Study On Preparation And Properties Of Geopolymers Reinforced By Whiskers

Geopolymer is an inorganic silicate material with three-dimensional network structure rising in recent years.It is a green and sustainable cementitious material.It is usually prepared by mixing alkaline solution with aluminosilicate powder.Because of its high early strength,fast setting speed and similar performance to cement,it is a green alternative to traditional Portland cement.Due to geopolymers have large brittleness,poor toughness and low flexural strength,fiber blending method is often used to modify geopolymers,its application and popularization are limited.Therefore,based on previous studies,this topic selects two kinds of whiskers(calcium sulfate whisker and potassium titanate whisker)as fillers and metakaolin as silicon aluminum raw materials to prepare whisker/metakaolin based geopolymer composites with different contents(1 wt%-5wt%),and the influence of whiskers on the properties of geopolymers were studied.The mechanical properties of two kinds of whiskers can be improved by directly adding them into geopolymer.The strength first increases and then decreases with the increase of whisker content,but the modification effect was not stable enough,and the deviation of the same group of test results were large.Among them,the optimum content of calcium sulfate whisker was 1 wt%,and the compressive and flexural strength of geopolymer composites cured for 28d were 52.9 MPa and 10.5 MPa,which were increased by 22.6%and 66.7%respectively compared with the blank group;Furthermore,after the surface modification of calcium sulfate whisker with trisodium phosphate was added into geopolymer,the optimal content was still 1 wt%.At this time,the compressive and flexural strength of Geopolymer Composites were 57.3 MPa and 11.75 MPa respectively after curing for 28d,which was 8.3%and 11.9%higher than that of calcium sulfate whisker/geopolymer without surface modification;The cumulative pore volume of the composite decreased from 176 mm~3/g to 145 mm~3/g,which was 25%lower than193 mm~3/g in the blank group.The durability of modified calcium sulfate whisker/Geopolymer in sulfate environment was further explored.After soaking in sulfate solution for 120d,the cumulative pore volume and the most probable pore diameter of modified calcium sulfate whisker/geopolymer were 161 mm~3/g and 10.09nm respectively.Under the same soaking time,the cumulative pore volume,and the most probable pore diameter of pure geopolymer are 178 mm~3/g and 12.68 nm respectively.The sulfate corrosion resistance of the composite is improved.The optimum content of potassium titanate whisker is 3 wt%.At this time,the compressive and flexural strength of Geopolymer Composites cured for 28 days are 66.5MPa and 8.2 MPa,which are increased by 54.1%and 30.0%respectively compared with the blank group.Furthermore,the effects of different whisker dispersion processes,such as adding different dispersants,ultrasonic after mixing whiskers with activators,and surface modification of whiskers,on the modification effect of geopolymers are studied.The results show that the effect of only using dispersants to assist the dispersion of whiskers in geopolymers is limited.Although the performance deviation of the same group is reduced,the overall performance is declining;On this basis,by adding ultrasonic dispersion process,the compressive and flexural strength are further improved by 9%and35%,but the properties are still lower than those of the samples directly doped with whiskers.Surface modification can significantly improve the dispersion of whiskers in geopolymer matrix,to improve the properties of geopolymer.By attaching hydroxyl groups or groups with hydroxyl groups to the whisker surface,it can not only reduce the repulsive force between whisker particles,but also promote the early hydration reaction of geopolymer under the condition of alkali excitation and enhance the adhesion between whisker and matrix.After surface modification,the two kinds of whiskers can better fill the pores of geopolymer,reduce the average pore diameter and cumulative pore volume of composites,improve the pore structure of geopolymer,give full play to the crack bridging effect of whiskers in the matrix,and play a reinforcing and toughening modification effect.The three-point bending model simulation is established by ANSYS software to verify the importance of whisker crack bridging in geopolymer matrix.The research results have certain theoretical and practical value for the research of whisker modified geopolymer.