Mix proportion design of phosphoric acid-activated cementitious materials and microstructure evolution at high temperature

To clarify the polymerization mechanism and high-temperature resistance of acid-activated cementitious materials. In this study, metakaolin was used as precursor, phosphoric acid solution served as the activator, and 0–6% of magnesia was incorporated to prepare acid-activated cementitious materials. The effects of P/Al (molar ratio of phosphorus to aluminum), L/S (mass ratio of activator to raw material) and magnesia dosage (W_MgO) on the mechanical properties and high temperature resistance of acid-activated cementitious materials were investigated. This study elucidated the effects, mechanism and microstructure evolution associated with phosphoric acid activation. The optimal formulation of the acid-activated cementitious material is characterized by a P/Al ratio of 0.8, an L/S ratio of 0.9, and a W_MgO of 4 %. Under these conditions, the compressive strength at 28 d can reach 101 MPa. Metakaolin is depolymerized in an acidic environment provided by phosphoric acid to produce oligomeric silicon and Al³⁺, which then undergo a polycondensation bonding process with PO₄³⁻ to form an amorphous gel. Magnesia reacts with phosphoric acid to form MgHPO₄ 3H₂O, which then adheres to the dealuminated silica layer of metakaolin to form an acid-activated cementitious material. After calcination at 1200 °C, the acid-activated cementitious material with a P/Al ratio of 0.6 and an L/S ratio of 0.9, and without magnesia, exhibited the best performance, with a residual strength of 39.7 MPa.