Dissolution behaviors and mechanisms of metakaolin in acidic activators

Aluminosilicate dissolution is critical for understanding the chemistry of silico-aluminophosphate (SAP) geopolymer. The dissolution behaviors of typical aluminosilicate (i.e., metakaolin, MK) in organic and inorganic acids were studied using optical microscopy as well as three-dimensional profilometry. The underlying mechanisms of MK dissolution in acid solutions were further revealed using FTIR, NMR, XPS, and SEM-EDS measurements and analyses combined with molecular dynamics (MD) simulations. The Results indicated that phosphoric acid (H3PO4, PA) and oxalic acid (H2C2O4, OA) were more likely to dissolve the Al phase in MK than citric acid (C6H8O7, CA), tartaric acid (C4H6O6, TA) and monoaluminum phosphate (Al(H2PO4)3, MAP). Compared to the inorganic acids, the organic acids were more prone to disrupting the Si-OH bonds in MK because of the highly polar functional groups (-COOH) in the organic acids. The dissolution of MK depended on the -COOH content of the employed acid. Furthermore, according to the MD simulation results, the chemical stability of Si-OH bonds was higher than that of Al-OH bonds in terms of the acid dissolution of MK. Acidic molecules were adsorbed on the surface of MK under an electrostatic field and reacted with the surface AlO bonds. Such action resulted in the dealumination of MK. Thus, the molar ratios of Al/P, Si/P and Al/Si in SAP gels can be tailored by adding organic acid.