Journal Article

Structural Study of Metakaolin-Phosphate Geopolymers Prepared with Wide Range of Al/P Molar Ratios

Pokorný, Václav, Krejsová, Jitka, Koňáková, Dana, Urbanová, Martina, Keppert, Martin, Breníková, Michala, Pommer, Vojtěch, Vejmelková, Eva, Šeděnková, Ivana, Brus, Jiří

2025

Overview

Geopolymers represent an innovative and environmentally sustainable alternative to traditional construction materials, offering significant potential for reducing anthropogenic CO2 emissions. Among these, phosphoric acid-activated metakaolin-based systems have attracted increasing attention for their chemical and thermal resilience. In this study, we present a comprehensive structural and mechanical evaluation of metakaolin-based geopolymers synthesized across a wide range of Al/P molar ratios (0.8–4.0). Six formulations were systematically prepared and analyzed using X-ray powder diffraction (XRPD), small-angle X-ray scattering (SAXS), Fourier-transform infrared spectroscopy (FTIR), solid-state nuclear magnetic resonance (ssNMR), and complementary mechanical testing. The novelty of this work lies in the integrated mapping of composition–structure–property relationships across the broad Al/P spectrum under controlled synthesis, combined with the rare application of SAXS to reveal composition-dependent nanoscale domains (~18–50 nm). We identify a stoichiometric window at Al/P ≈ 1.5, where complete acid consumption leads to a structurally homogeneous AlVI–O–P network, yielding the highest compressive strength. In contrast, acid-rich systems exhibit divergent flexural and compressive behaviors, with enhanced flexural strength linked to hydrated silica domains arising from metakaolin dealumination, quantitatively tracked by 29Si MAS NMR. XRPD further reveals the formation of uncommon Si–P crystalline phases (SiP2O7, Si5P6O25) under low-temperature curing in acid-rich compositions. Together, these findings provide new insights into the nanoscale structuring, phase evolution, and stoichiometric control of silica–alumino–phosphate geopolymers, highlighting strategies for optimizing their performance in demanding thermal and chemical environments.

• Publisher: MDPI AG
• Subject: Acids / Air quality management / Clay / Composition / Compressive properties / Compressive strength / Curing / Density / Diffraction / Flexural strength / Fourier transforms / Geopolymers / Infrared spectroscopy / Low temperature / Mechanical tests / Metakaolin / NMR spectroscopy / Particle size / Phosphates / Phosphoric acid / Resonance scattering / Silica / Silicon dioxide / Small angle X ray scattering / Spectrum analysis / Stoichiometry / X ray powder diffraction / X-rays