Preparation And Properties Of Phosphoric Acid-based Geopolymer Foams

Inorganic foams cannot combust when exposed to fire and may be a more suitable substitutefor cement concrete cover in fire protection.Geopolymers,especially acid-base geopolymershave attracted considerable attention due to their environmental friendliness,hightemperature stability and superior mechanical property.Meanwhile,phosphoric acid-basedgeopolymer foams can not only inherit the advantages of acid-based geopolymers but alsodecrease the thermal conductivity and self-weight to a large extent,owing to the presence othe huge number of pores.The remarkable high temperature stability makes phosphoricacid-based geopolymer foams strong candidates in preparation of inorganic foams for fireprotection.Therefore,phosphoric acid-based geopolymer foams are fabricated frommetakaolin and phosphoric acid with hydrogen peroxide as a foaming agent in this researchMeanwhile,the performance optimization of phosphoric acid-based geopolymer foams wasexplored by changing the liquid-solid ratio and adding manganese dioxide or fibres,whichprovided the research and experimental basis of phosphoric acid-based geopolymer foams insecurity field.The effect of hydrogen peroxide as foaming agent on the preparation of phosphoric acid-based geopolymer foams was studied by changing the amount of hydrogen peroxide.The effect was characterized by physical performance test and fire resistance test.Different content of hydrogen peroxide affected the number of bubbles formed in the slurry,leading to the change of pore structure and the properties of phosphoric acid-based geopolymer foams.To be specific,the compressive strength of the sample was up to 0.7 MPa and the thermal conductivity was lower than 0.2 W /(m·K),when the mass fraction of hydrogen peroxide was 2-3.5%.After 2 h fire resistance tests,the reverse-side temperatures except the phosphoric acid-based geopolymer foams with 4% mass fraction of hydrogen peroxide still maintain around 220 °C.From the analysis of the mineral phase,molecular structure,the amorphous gel phase was changed to calcite and aluminum phosphate phase after fire resistance.In order to optimize the pore structure of phosphoric acid-based geopolymer foams,the adjustment of liquid-solid ratio and adding the manganese dioxide were studied in this paper.The effects of both of that on the performance of samples were discussed by combining with the physical properties,fire resistance test,mineral phase and molecular structure analysis.Due to the liquid-solid ratio and manganese dioxide could change the viscosity of slurry and foaming rate of hydrogen peroxide respectively,the bubbles in the slurry were influenced by both of that.It would improve the pore structure and optimize the properties of phosphoric acid-based geopolymer foams.When the liquid-solid ratios were 1.66 and 1.71,the samples showed lower thermal conductivity and higher compressive strength,about 0.18 W/(m·K)and 2.7 MPa,respectively.On this basis,the thermal conductivity of the sample decreased to 0.17 W/(m·K),and the strength remained 2.09 MPa,when the content of manganese dioxidwas 0.1%.After 3 h fire resistance tests,the reverse-side temperatures with liquid-solid ratiof 1.71 and 0.1% content of manganese dioxide both still maintain around 260 °CMeanwhile,the addition of manganese dioxide introduced manganese element into thsample,resulting in manganese phosphate phase,but it did not affect the phase change of thsamples which had fire resistence test.To enhance the performance of phosphoric acid-based geopolymer foams,the enhancement effect of the addition of basalt fiber and glass fiber on the sample through the physical properties fire resistance test,morphology analysis and physical phase change had been discussed.The addition of basalt fiber and glass fiber can enhance the properties of phosphoric acid-based geopolymer foams,maintain the integrity of samples.With the addition of basalt fiber and glass fiber,the flexural strength increased from 0.6867 MPa to 0.7867 MPa and 0.825 MPa respectively.After 3 h fire resistance tests,the reverse-side temperatures with 0.75% content of basalt fibre and 1.33% content of glass fibre both are reduced by about 20 °C,and the cracks on the surface of the sample were significantly reduced and disappeared after the fire resistance.Moreover,microstructure analysis showed that the fibers were wrapped by the gel phase,which made the bridging effect and deflection capacity to cracks better play in samples to realize the enhancement of the properties of phosphoric acid-based geopolymer foams.Therefore,phosphoric acid-based geopolymer foams are successfully fabricated using hydrogen peroxide as a foaming agent.Moreover,adding fibers and increasing the liquid-solid ratio properly can effectively enhance and optimize the performance of phosphoric acid-based geopolymer foams,and improve the potential for fire resistance.