Synthesis,High-temperature And Corrosion Ristance Of Phosphoric Acid Based Geopolymer

The phosphoric acid based geopolymer with three-dimensional network gel structure,formed by the reaction of divalent or trivalent metal oxide with phosphoric or phosphate solution,has excellent mechanical properties,high temperature resistance and corrosion resistance.In this study,metakaolin and phosphoric acid are used to prepare phosphoric acid based geopolymer（MGP）,and the optimal concentration of phosphoric acid solution and solid-liquid ratio are discussed.Meanwhile,MgO and Fe₃O₄ are selected as modifier to optimize the properties of phosphoric acid based geopolymer,and the effect of MgO and Fe₃O₄ dosage on the mechanical properties of phosphoric acid based geopolymers is discussed.Further more,the high temperature and corrosion resistance of phosphoric acid based geopolymer（MGP）,MgO-phosphoric acid based geopolymer（MgO-MGP）and Fe₃O₄-phosphoric acid based geopolymer（Fe₃O₄-MGP）are investigated.The main conclusions could be generalized as follows:（1）The optimal concentration of phosphoric acid solution,solid-liquid ratio,MgO and Fe₃O₄content for preparing phosphoric acid based geopolymer are discussed.The results show that the compressive strength of MGP could reach 42.45 MPa after sealed for 28 days when the concentration of phosphoric acid solution and solid-liquid ratio are 3:2 and 1:1.3,respectively.When the dosages of MgO and Fe₃O₄ are 6.25%and 9.38%,MgO-MGP and Fe₃O₄-MGP could reach the maxmum compressive strength 58.03 MPa and 62.81 MPa,respectively.The heat of neutralization between MgO and phosphate ions increases the temperature of reaction system,accelerates the depolymerization and polycondensation,increases the geopolymerization,and enhances the mechanical properties of the geopolymer.The gel phase generated in Fe₃O₄-MGP increases with the increase of Fe/P,which has the positively effects on the compressive strength.XRD,FT-IRand SEM results show that MGP,MgO-MGP and Fe₃O₄-MGP are mainly composed of amorphous gel,MgO and Fe₃O₄ increased the density of the geopolymer structure and improved the mechanical performance.（2）The high temperature resistance of MGP,MgO-MGP and Fe₃O₄-MGP are discussed under calcined at 600,800,1000,1100 and 1200oC.The results show that there is no melting occurred during MGP,MgO-MGP,and Fe₃O₄-MGP after calcined at 1200oC,and the compressive strength retention rates could reach 95.43%,88.04%104.14%,respectively.The high compressive strength retention rate of MGP and Fe₃O₄-MGP after 1200°C calcined is related to the densification of structure caused by the sintering reaction,while the continuous decomposition of hydration products of the MgO-MGP leads to a significant reduction in the compressive strength.According to the XRD,FT-IR,SEM and TG-DTA analyses,the microcracks and porosity caused by water loss decreased the compressive strength of MGP when the calcination temperature is 600oC;while MgO-MGP and Fe₃O₄-MGP samples formed a high-temperature autoclave due to the dense structure and leads to further hydration of unreacted substances,increasing the compressive strength of the geopolymer.At 800oC and 1000oC,the decomposition of hydration products leads to the decrease of compressive strength.The temperature furtherly increase,the sintering reaction occurs,resulting in a slight rebound in compressive strength of MGP and Fe₃O₄-MGP;incontrast,the compressive strength of MgO-MGP is further reduced due to the continued decomposition of hydration products.（3）The corrosion resistance of MGP,MgO-MGP and Fe₃O₄-MGP in different immersion media（deionized water,5 wt%HCl,5 wt%NaOH and 5 wt%Na₂SO₄）is discussed.The results show that MGP,MgO-MGP and Fe₃O₄-MGP have excellent corrosion resistance to water and salt;however,the corrosion resistance in acid-base is poor due to the decomposition of geopolymer structure which caused by the erode of H⁺and OH^-.The structure of MgO-MGP is destroyed after immersed to the media,which is resulted from the internal stress generated from the continues reaction between MgO and the solution.The corrosion resistance of Fe₃O₄-MGP is superior to MgO-MGP,because the density and the cohesion between the geopolymeric structures are increased after adding Fe₃O₄,which hinders the corrosion of H⁺and OH^-.The good mechanical properties,high temperature resistance,water and salt corrosion resistance of phosphoric acid based geopolymer lead a good application potential and prospects in the fields of construction,ceramics,and safe disposal of nuclear waste.