Effect Of Water On Microstructure And Properties Of Alkali-activated Geopolymers

As a reactive medium,water,which is important part of synthetic geopolymer materials,participates in the entire geopolymerization process and has a significant effect on the microstructure and mechanical properties of geopolymers.In this paper,Metakaolin-based geopolymers(?)and(?)were prepared with the using of metakaolin as raw material,and water glass and sodium hydroxide as activators.The slag-based geopolymers were prepared by using slag as raw material,sodium hydroxide as activators.The effects of alkali activator and H2O/Na2O mole ratio on the properties of alkali-activated geopolymers were studied.In order to explore the role of water in geopolymers,water was removed from geopolymers by vacuum dehydration.The effects of different molar ratios of H2O/Na2O on the properties and microstructure of geopolymers before and after vacuum dehydration were characterized by compressive strength,XRD,BET,and SEM.The main results are as follows:(1)The metakaolin-based geopolymers(?),(?)and slag-based geopolymers obtained the highest mechanical properties when the Na2O/Al2O3 molar ratios were 0.67,0.85,and 0.87,respectively.The XRD showed that the metakaolin-based geopolymer(?)gel phase was mainly amorphous structure;the metakaolin-based geopolymer(?)exhibited Na-A zeolite structure;in addition to the amorphous structure in the slag-based polymer,hydration products such as calcium silicate and hydrocalumite were also found.(2)In the process of increasing the molar ratio of H2O/Na2O of geopolymers:the compressive strength of metakaolin-based geopolymer(?)increased first and then decreased.The compressive strength of metakaolin-based geopolymer(?)and slag-based geopolymer decreased gradually.The setting time and gel content of three samples increased with the increase of molar ratio of H2O/Na2O.(3)Combined with BET and SEM analysis,the porosity of alkali-activated geopolymers increased and obtained more pore structures with the increase of water content in geopolymers.While the molar ratio of water to sodium is increased,the geopolymers can obtain a larger specific surface area after losing water in a vacuum.(4)Metakaolin-based geopolymer(?)and(II)were affected by the process of geopolymerization,more water would be removed after curing 3 days.The slag-based geopolymers were affected by hydration products,more water would be removed after curing 1 days.However,the loss of water in the early curing stage of geopolymers caused the early strength of geopolymers to shrink in the later stage,which had a negative impact on the whole geopolymerization.(5)The final residual water(vacuum dehydration at 120 ?,curing for 3 days)of Metakaolin-based geopolymer(?),(?)and slag-based geopolymers were:4.24%,1.08%,7.91%,respectively.The geopolymers after water loss also maintain stable mechanical properties,which were not lower than the samples undehydrated treatment.