Competitive complexation mechanisms between Ca, Al, Fe, Mg, Si and triisopropanolamine (TIPA) at pH 13

Triisopropanolamine (TIPA) can affect reactivity and hydration process of clinker phases and supplementary cementitious materials by forming aqueous complexes with the elements in the pore solution, yet competitive complexation mechanisms remain uncharacterized at the molecular level. This study establishes TIPA's competitive binding with five elements (Ca, Al, Fe, Mg, Si) at pH 13 through 1 H NMR spectroscopy, ESI-MS spectrometry, and ICP-OES analysis. Single-metal systems reveal a binding hierarchy where Al and Fe form kinetically stable complexes with distinct stoichiometries, while Ca and Mg exhibit weak binding. TIPA provides the sole Fe solubilization pathway (510-fold enhancement), whereas Al maintains alternative solubility through Al(OH)₄ − formation. Multi-metal competition produces four element-selective, non-cumulative effects: mutual reduction in Ca + Al systems; synergistic enhancement in Ca + Al + Fe systems; selective Mg antagonism disrupting Fe-TIPA while preserving Al-TIPA coordination; Si-triggered co-precipitation removing Ca and Al while Fe-TIPA remains stable. These phenomena arise from parallel equilibria rather than sequential displacement, explaining TIPA's unpredictable performance. These mechanisms enable rational admixture optimization in chemically diverse cementitious systems.