Unveiling protonated Al–O–P and Si–O–Si domains in phosphoric acid–activated metakaolin by solid-state NMR: temperature-dependent network/strength development

Phosphoric acid–activated metakaolin forms silico-alumino-phosphate (SAP) geopolymers through acid-driven transformation of metakaolin and subsequent condensation of amorphous Al–O–P and Si–O–Si networks. In this work, where we focus on finding molecular structure-property relations, SAP geopolymers with an Al/P molar ratio of 1.5 were cured at 20–70 °C and comprehensively studied by setting-time measurements, isothermal calorimetry, mechanical testing, FTIR spectroscopy, XRD, and multinuclear solid-state NMR. Increasing curing temperature strongly accelerated hardening, reducing the final setting time from 430 h at 20 °C to 4.6 h at 70 °C. However, the highest calorimetric reaction rate and the highest 28-day compressive strength were obtained at 60 °C, whereas curing at 70 °C promoted faster early hardening but resulted in lower final strength. XRD and porosimetry showed predominantly amorphous and similarly compact matrices, indicating that strength differences arise mainly from local structural evolution rather than porosity. Solid-state NMR revealed the progressive transformation of mobile phosphate rich environments into rigid proton proximal Al VI –O–P domains, accompanied by condensation of the silicate network toward Si–O–Si-rich structures. FTIR and ¹H NMR further showed that curing temperature and aging reduce the content of mobile/protonated OH environments. These results demonstrate that the mechanical performance of SAP geopolymers is closely linked to temperature-controlled evolution of amorphous protonated Al–O–P and Si–O–Si domains.