| Geopolymer materials have been widely used in construction due to their excellent mechanical properties and low carbon dioxide emissions.However,the stability of geopolymer under harsh environments remains unclear.Here,we studied the physico-mechanical stability of metakaolin geopolymer(MKG)mortars with different mixing ratios(SiO2/Al2O3=4/4.5,Na2O/A12O3=0.8/1)soaked in different sulfate solutions(5%-wt Na2SO4&MgSO4,10%-wt Na2SO4&MgSO4),and change of pH value and ion concentration of sulfate solutions were also determined.For MKG mortar specimens,the change of appearance,mass and length,strength,microstructure and chemical composition of specimens under sulfate attack were observed,and it was determined that MKG mortar specimens soaked in 5%-wt sulfate solutions showed no deterioration visual,mass and length.During exposure process,the strength and microstructure of the MKG specimen will be gradually optimized by continuous polymerization reaction,and no new crystals was produced in the specimen after sulfate exposure.Particularly,MKG mortar specimens soaked in the magnesium sulfate solutions for 30 days got white‘rime’,a mixture of Mg(OH)2,MgSO4,MgCO3 crystals and M-S-H gel and M-S-Al-H gel,showed on surface.The white‘rime’gradually disappear due to the reaction of Mg(OH)2 and sodium silicate,forming M-S-H gel.For MKG paste specimens,the changes in pH value and ion concentration of sulfate solution,as well as the changes in compressive strength,microstructure and chemical composition of the specimens were studied.It was confirmed that there was material exchange between the MKG paste specimen and the sulfate solution.The alkalis(OH-)and Na+ in the specimens would gradually seep into the solution,causing the increase of the pH value and Na+concentration of sulfate solution.Mg2+in magnesium sulfate solution would gradually permeate into the specimen and exchange with Na+to generate M-S-Al-H gel.Increasing the concentration of sulfate solutions from 5%-wt to 10%-wt,the crystallization of sulfate would be more obvious.When soaked in sodium sulfate,the strength of MKG paste specimen would fluctuate due to the crystallization of sodium sulfate,and the microstructure would become loose respectively.The strength of MKG paste specimen showed continuous increasing under magnesium sulfate exposure.During exposure,the vibration of O-H bond and H-O-H bond inside the specimen would become stronger due to the migration of cations.The infrared absorption peak of Si-O-T(Si/Al)bond would become stronger due to the continuous polymerization reaction of the specimen.The influence of SiO2/Al2O3 ratio and Na2O/Al2O3 ratio on MKG mortar specimens and paste specimens was different.For mortar specimens,higher SiO2/Al2O3 ratio would enhance the reactions between magnesium sulfate and MKG specimens,while the strength of the specimens with higher SiO2/Al2O3 ratio soaked in magnesium sulfate would fluctuate.However,increasing Na2O/Al2O3 ratio would enhance the physical and chemical stability of MKG mortar specimens under sulfate attack.For MKG paste specimens,the influence of SiO2/Na2O(Ms)ratio of alkali activator on the resistance to sulfate corrosion was greater.When Ms increases from 1.5 to 2.0,the MKG paste specimens got better mechanical properties under sulfate immersion.Based on the results of this study,it was concluded that the MKG specimen had good physical and chemical stability against sulfate attack.The content of alkali(Na2O)in alkaline activator influenced the sulfate resistance of MKG specimens,and the presence of aggregate had a certain effect on the sulfate resistance of MKG specimens.In future,MKG specimens with high strength and high resistance to sulfate corrosion should be prepared with a comprehensive consideration of the mixture ratio. |
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