Preparation and Reaction Mechanism of Ambient-Cured Silico-Aluminate Phosphate Geopolymer Modified by Magnesium Oxide

Abstract

Silico-aluminate phosphate (SAP) geopolymers generally require heat curing because of their slow reaction kinetics at ambient temperature. To enable ambient curing, this study couples geopolymerization with the exothermic reaction between magnesium oxide (MgO) and potassium dihydrogen phosphate (KDP). The workability, setting time, strength development and reaction mechanism were characterized by isothermal calorimetry, ICP-OES, FTIR, solid-state MAS NMR, TG-DTG, MIP and kinetic modeling. The results indicate that the hydration heat released by MgO accelerates metakaolin dissolution and promotes the formation of a crystalline–gel interpenetrating network. The optimum formulation (MK/MgO = 7/3, 35 wt% KDP) reaches 31.2 MPa after 28 d of ambient curing. These findings provide an energy-efficient route for the ambient-temperature preparation of acid-based geopolymers.