Fire resistance of phosphoric acid activated metakaolin geopolymer

Fire resistance is a key advantage of geopolymer (GP) materials. While most research focuses on alkali-activated GPs, this study examines the fire resistance of phosphoric acid-activated GPs with varying phosphorus-toaluminum (P/Al) molar ratios. By activating metakaolin with different ortho-phosphoric acid (H3PO4) solutions, it was found that GPs with P/Al > 0.59 exhibited intumescence during fire tests, resulting in a 110( degrees)C lower backside temperature (310( degrees)C) compared to non-intumescent GPs (420( degrees)C). Physico-chemical changes were analyzed before and after fire tests using DMA, TGA NMR, EPMA, and XRD. The study revealed dehydration, crystallization, and rheological changes, with NMR showing transitions from Al(VI) to Al(IV), full condensation of phosphorus species, and Si-O-Al to Si-O-Si bonds. Intumescence was linked to GP softening between 100 and 160( degrees)C, achievable only with a P/Al ratio >= 0.74, with no additional benefits from excess phosphorus.