDS). The findings reveal that both single and compound modifiers positively influence the FMOSC systems. The compressive strength, water resistance and pore structure of the specimens were significantly optimized when the dosage of compound modifier was 0.75 % m(HEDP):m(KDP)=2:1 compared to the single modifier. HEDP and KDP stabilize [Mg(OH)(H2O)x]+ through chelation and adsorption, respectively, thereby promoting the generation of the 5·1·7 phase. Furthermore, a positive synergistic effect generates more 5·1·7 phases when both are added simultaneously. Additionally, with the aid of anionic surfactants, the 5·1·7 phase in the bubble wall region grows more densely compared to the 5·1·7 phase in the triangle region. The results of this study show that adding modifiers to FMOSC can expand the scope of its applications, increasing its potential as a structural building material and contributing to building energy efficiency and sustainable development.