Improving the water resistance of phosphoric acid-based geopolymers by optimizing the Al/Si molar ratio method

Phosphoric acid-based geopolymers (PGEOs) have a special application in acid environment, but its poor water resistance caused concern in engineering. Al³⁺ can improve the mechanical performance of PGEOs; it may improve the water resistance of PGEOs. In this article, the impact of Al/Si molar ratios on the water resistance of PGEOs and related mechanisms were investigated using unconfined compression strength, scanning electron microscope, X-ray diffraction, and mercury intrusion testing. Results indicate that the Al/Si molar ratio is a more important parameter affecting water resistance compared to the P/Si molar ratio, high Al/Si molar ratio is conducive to the enhancement of soaked strength of PGEOs, the PGEO's specimens with an Al/Si molar ratio of 0.664 undergoes varying degrees of damage at 280d soaking, but no cracks were found if the Al/Si molar ratios are > 0.669. Especially when the Al/Si molar ratio reaches 0.687, there is little change in the strength of PGEOs after soaking. When Al/Si molar ratios increased, some crystalline compounds with higher Al content, such as AlPO₄, are generated, which encapsulate the gels with Si–O–P linkages and restrict its dissolution in water. Therefore, the water resistance of geopolymer is improved by high Al/Si molar ratios. Moreover, high Al³⁺ content can reduce the geopolyerization rate, making the proportion of harmful pores reduce.